EP4713085A1 - Retainer for a medical device delivery system - Google Patents

Abstract

A medical system including a receptacle device configured to hold an implantable medical device (IMD) within a receptacle volume. The receptacle device includes a retainer configured to engage the IMD to limit a translation of the IMD relative to the receptacle device. The retainer is configured to disengage from the IMD when the IMD is rotated relative to the receptacle device. In examples, the retainer is configured to disengage from the IMD when the IMD rotates to cause substantial alignment of a retention finger of the retainer and a notch defined by the IMD. In examples, the retainer is configured to allow alignment of the retention finger and the notch when the IMD rotates in a primary rotational direction and substantially prevent the alignment when the IMD rotates in a second rotational direction.

Description

Atty Ref. No: A0010569WO01 RETAINER FOR A MEDICAL DEVICE DELIVERY SYSTEM [0001] This application claims the benefit of U.S. Provisional Patent Application Serial No.63/502,775, filed May 17, 2023, the entire content of which is incorporated herein by reference. TECHNICAL FIELD [0002] This disclosure is related to systems for delivery and/or retrieval of implantable medical devices. BACKGROUND [0003] Various types of implantable medical devices have been implanted for treating or monitoring one or more conditions of a patient. Such implantable medical devices may be adapted to allow medical devices to monitor and/or treat conditions or functions relating to heart, muscle, nerve, brain, stomach, endocrine organs or other organs and their related functions. The implantable medical devices may be implanted at target locations selected to detect a physiological condition of the patient and/or deliver one or more therapies. For example, implantable medical devices may be delivered to locations within an atrium or ventricle of a heart to sense intrinsic cardiac signals and deliver pacing or antitachyarrhythmia shock therapy. [0004] Some implantable medical devices are sized to be completely implanted within one of the chambers of the heart and/or another anatomical volume of the patient to detect a physiological condition and/or deliver one or more therapies. Such implantable medical devices may utilize delivery and/or retrieval systems to allow a clinician to navigate the implantable medical device (e.g., through vasculature of the patient) to the target location, and/or to retrieve the implantable medical device from the patient. In some examples, the implantable medical device may include one or more anchoring components intended to engage tissues at the target location (e.g., for implantation) and/or disengage from tissue at the target location (e.g., for retrieval). SUMMARY [0005] The disclosure describes a medical system configured to deliver, position, Atty Ref. No: A0010569WO01 retrieve, and/or otherwise re-orient an implantable medical device (“IMD”) within an anatomical volume (e.g., a chamber of a heart) within a patient. The medical system including a receptacle device configured to hold the IMD within a receptacle volume. The receptacle device includes a retainer configured to engage the IMD to limit a translation of the IMD relative to the receptacle device such that, for example, the IMD remains within the receptacle volume during transit through vasculature to an anatomical chamber of a patient, such as a heart chamber. The retainer is configured to disengage from the IMD when the IMD is rotated relative to the receptacle device (e.g., rotated by a clinician). The retainer thus serves to limit translation of the IMD relative to the receptacle device as the retainer and IMD transit through vasculature. Once in proximity to a target site, for example, a rotation of the IMD (e.g., by a clinician) within the receptacle volume causes disengagement of the IMD and retainer, such that the IMD may be extended from or exit the receptacle volume in support of an implantation at the target site. [0006] In an example, a medical system comprises a receptacle device configured to be positioned within an anatomical volume defined by a body of a patient, wherein the receptacle device includes a receptacle wall defining a receptacle volume configured to receive at least a portion of an implantable medical device, the receptacle wall at least partially surrounding a longitudinal axis extending through the receptacle volume, wherein the receptacle device defines a receptacle opening which opens to the receptacle volume at a distal end of the receptacle device, the receptacle opening configured to allow the implantable medical device to pass from the receptacle volume and through the receptacle opening, wherein the receptacle device includes a retainer positioned within the receptacle volume, the retainer defining a retention finger extending in a direction from an inner surface of the receptacle wall toward the longitudinal axis, wherein the retention finger is configured to engage the implantable medical device within the receptacle volume to limit a translation of the implantable medical device relative to the receptacle wall in at least a distal direction of the receptacle device, and wherein the retention finger is configured to disengage from the implantable medical device when the implantable medical device is rotated relative to the receptacle wall. [0007] In an example, a method comprises: engaging, using a retention finger of a retainer, an implantable medical device to limit a translation of the implantable medical device relative to a receptacle device in at least a distal direction of the receptacle device, Atty Ref. No: A0010569WO01 the implantable medical device within a receptacle volume defined by a receptacle wall surrounding a longitudinal axis extending through the receptacle volume, wherein the retention finger extending in a direction from an inner surface of the receptacle wall toward the longitudinal axis, and wherein the receptacle device defining a receptacle opening which opens to the receptacle volume at a distal end of the receptacle device, the receptacle opening configured to allow the implantable medical device to pass from the receptacle volume and through the receptacle opening; and disengaging the retention finger and the implantable medical device using a rotation of the implantable medical device relative to the receptacle wall. [0008] The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF DRAWINGS [0009] FIG.1 is a conceptual diagram illustrating an example medical system and receptacle device within a heart. [0010] FIG.2 is a perspective view an example receptacle device including a retainer engaged with an implantable medical device. [0011] FIG.3 is a perspective illustration of an example implantable medical device and a retainer. [0012] FIG.4 is a cross-sectional schematic diagram of a retainer engaging an implantable medical device, with the cutting plane passing through a longitudinal axis of the receptacle device and parallel to the page. [0013] FIG.5 is a cross-sectional schematic diagram with an implantable medical device in a first rotational position relative to a receptacle wall, with the cutting plane perpendicular to the longitudinal axis of the receptacle device and parallel to the page. [0014] FIG.6 is a cross-sectional schematic diagram with an implantable medical device in a second rotational position relative to a receptacle wall, with the cutting plane perpendicular to the longitudinal axis of the receptacle device and parallel to the page. [0015] FIG.7 is a perspective view of an implantable medical device including a device structure. [0016] FIG.8 is a perspective view of an implantable medical device including a slip Atty Ref. No: A0010569WO01 portion and an interference portion. [0017] FIG.9 is a cross-sectional schematic diagram of a first section of a slip portion of an implantable medical device within a groove of a retainer, with the cutting plane passing through a longitudinal axis of the receptacle device and parallel to the page. [0018] FIG.10 is a cross-sectional schematic diagram of a second section of the slip portion of FIG.9 within the groove of the retainer, with the cutting plane passing through a longitudinal axis of the receptacle device and parallel to the page. [0019] FIG.11 is a cross-sectional schematic diagram of an interference portion of the implantable medical device of FIG.9 and FIG.10 within the groove of the retainer, with the cutting plane passing through a longitudinal axis of the receptacle device and parallel to the page. [0020] FIG.12 is a cross-sectional schematic diagram of a retainer positioned within a gap defined by an implantable medical device, with the cutting plane passing through a device axis of the implantable medical device and parallel to the page. [0021] FIG.13 is a cross-sectional schematic diagram of a retainer establishing an interference fit with the implantable medical device, with the cutting plane passing through the device axis of the implantable medical device and parallel to the page. [0022] FIG.14 illustrates an example technique for engaging an implantable medical device using a retainer. DETAILED DESCRIPTION [0023] This disclosure describes a medical system configured to deliver, position, and/or retrieve an implantable medical device (“IMD”) within an anatomical volume (e.g., a chamber of a heart) within a patient. In examples, the medical system includes a receptacle device configured to hold the IMD during, for example, transit through vasculature of a patient. The medical system is configured to constrain movement of the IMD relative to the receptacle device to limit incidental movement between the IMD and the receptacle device which might otherwise occur. For example, the medical system may constrain relative motions between the IMD and the receptacle device that otherwise might occur as the receptacle device and IMD transit over a curving path through vasculature enroute to a target site. Such relative motions may cause undesired repositioning of the IMD within the receptacle device during the transit. For example, such Atty Ref. No: A0010569WO01 relative motions might cause a slight extension of the IMD from a distal end of the receptacle device, and/or cause other repositioning of the IMD enroute to the target site. [0024] The medical system includes a receptacle device (e.g., a device cup) configured to hold the IMD within a receptacle volume (e.g., a cup volume) during transit to and deployment of the IMD within the anatomical volume of the patient. The receptacle device includes a retainer positioned within the receptacle volume. The retainer is configured to engage the IMD to substantially limit translation of the IMD relative to the receptacle device (e.g., relative to a receptacle wall defining the receptacle volume) to, for example, substantially maintain the IMD within the receptacle volume. The retainer is configured such that rotation of the IMD relative to the receptacle wall causes the retainer to disengage from the IMD, such that the IMD may move (e.g., in a distal direction) relative to the receptacle device. For example, when the retainer is engaging the IMD, a clinician may cause a rotation of the IMD relative to the receptacle wall using a delivery system coupled to the IMD. The rotation may cause the retainer to disengage from the IMD, such that the clinician may cause the IMD (e.g., using the delivery system) to translate distally relative to the receptacle device. As used herein, translation of the IMD relative to the receptacle device and/or receptacle wall may refer to movement of the IMD relative to the receptacle device and/or receptacle wall in a distal direction of the receptacle device and/or a proximal direction of the receptacle device. [0025] The receptacle device includes a receptacle opening at a distal end configured to allow the IMD to pass therethrough during deployment of the IMD. The receptacle device may be configured such that, when the retainer is disengaged from the IMD, the IMD may be translated (e.g., by a clinician) in a distal direction to extend and/or pass through the receptacle opening as, for example, the clinician positions the IMD for deployment, evaluates the target site, and/or for other reasons. In examples, the receptacle device is supported at a proximal end by a delivery catheter defining a delivery lumen. The delivery lumen may open into the receptacle volume. The delivery system may extend through the delivery lumen. The clinician may rotate the IMD within the receptacle volume using the delivery system to cause disengagement of the retainer and the IMD. Once disengaged, the clinician may cause distal and/or proximal movement of the IMD relative to the receptacle device using the delivery system. In examples, the clinician may cause an attachment member of the IMD to engage tissues at the target site using the Atty Ref. No: A0010569WO01 delivery system. [0026] The retainer is configured to be stationary relative to the receptacle wall defining the receptacle volume, such that the IMD rotates relative to the retainer when the IMD rotates relative to the receptacle wall. The retainer includes a retention finger extending substantially from an inner surface of the receptacle wall toward a longitudinal axis extending through the receptacle volume. The retention finger is configured to engage the IMD to limit translation of the IMD in at least a distal direction relative to the receptacle wall. In examples, the retention finger is configured to engage a device structure of the IMD such as a retrieval structure (“IMD retrieval structure). [0027] For example, the device structure may include a stem portion extending from a housing of the IMD (“IMD housing”) and supporting a crown portion. The retention finger may be configured to engage the crown portion to limit translation of the IMD. In some examples, the device structure defines a gap between the IMD housing and the crown portion. The retention finger may be configured to substantially insert within the gap, such that translation of the IMD at least in a distal direction causes the retention finger to encounter the crown portion. In examples, the retention finger is configured to exert a reaction force against the IMD (e.g., the crown portion) to limit translation of the IMD relative to the receptacle wall. [0028] In example, the retainer defines a groove proximal to the retention finger. The groove may be configured to receive a portion of the IMD (“IMD portion”), such as a portion of the head defining an outer perimeter of the head. In examples, the retainer defines the groove substantially between the retention finger and a backing surface proximal to the retention finger. The groove may be configured such that the IMD portion substantially rotates through the groove when then IMD rotates relative to receptacle wall. The IMD may be configured such that, with the IMD portion in the groove, at least a distal translation of the IMD relative to the receptacle wall causes the retention finger to encounter the IMD portion, limiting translation of the IMD. [0029] In examples, the IMD may define a notch configured to allow the IMD to translate relative to the receptacle wall. For example, the device structure may define the notch to substantially pass through the IMD portion, such that when notch is aligned with the retention finger and the IMD translates in the distal direction relative to the receptacle wall, the retention finger passes through the notch and the IMD may translate distally Atty Ref. No: A0010569WO01 relative to the receptacle wall relatively unencumbered by the retention finger. The IMD may be rotated (e.g., by a clinician) to cause the notch to align with the retention finger, such that the retention finger disengages with the IMD. Hence, the retention finger may engage the IMD to limit translation of the IMD relative to the receptacle wall during, for example, transit through vasculature. Once in proximity to a target site, the IMD may be rotated (e.g., by a clinician) to substantially align the notch with the retention finger, such that IMD be translated (e.g., by the clinician) in at least a distal direction relative to the receptacle wall. [0030] In some examples, the receptacle device includes a distal portion which defines the receptacle opening and a proximal portion defining a delivery opening configured to open into the delivery lumen of a delivery catheter. The receptacle device may be configured to support the retainer within the proximal portion. In some examples, the receptacle device (e.g., the receptacle wall) defines one or more flow channels configured to provide a flow path for a fluid from the delivery lumen into the receptacle volume. For example, a flow channel may define a flow path for the fluid from the delivery lumen and substantially past the retainer and into the receptacle volume when the retainer is engaged with the IMD. The flow channel may thus assist a fluid such as a fluorescent dye, or radiopaque fluid, or saline, or other fluid to flow from the delivery lumen and, for example, exit the receptacle volume through the receptacle opening or another passage of the receptacle device when the retainer is engaging the IMD. In similar manner, the defined flow path may reduce hydrodynamic forces (e.g., a fluid pressure) on the IMD when the retainer engages the IMD and the fluid flows from the delivery lumen. [0031] Hence, the medical system includes a receptacle device configured to hold an IMD within a receptacle volume during transit to and deployment within an anatomical volume (e.g., a heart chamber) of a patient. The receptacle device includes a retainer configured to engage the IMD to limit translation of the IMD relative to a receptacle wall of the receptacle device. The retainer is configured such that rotation of the IMD (e.g., by a clinician) within the receptacle volume causes the retainer to disengage from the IMD, such the IMD may be translated (e.g., by the clinician) in at least a distal direction relative to the receptacle wall. The retainer may thus serve to limit translation of the IMD relative to the receptacle device as the retainer and IMD transit through vasculature. In proximity to a target site, rotation of the IMD within the receptacle volume may cause the retainer Atty Ref. No: A0010569WO01 may disengage such that the IMD may be extended from or exit the receptacle volume in support of an implantation at the target site. [0032] FIG.1 is a conceptual diagram illustrating an example medical system 100 within a right atrium (“RA”) of a heart 101. Medical system 100 is configured to deliver and/or retrieve an implantable medical device 102 (“IMD 102”) to and/or from the vicinity of a target site 104 of heart 101. Medical system 100 includes a delivery catheter 106 supporting a receptacle device 108. Receptacle device 108 is configured to hold IMD 102 during delivery, deployment, and/or retrieval of IMD 102. Receptacle device 108 includes a receptacle wall 110 defining a receptacle volume 112 configured to hold and/or support IMD 102 during the delivery, deployment, and/or retrieval of IMD 102. In examples, receptacle device 108 defines an opening 109 (“receptacle opening 109”) which opens into receptacle volume 112. Receptacle opening 109 may be configured to allow at least IMD 102 to pass therethrough. In examples, medical system 100 is configured to deploy IMD 102 from receptacle device 108 (e.g., from a position within receptacle volume 112) and through receptacle opening 109 to cause IMD 102 to engage tissues within target site 104. In examples, medical system 100 is configured to cause IMD 102 to disengage from tissues within target site 104 to, for example, retrieve IMD 102 from and/or reposition IMD 102 within heart 101. [0033] Delivery catheter 106 is configured to deliver receptacle device 108 and/or IMD 102 to an anatomical volume of the patient (e.g., the RA). Optionally, delivery catheter 106 may be advanced to the anatomical volume of the patient through a surrounding tubular member (not shown), such as a sheath or guide catheter, which may be placed with its distal end in the anatomical volume before delivery catheter 106 is advanced through the surrounding tubular member. In examples, delivery catheter 106 is configured to retrieve receptacle device 108 and/or IMD 102 from the anatomical volume of the patient. Delivery catheter 106 may include a distal portion 114 (“delivery catheter distal portion 114”) configured to be intracorporeal to the patient and a proximal portion 116 (“delivery catheter proximal portion 116”) which may be extracorporeal to the patient when delivery catheter distal portion 114 is intracorporeal. In examples, delivery catheter distal portion 114 supports (e.g., is attached to and/or is a substantially unitary component with) receptacle device 108. In examples, delivery catheter 106 is configured to deliver and/or retrieve receptacle device 108 and/or IMD 102 using vasculature of a patient, such Atty Ref. No: A0010569WO01 as an SVC or other vasculature leading to the anatomical volume. In examples, delivery catheter 106 defines a lumen 118 (“delivery lumen 118”) which opens to receptacle volume 112. [0034] In examples, medical system 100 includes a delivery system 120 configured to engage IMD 102 when, for example, IMD 102 is positioned within receptacle volume 112. Delivery system 120 includes a driver 122 that may include a distal portion 124 (“driver distal portion 124”) configured to be intracorporeal to the patient and a proximal portion 126 (“driver proximal portion 126”) which may be extracorporeal to the patient when driver distal portion 124 is intracorporeal. In examples, delivery system 120 includes a head portion 128 supported by driver distal portion 124 and configured to engage IMD 102. At least driver 122 may be configured to slidably translate and/or rotate within delivery lumen 118 (e.g., translate and/or rotate relative to delivery catheter 106). Delivery system 120 may be configured such that the translation and/or rotation of driver 122 causes driver 122 and/or head portion 128 to impart translational and/or rotational forces on IMD 102, such that IMD 102 translates and/or rotates relative to receptacle wall 110. For example, delivery system 120 may be configured to translate within delivery lumen 118 to impart (e.g., via head portion 128) a translational force on IMD 102 causing IMD 102 to translate (e.g., within receptacle volume 112) in the proximal direction P or the distal direction D relative to receptacle wall 110. Delivery system 120 may be configured to rotate within delivery lumen 118 to impart (e.g., via head portion 128) a rotational torque on IMD 102 causing IMD 102 to rotate (e.g., within receptacle volume 112) about a device axis LD defined by IMD 102. In some examples, head portion 128 and driver 122 may be substantially separate components. In some examples, head portion 128 may be substantially contiguous with driver 122, such that head portion 128 and driver 122 define a unified component. In FIG.1, portions of driver 122 and/or head portion 128 positioned within delivery lumen 118 and/or receptacle volume 112 are depicted with dashed lines. [0035] Medical system 100 may be configured to position IMD 102 in proximity to target site 104 such that IMD 102 may be anchored to tissues within target site 104. In examples, IMD 102 includes an attachment member 132 configured to engage tissues within target site 104. For example, delivery catheter 106 may be configured (e.g., under the influence of a clinician) to traverse vasculature of the patient to position receptacle Atty Ref. No: A0010569WO01 device 108 and IMD 102 in proximity to target site 104. Medical system 100 (e.g., driver 122 and/or head portion 128) is configured to impart a torque to IMD 102 to cause attachment member 132 to engage tissues (e.g., tissue within target site 104) when attachment member 132 is within or in proximity to target site 104. In examples, medical system 100 (e.g., driver 122 and/or head portion 128) is configured to impart a force (e.g., in the distal direction D) to IMD 102 to cause attachment member 132 to engage tissues (e.g., tissue within target site 104) when attachment member 132 is within or in proximity to target site 104. Medical system 100 may be configured such that driver 122 and/or head portion 128 may be disengaged from IMD 102 as IMD 102 remains anchored to tissues within or in proximity to target site 104. Delivery catheter 106, receptacle device 108, and delivery system 120 may subsequently be withdrawn from the patient (e.g., via vasculature of the patient). [0036] In examples, medical system 100 is configured to deliver therapy to a patient and/or sense physiological signals of the patient received when receptacle device 108 is positioned (e.g., by a clinician) within heart 101. In some examples, medical system 100 may be configured to sense an indication of an intrinsic cardiac electrical signal produced by heart 101. Medical system 100 may be configured to process and/or condition a sensed signal to provide, for example, an indication of a location of receptacle device 108 and/or IMD 102 within heart 101, to conduct pace mapping for deployment of IMD 102, to provide indications indicative of the deployment and/or attachment of IMD 102 within heart 101, or for other reasons. In examples, medical system 100 includes processing circuitry 134 configured to deliver therapy and/or sense physiological signals. In examples, processing circuitry 134 includes processing circuitry such as device processing circuitry 131, configured to be mechanically supported by an external device 136 and/or another device of medical system 100. External device 136 and/or device processing circuitry 131 may be configured to be extracorporeal to the patient and/or otherwise displaced from receptacle device 108 and/or IMD 102 when, for example, receptacle device 108 and/or IMD 102 are intracorporeal to the patient. In examples, processing circuitry 134 includes processing circuitry such as IMD processing circuitry 133, configured to be mechanically supported by a housing of IMD 102 and/or another device of medical system 100. IMD processing circuitry 133 may be configured to be intracorporeal to the patient when, for example, receptacle device 108 and/or IMD 102 are Atty Ref. No: A0010569WO01 intracorporeal to the patient. Processing circuitry 134 may be configured to deliver therapy and/or sense physiological signals using one or more electrodes of medical system 100, such as one or more electrodes supported by IMD 102, one or more electrodes supported by receptacle device 108, one or more electrodes supported by delivery system 120, an external electrode 138 configured to be extracorporeal to the patient, and/or one or more other electrodes in communication with and/or supported by medical system 100. In examples, processing circuitry 134 is configured to communicate with one or more electrodes using one or more communication links, such as communication link 135, communication link 137, and/or other communication links. [0037] Receptacle device 108 includes a retainer 140 positioned within receptacle volume 112. Retainer 140 may engage IMD 102 to limit translation of IMD 102 relative to receptacle device 108 during, for example, transit to target site 104 through vasculature, and/or at other times when desired by a clinician. Retainer 140 is configured such that a rotation of IMD 102 (e.g., by the clinician using, for example, delivery system 120) about device axis LD causes retainer 140 to disengage from IMD 102, allowing relatively unencumbered translation of IMD 102 within receptacle volume 112. For example, in proximity to target site 104, and with retainer 140 engaged with IMD 102 and limiting translation of IMD 102, IMD 102 may be rotated (e.g., by the clinician) to disengage retainer 140 and IMD 102, such IMD 102 may be translated toward receptacle opening 109 and/or target site 104 relative to receptacle wall 110. In examples, delivery system 120 (e.g., driver 122 and/or head portion 128) is configured to transfer a torque to IMD 102 to cause the rotation of IMD 102. When retainer 140 is disengaged from IMD 102, delivery system 120 may exert a force on IMD 102 to cause the translation of IMD 102 in the distal direction D. [0038] Retainer 140 is configured to be stationary relative to receptacle wall 110, such that IMD 102 rotates relative to retainer 140 when IMD 102 rotates relative to receptacle wall 110. In examples, retainer 140 includes a retention finger (e.g., retention finger 160 (FIG.2)) configured to engage IMD 102 to limit translation of IMD 102 relative to receptacle wall 110. In examples, the retention finger is configured to engage a device structure (e.g., device structure 168 (FIG.2)) of IMD 102, such as a retrieval structure of IMD 102. [0039] Retainer 140 is configured to such that IMD 102 may translate in the distal Atty Ref. No: A0010569WO01 direction D and/or proximal direction P when retainer 140 is disengaged from IMD 102. In examples, IMD 102 defines a notch (e.g., notch 166 (FIG.3)) configured to cause disengagement of retainer 140 from IMD 102. In examples, the device structure defines the notch. In examples, IMD 102 is configured such that the rotation of IMD 102 relative to receptacle wall 110 allows the notch to substantially align with retainer 140 (e.g., to align with the retention finger). At least some portion of retainer 140 (e.g., retention finger 160) is configured to substantially pass through the notch when the notch aligns with retainer 140 and IMD 102 translates relative to receptacle wall 110. For example, when the retainer 140 is engaged with IMD 102 to limit translation of IMD 102 relative to receptacle wall 110, IMD 102 may be rotated to align the notch with a portion of retainer 140 (e.g., the retention finger), such that retainer 140 disengages from IMD 102 and passes through the notch when IMD 102 is subsequently caused (e.g., by a clinician) to translate relative to receptacle wall 110. Hence, retainer 140 may engage IMD 102 to limit translation of IMD 102 relative to receptacle device 108 during, for example, transit to target site 104 through vasculature. In proximity to target site 104, IMD 102 may be rotated (e.g., by a clinician) to disengage retainer 140 and IMD 102, such IMD 102 may translate (e.g., translate toward receptacle opening 109 and/or target site 104) relative to receptacle wall 110. [0040] In some examples, receptacle device 108 defines one or more flow channels (e.g., flow channel 186 (FIG.4)) configured to provide a flow path for a fluid from delivery lumen 118 into receptacle volume 112. A flow channel may define the flow path from delivery lumen 118 and substantially past retainer 140 and/or some portion of IMD 102 when retainer 140 is engaged with IMD 102, such that potential flow obstruction by retainer 140 and/or IMD 102 is limited and/or reduced. The flow channel may thus assist the flow of a fluid such as a fluorescent dye, or radiopaque fluid, or saline, or other fluid from delivery lumen 118 when retainer 140 is engaged with IMD 102. In examples, the flow channel is configured to limit and/or reduce hydrodynamic forces (e.g., a fluid pressure) on IMD 102 when retainer 140 is engaged with IMD 102 and the fluid flows from delivery lumen 118. [0041] In examples, delivery system 120 (e.g., head portion 128) is configured to engage IMD 102 to transfer a torque to IMD 102. Driver 122 may be configured to receive a torque (e.g., from a clinician) and transfer the torque to head portion 128. Atty Ref. No: A0010569WO01 Delivery system 120 may be configured to engage with IMD 102 to, for example, implant IMD 102 within an anatomical volume, retrieve IMD 102 from an anatomical volume, re- position IMD 102 within an anatomical volume, and/or re-orient IMD 102 within an anatomical volume. In examples, IMD 102 includes a distal portion 142 (“IMD distal portion 142”) supporting attachment member 132. In some examples, attachment member 132 is configured (e.g., as a helix) such that rotation of IMD 102 about device axis LD causes attachment member to engage and/or disengage tissues within target site 104. [0042] For example, attachment member 132 may be configured such that rotation of IMD 102 in a first rotational direction W1 about device axis LD causes attachment member 132 to engage (or alternately, disengage from) tissues within target site 104. Attachment member 132 may be configured such that rotation of IMD 102 about device axis LD in a second rotational direction W2 substantially opposite first rotational direction W1 causes attachment member 132 to disengage from (or alternately, engage) tissues within target site 104. In examples, IMD 102 includes one or more components (e.g., a communication antenna, a sensor, or another component) configured to rotate around and or revolve about device axis LD when IMD 102 rotates about device axis LD. Medical system 100 may cause IMD 102 to rotate about device axis LD to cause one or more of the components to substantially establish a specific orientation with respect to the anatomy of the patient, another device implanted within or worn by the patient, another device external to the patient, and/or other devices. Medical system 100 may cause rotation of IMD 102 in the first rotational direction W1 and/or the second rotational direction W2 to, for example, implant IMD 102, retrieve IMD 102, re-position IMD 102, and/or re-orient IMD 102 within an anatomical volume such as the RA. In some examples, medical system 100 may include a snare configured to engage IMD 102 (e.g., IMD retrieval structure 148). [0043] Although the examples herein discuss delivery, retrieval, and/or positioning of IMD 102 within the RA of heart 101, medical system 100 may be configured to deliver, retrieve, and/or position IMD 102 in any of the other chambers of heart 101 and/or in other anatomical volumes of a patient in a like manner as that described for the RA of heart 101. Further, although the examples herein discuss attachment member 132 defining a helix, attachment member 132 may defines other structures, such as one or more elongated tines extending from, for example, IMD distal portion 142. Further, delivery system 120 may Atty Ref. No: A0010569WO01 be configured to exert a translational force (e.g., in the distal direction D) on IMD 102 to cause attachment member 132 to engage tissues. The translational force exerted by delivery system 120 may cause IMD 102 to move in the distal direction D relative to receptacle device 108 such that, for example, attachment member 132 engages the tissues. Target site 104 may include an appendage of the RA, or the triangle of Koch region of the RA, or some other portion of heart 101, or some other location within a body of a patient. [0044] FIG.2 is a perspective view of a portion of medical system 100 including receptacle device 108 including retainer 140, with IMD 102 positioned within receptacle volume 112. In FIG.2, retainer 140 is engaged with IMD 102 and receptacle wall 110 is illustrated as transparent. FIG.3 is a perspective view illustrating IMD 102 and retainer 140 supported by receptacle wall 110, with IMD 102 and retainer 140 disengaged and receptacle wall 110 illustrated as transparent. FIG.4 is a cross-sectional plan view of receptacle device 108 with retainer 140 engaged with IMD 102, with the cutting plane taken parallel to the page. FIG.5 illustrates a cross-sectional schematic view of receptacle device 108 viewed from and in the direction of cutting plane A-A’ of FIG.4. In FIG.5, IMD 102 is shown in a first rotational position relative to retainer 140 and represented with dashed lines. FIG.6 illustrates a cross-sectional schematic view of receptacle device 108 viewed from and in the direction of cutting plane A-A’ of FIG.4 with IMD 102 shown in a second rotational position relative to retainer 140. [0045] Receptacle wall 110 includes a body 144 (“wall body 144”) (FIG.4) defining an inner surface 146 (“wall inner surface 146”). Wall inner surface 146 may define at least a portion of a boundary defining receptacle volume 112. In examples, wall inner surface 146 at least partially and/or substantially completely surrounds a longitudinal axis L defined by receptacle device 108. Wall body 144 may be configured such that wall inner surface 146 substantially faces in a direction substantially toward longitudinal axis L. For example, at least some portion of wall inner surface 146 may be configured such that a vector V1 normal to and extending away from wall inner surface 146 extends in a direction from wall inner surface 146 toward longitudinal axis L. In some examples, vector V1 is substantially perpendicular to longitudinal axis L. [0046] Wall body 144 may define an outer surface 148 (“wall outer surface 148”) substantially opposite wall inner surface 146. In examples, at least some portion of wall body 144 is between wall outer surface 148 and wall inner surface 146. In examples, wall Atty Ref. No: A0010569WO01 outer surface 148 at least partially and/or substantially completely surrounds longitudinal axis L and/or wall inner surface 146. Wall body 144 may be configured such that wall outer surface 148 substantially faces in a direction away from longitudinal axis L, wall inner surface 146, and/or receptacle volume 112. At least some portion of wall outer surface 148 may be configured such that a vector V2 normal to and extending away from wall outer surface 148 extends in a direction from wall outer surface 148 away from longitudinal axis L, wall inner surface 146, and/or receptacle volume 112. In some examples, vector V2 is substantially perpendicular to longitudinal axis L. In some examples, vector V2 is substantially parallel to vector V1. [0047] In examples, wall body 144 defines receptacle opening 109. Receptacle device 108 may define receptacle opening 109 substantially at a distal end 150 of receptacle device 108 (“receptacle distal end 150”). In examples, wall body 144 defines receptacle distal end 150. Receptacle opening 109 may be configured such that IMD 102 may egress from within receptacle volume 112 through receptacle opening 109 and/or ingress into receptacle volume 112 through receptacle opening 109. In examples, longitudinal axis L extends through at least some portion of receptacle volume 112 and extends through receptacle opening 109. In examples, receptacle volume 112 is bounded at least partially by receptacle opening 109. [0048] IMD 102 may define a proximal portion 143 (“IMD proximal portion 143”) opposite IMD distal portion 142. IMD 102, which in some examples can comprise a pacemaker such as a leadless and/or wholly intracardiac pacemaker, may include one or more electrodes such as electrode 152 supported by attachment member 132, electrode 154 (e.g., an atrial electrode) supported by a housing 156 of IMD 102 (“IMD housing 156”), and/or electrode 158 (e.g., a return electrode) supported by IMD housing 156. One or more of electrodes 152, 154, 158 may be electrically connected to processing circuitry 134. Processing circuitry 134 may be configured to deliver therapy to a patient and/or sense physiological signals of the patient using electrodes 152, 154, 158 and/or other electrodes of medical system 100. [0049] Receptacle device 108 includes retainer 140 configured to limit translation of IMD 102 at least in the distal direction D when retainer 140 is engaged with IMD 102. Retainer 140 is configured to be substantially stationary relative to receptacle wall 110. In examples, retainer 140 is configured such that, when IMD 102 rotates relative to Atty Ref. No: A0010569WO01 receptacle wall 110, IMD 102 rotates relative to retainer 140. In examples, when retainer 140 is engaged with IMD 102 (e.g., contacting some portion of IMD 102), retainer 140 is configured to exert a reaction force on IMD 102 to limit translation of IMD 102 relative to receptacle wall 110 (e.g., limit translation at least in the distal direction D). [0050] For example, retainer 140 may be configured to contact IMD 102 when IMD 102 translates in the distal direction D relative to receptacle wall 110. Retainer 140 may be configured such that, when IMD 102 contacts and exerts a force in the distal direction D on retainer 140, retainer 140 exerts a reaction force on IMD 102 in the proximal direction P. Retainer 140 may be configured such that (e.g., when retainer 140 is engaged with IMD 102) the reaction force limits and/or ceases movement of IMD 102 in the distal direction D, such that retainer 140 limits translation of IMD 102 at least in the distal direction D. In examples, retainer 140 is configured to limit translation of IMD 102 in the proximal direction P relative to receptacle wall 110. Retainer 140 may be configured such that, when IMD 102 contacts and exerts a force in the proximal direction P, retainer 140 exerts a reaction force on IMD 102 in the distal direction D to limit and/or ceases movement of IMD 102 in the proximal direction P. [0051] Retainer 140 includes a retention finger 160 extending substantially into receptacle volume 112. In examples, retention finger 160 extends in a direction from wall inner surface 146 to longitudinal axis L. Retention finger 160 may be configured to engage IMD 102 to limit the translation of IMD 102 relative to receptacle wall 110. In examples, retention finger 160 is configured to contact IMD 102 when IMD 102 translates relative to receptacle wall 110 in the distal direction D. Retention finger 160 may be configured such that, when IMD 102 exerts a force on the distal direction D on retention finger 160, retention finger 160 exerts a reaction force in the proximal direction P on IMD 102 to limit and/or cease movement of IMD 102 in the distal direction D. [0052] Retainer 140 is configured to allow rotation of IMD 102 relative to receptacle wall 110 as retention finger 160 engages IMD 102. For example, retainer 140 is configured to allow rotation of IMD 102 about device axis LD as retention finger 160 acts to limit and/or cease translation of IMD 102 in the distal direction D (e.g., by exerting a reaction force on IMD 102 in the proximal direction P). In examples, retention finger 160 defines a bearing surface 162 (FIG.3) configured to engage IMD 102 to exert the reaction force in the proximal direction P. Bearing surface 162 may be configured to engage an Atty Ref. No: A0010569WO01 IMD surface 164 defined by IMD 102. In examples, bearing surface 162 is configured such that retention finger 160 transmits the reaction force to IMD 102 (e.g., to IMD surface 164) via bearing surface 162. [0053] In examples, bearing surface 162 is configured such that IMD 102 substantially slides over bearing surface 162 in a rotational direction around device axis LD when IMD 102 rotates relative to receptacle wall 110 and/ retainer 140. Bearing surface 162 may be configured to slidably engage IMD 102 (e.g., IMD surface 164) when IMD 102 rotates relative to receptacle wall 110 and/ retainer 140. Bearing surface 162 is configured such that IMD 102 may substantially slide over bearing surface 162 in a rotational direction as retention finger 160 exerts the reaction force in the proximal direction P on IMD 102. Hence, retainer 140 (e.g., bearing surface 162) is configured to limit and/or cease translation of IMD 102 at least in the distal direction D as IMD 102 rotates (e.g., is caused to rotate by a clinician) about device axis LD relative to receptacle wall 110. In some examples, IMD 102 defines a gap G and retainer 140 is configured to substantially insert within gap G as IMD 102 rotates about device axis LD, such that retention finger 160 may engage IMD 102 to limit and/or cease translation of IMD 102 relative to receptacle wall 110. [0054] Retention finger 160 is configured to disengage from IMD 102 when IMD 102 is rotated to a particular rotational position within receptacle volume 112. In examples, retention finger 160 is configured such that retention finger 160 (e.g., bearing surface 162) is substantially displaced from IMD 102 (e.g., IMD surface 164) when IMD 102 is rotated to the particular rotational position. When IMD 102 rotates to the particular rotation position about device axis LD, retention finger 160 may substantially break contact with IMD 102, such that retention finger 160 no longer exerts a reaction force in the proximal direction P in response to a force in the distal direction D exerted by IMD 102. [0055] For example, IMD 102 may define a notch 166 configured to displace retention finger 160 and IMD 102 when IMD 102 rotates to a particular rotational position (e.g., a particular rotational position relative to receptacle wall 110). Retention finger 160 may be configured such that, when IMD 102 is rotated (e.g., by a clinician) to a particular rotational position aligning notch 166 and retention finger 160, retention finger 160 substantially passes through notch 166 when IMD 102 translates in the distal direction D relative to receptacle wall 110 and/or retainer 140. Thus, in examples, when notch 166 is Atty Ref. No: A0010569WO01 aligned with retention finger 160, IMD 102 may translate in the distal direction D relative to receptacle wall 110 in a manner relatively unencumbered by retention finger 160. Hence, retention finger 160 may be configured to engage IMD 102 to limit translation of IMD 102 relative to receptacle wall 110 during, for example, transit through vasculature. In proximity to a target site, IMD 102 may be rotated (e.g., by a clinician, using delivery system 120) to substantially align notch 166 with retention finger 160, such IMD 102 may be translated (e.g., by the clinician, using delivery system 120) in at least the distal direction D relative to receptacle wall 110 to, for example, cause IMD 102 to protrude through and/or exit via receptacle opening 109. [0056] For example, IMD 102 may include a device structure 168 defining notch 166. In some examples, device structure 168 includes a stem portion 170 extending from IMD 102 (e.g., IMD proximal portion 143) and supporting a crown portion 172. IMD 102 may define notch 166 using crown portion 172. In examples, device structure 168 (e.g., crown portion 172) defines IMD surface 164. Retention finger 160 may be configured to engage device structure 168 (e.g., crown portion 172) to limit translation of IMD 102 relative to receptacle wall 110. In some examples, device structure 168 defines gap G between, for example, IMD proximal portion 143 and crown portion 172. Retention finger 160 may be configured to substantially insert into gap G such that translation of IMD 102 at least in the distal direction D causes retention finger 160 to encounter crown portion 172. In examples, retention finger 160 is configured to exert a reaction force against crown portion 172 to limit translation of IMD 102 relative to receptacle wall 110. In examples, device structure 168 is a retrieval structure configured to releasably couple with delivery system 120 (e.g., head portion 128) to assist in, for example, delivery, deployment, and/or retrieval of IMD 102. [0057] Delivery system 120 may be configured to impart a torque on IMD 102 (e.g., on device structure 168) to cause rotation of IMD 102 about device axis LD. Delivery system 120 may be configured to impart a translational force in the distal direction D and/or the proximal direction P on IMD 102 (e.g., on device structure 168) to cause IMD 102 to translate relative to receptacle wall 110 when retention finger 160 is disengaged with IMD 102. For example, delivery system 120 may be configured to transmit a torque and/or translational force to IMD 102 when head portion 128 engages IMD 102 and a clinician imparts a torque and/or translational force on driver proximal portion 126 (FIG. Atty Ref. No: A0010569WO01 1). The clinician may impart the torque to cause IMD 102 to rotate from a first rotational position wherein retainer 140 (e.g., retention finger 160) acts to limit translation of IMD 102 to a second rotational position wherein retainer 140 (retention finger 160) disengages from IMD 102 to allow translation of IMD 102. [0058] For example, FIG.5 illustrates retainer 140 and IMD 102 with IMD 102 in the first rotational position with respect to receptacle wall 110 and/or retainer 140. IMD 102 is illustrated with dashed lines for clarity. The distal direction D proceeds out of the page and the proximal direction P proceeds into the page. Retention finger 160 is configured such that, with IMD 102 in the first rotational position, retention finger 160 is distal to some portion of IMD 102 (e.g., crown portion 172), such that retention finger 160 engages (e.g., contacts) IMD 102 to limit and/or cease translation of IMD 102 in the distal direction D relative to receptacle wall 110. FIG.6 illustrates retainer 140 and IMD 102 with IMD 102 in the second rotational position with respect to receptacle wall 110 and/or retainer 140, with notch 166 substantially aligned with retention finger 160. In FIG.6, retention finger 160 is substantially disengaged (e.g., displaced from) IMD 102, such that retention finger 160 may substantially pass through notch 166 when IMD 102 translates in the distal direction D relative to receptacle wall 110 and/or retainer 140. [0059] IMD 102 is configured such that a torque on IMD 102 (e.g., on device structure 168) may cause IMD 102 to rotate about device axis LD to transition from the first rotational position of FIG.5 to the second rotational position of FIG.6. Delivery system 120 may be configured to transmit the torque to IMD 102 to cause IMD 102 to transition from the first rotational position to the second rotational position, such that IMD 102 may translate distally in a manner relatively unencumbered by retention finger 160. Hence, a clinician may impart a torque on delivery system 120 (e.g., on driver proximal portion 126 (FIG.1)) to cause delivery system 120 (e.g., head portion 128) to transfer the torque to IMD 102, causing IMD 102 to transition from the first rotational position to the second rotational position (and vice-versa). When notch 166 is substantially aligned with retention finger 160, the clinician may impart a force in the distal direction on delivery system 120 (e.g., on driver proximal portion 126) to cause delivery system 120 (e.g., head portion 128) to transfer the force to IMD 102 such that, for example, IMD 102 may be translated toward receptacle opening 109 (FIG.2). [0060] In examples, retention finger 160 is configured to extend partially around a Atty Ref. No: A0010569WO01 wall perimeter defined by wall inner surface 146. Retention finger 160 may extend only partially around the wall perimeter such that, for example, retention finger 160 may substantially pass through notch 166 when IMD 102 rotates to substantially align notch 166 and retention finger 160. For example, as illustrated in FIG.5, wall inner surface 146 may define a wall perimeter WP at least partially surrounding longitudinal axis L. In examples, wall perimeter WP surrounds longitudinal axis L. In some examples, wall perimeter WP is substantially perpendicular to longitudinal axis L. In some examples, wall perimeter WP completely surrounds longitudinal axis L. Retainer 140 may be configured such that retention finger 160 extends only partially around wall perimeter WP. [0061] For example, retention finger 160 may substantially define a protrusion extending in the direction from wall inner surface 146 toward longitudinal axis L. The protrusion may define an arc S, with the arc S coincident with an arc of a closed curve (e.g., a circular curve) surrounding longitudinal axis L. In examples, arc S is within a plane substantially perpendicular to longitudinal axis L. In examples, arc S extends from a first endpoint E1 defined by retention finger 160 to a second endpoint E2 defined by retention finger 160. Endpoint E1 and endpoint E2 may define an angle B having a vertex on longitudinal axis L. In some examples, endpoint E1 and endpoint E2 are displaced from longitudinal axis L by a radial length, and arc S has an arc-length substantially equal to the product of the angle B and the radial length. Endpoint E1 and Endpoint E2 may define angle B to have any value, and/or retainer 140 may be configured to define the radial length to have any value. In some examples, wall perimeter WP extends around longitudinal axis L a full 360 degrees, and angle B subtends less than 360 degrees. In some examples, angle B subtends less than 180 degrees. In some examples, angle B subtends less than 90 degrees, in some examples less than 60 degrees, in some examples less than 30 degrees, and/or in some examples less than 15 degrees. [0062] As illustrated in, for example, FIGS.2–4, retainer 140 may define a groove 174 configured to receive some portion of IMD 102 when retention finger 160 engages IMD 102 and IMD 102 rotates about device axis LD. For example, groove 174 may be configured to receive a portion of device structure 168, such as a portion of crown portion 172. In examples, retainer 140 defines a backing surface 176 proximal to retention finger 160. Retainer 140 may define groove 174 substantially between retention finger 160 and backing surface 176. Groove 174 may be configured such that the portion of IMD 102 Atty Ref. No: A0010569WO01 (e.g., crown portion 172) substantially rotates through groove 174 when IMD 102 rotates relative to receptacle wall 110. Retention finger 160 may be configured such that, with the IMD portion within groove 174, distal translation of IMD 102 relative to receptacle wall 110 and/or retainer 140 causes retention finger 160 to encounter IMD 102 to limit and/or cease the distal translation. [0063] For example, groove 174 may be configured to receive a device perimeter PC defined by IMD 102. Groove 174 and/or IMD 102 may be configured such that device perimeter PC substantially rotates through groove 174 when IMD 102 rotates relative to receptacle wall 110. In examples, device structure 168 defines device perimeter PC. In some examples, crown section 172 defines device perimeter PC. In examples, IMD 102 (e.g., crown portion 172) defines a first edge 165 and a second edge 167 (FIG.8, FIG.9). Notch 166 may extend substantially from first edge 165 to second edge 167. In examples, device perimeter PC extends from first edge 165 to second edge 167 such that, for example, notch 166 establishes a break (e.g., a discontinuity) in device perimeter PC. In examples, device perimeter PC defines a curved, curvilinear, or polygonal shape extending at least partially around device axis LD. [0064] Backing surface 176 may be configured to assist in substantially maintaining and/or establishing device perimeter PC within groove 174 when IMD 102 is positioned within receptacle volume 112. For example, backing surface 176 may be configured to contact IMD 102 when IMD 102 translates in the proximal direction P relative to receptacle wall 110 and/or retainer 140. Backing surface 176 may be configured such that, when IMD 102 contacts and exerts a force in the proximal direction P on backing surface 176, backing surface 176 exerts a reaction force on IMD 102 in the distal direction D. The reaction force in the distal direction D may limit and/or cease movement of IMD 102 in the proximal direction P. In examples, the reaction force in the distal direction D from backing surface 176 and/or the reaction force in the proximal direction P from retention finger 160, or both acting substantially simultaneously on IMD 102, assist in substantially maintaining and/or establishing device perimeter PC within groove 174. [0065] In examples, retainer 140 is configured such that IMD 102 substantially slides over backing surface 176 in a rotational direction around device axis LD when IMD 102 rotates relative to receptacle wall 110 and/ retainer 140. Backing surface 176 may be configured to slidably engage IMD 102 when IMD 102 rotates relative to receptacle wall Atty Ref. No: A0010569WO01 110 and/ retainer 140. In examples, backing surface 176 is configured such that IMD 102 may substantially slide over backing surface 176 (e.g., in the rotational direction) as backing surface 176 exerts the reaction force in the distal direction D on IMD 102. In examples, backing surface 176 is configured to engage a second IMD surface 178 defined by IMD 102. Backing surface 176 may be configured to transmit the reaction force to IMD 102 via second IMD surface 178. In some examples, retainer 140 is configured such that backing surface 176 may exert a reaction force in the distal direction D on second IMD surface 178 as retention finger 160 (e.g., bearing surface 162) exerts a reaction force in the proximal direction P on IMD surface 164. [0066] Backing surface 176 may be configured to substantially urge IMD 102 (e.g., device structure 168) into groove 174. For example, backing surface 176 may be configured such that, when IMD 102 (e.g., IMD surface 164) is displaced from retention finger 160, IMD 102 tends to substantially slide (e.g., in the distal direction D) over backing surface 176 toward retention finger 160. In some examples, backing surface 176 is a ramping surface configured to urge IMD 102 into groove 174 and/or toward retention finger 160. For example, backing surface 176 may be configured to slope away from longitudinal axis L as backing surface 176 extends in the distal direction D to, for example, urge IMD 102 in the distal direction D toward retention finger 160. In some examples, backing surface 176 is configured to define a positive slope with respect to longitudinal axis L when positive rise is defined in a direction from backing surface 176 toward longitudinal axis L and positive run is defined in the proximal direction P. When backing surface 176 defines the positive slope, backing surface 176 may define a positive rise as backing surface 176 positively runs in the proximal direction P. [0067] In examples, device structure 168 (e.g., IMD crown portion 172) defines second IMD surface 178. For example, device structure 168 may define IMD surface 164 on a first side of device structure 168 (e.g., a first side of crown portion 172) and define second IMD surface 178 on a second side of device structure 168 (e.g., a second side of crown portion 172) substantially opposite the first side. In examples, IMD 102 may be configured such that IMD surface 164 substantially and/or generally faces in the distal direction D and second IMD surface 178 substantially and/or generally faces in the proximal direction P. In some examples, device perimeter PC separates IMD surface 164 and second IMD surface 178. In some examples, IMD 102 is configured such that IMD Atty Ref. No: A0010569WO01 surface 164 may receive a reaction force in the proximal direction P from retainer 140 (e.g., bearing surface 162) as second IMD surface 178 receives a reaction force in the distal direction D from retainer 140 (e.g., backing surface 176). In examples, IMD surface 164 and second IMD surface 178 are portions of a contiguous and/or continuous surface defined by IMD 102 (e.g., a contiguous and/or continuous surface defining device perimeter PC). [0068] Receptacle device 108 may define a delivery opening 180 (e.g., FIG.2, FIG.4) configured to open to delivery lumen 118. Delivery opening 180 may be configured to establish and/or maintain fluidic communication between delivery lumen 118 and receptacle volume 112 when delivery catheter 106 supports receptacle device 108. In examples, receptacle device 108 is configured such that receptacle volume 112 extends in the proximal direction P from receptacle distal end 150 and/or receptacle opening 109 to delivery opening 180. In examples, receptacle volume 112 is bounded at least in part by receptacle opening 109, delivery opening 180, and wall inner surface 146. [0069] Retainer 140 may be positioned in any portion of receptacle volume 112. For example, retainer 140 may be positioned in a distal portion 182 of receptacle device 180 (“receptacle distal portion 182”) or in a proximal portion 184 of receptacle device 108 (“receptacle proximal portion 184”) which is proximal to receptacle distal portion 182. In examples, receptacle distal portion 182 includes receptacle distal end 150 and/or receptacle opening 109. Receptacle proximal portion 184 may include delivery opening 180. In some examples, receptacle device 108 defines a midpoint M halfway between delivery opening 180 and receptacle distal end 150 and/or receptacle opening 109. Receptacle distal portion 182 may be distal to midpoint M and/or receptacle proximal portion 184 may be proximal to midpoint M. In some examples, retainer 140 is positioned in receptacle distal portion 182 to, for example, facilitate engagement with device structure 168 or another device structure supported by IMD distal portion 142. [0070] In some examples, receptacle device 108 defines one or more flow channels, such as flow channel 186. Flow channel 186 may be configured to provide a flow path for a fluid from delivery lumen 118 into receptacle volume 112 such that, for example, potential flow obstruction by retainer 140 and/or IMD 102 is limited and/or reduced. In examples, flow channel 186 defines a flow path from delivery lumen 118 and to receptacle volume 112 which substantially bypasses retainer 140 and/or some portion of IMD 102 Atty Ref. No: A0010569WO01 when retainer 140 is engaged with IMD 102. For example, flow channel 186 may define a flow path from delivery lumen 118 and substantially past device structure 168 and/or crown portion 172. Flow channel 186 may thus assist the flow of a fluid such as a fluorescent dye, or radiopaque fluid, or saline, or other fluid from delivery lumen 118 when retainer 140 is engaged with IMD 102. In examples, flow channel 186 is configured to limit and/or reduce hydrodynamic forces (e.g., a fluid pressure) on IMD 102 when retainer 140 is engaged with IMD 102 and the fluid flows from delivery lumen 118. [0071] Receptacle device 108 may have any number of flow channels. For example, in addition to or instead of flow channel 186, receptacle device may define other flow channels such as flow channel 187 and/or flow channel 188 (FIG.5–6). Flow channel 187, 188 may be configured to provide a flow path for a fluid from delivery lumen 118 into receptacle volume 112 to, for example, reduce potential flow obstruction by retainer 140 and/or IMD 102 and/or limit and/or reduce hydrodynamic forces on IMD 102. [0072] In examples, flow channel 186 is an open channel defined by receptacle wall 110 and/or another portion of receptacle device 108. In other examples, flow channel 186 is a closed channel defined by receptacle wall 110 and/or another portion of receptacle device 108. Flow channel 186 may be an open channel substantially recessed into wall inner surface 146. In examples, flow channel 186 defines an inlet 190 (“channel inlet 190”) configured to receive a fluid within and/or exiting delivery lumen 118 (e.g., via delivery opening 180). Flow channel 186 may define an outlet 192 (“channel outlet 192”) configured to discharge fluid entering channel inlet 190. Flow channel 186 may be configured such that a fluid substantially bypasses some portion of retainer 140 and/or IMD 102 when the fluid flows from channel inlet 190 to channel outlet 192. In examples, channel inlet 190 is proximal to retention finger 160. Channel outlet 192 may be distal to groove 174. In some examples, channel inlet 190 is proximal to a portion of retainer 140 and channel outlet 192 is distal to the portion of retainer 140. The portion of retainer 140 may be, for example, bearing surface 162, groove 174, backing surface 176, and/or another portion of retainer 140. In some examples, when retention finger 160 is engaged with IMD 102, channel inlet 190 is proximal to crown portion 172 and channel outlet 192 is distal to crown portion 172. In some examples, retention finger 160 extends around a first portion of wall perimeter WP and flow channel 186 extends around a second portion of wall perimeter WP different from the first portion. Atty Ref. No: A0010569WO01 [0073] Retainer 140 and/or IMD 102 may be configured such that, when retainer 140 is engaged with IMD 102, IMD 102 must be rotated in a primary rotational direction about device axis LD (e.g., first rotational direction W1) in order to cause disengagement of retainer 140 and IMD 102. Retainer 140 and/or IMD 102 may be configured such that retainer 140 is substantially prevented from disengaging from IMD 102 when IMD 102 is rotated in a secondary rotational direction opposite the primary rotational direction (e.g., second rotational direction W2). In examples, retainer 140 and/or IMD 102 are configured to substantially prevent IMD 102 from aligning notch 166 and retention finger 160 when IMD 102 is rotated relative to receptacle wall 110 in the secondary rotational direction. [0074] For example, retainer 140 and/or IMD 102 may be configured such that, when IMD 102 is in the first rotational position of FIG.5, IMD 102 must be rotated in the first rotational direction W1 to achieve the second rotational position of FIG.6, where notch 166 and retention finger 160 are substantially aligned. Retainer 140 and/or IMD 102 may be configured to limit the rotation of IMD 102 in the second rotational direction W2 when IMD 102 is in the first rotational position of FIG.5, such that IMD 102 is substantially prevented from aligning notch 166 and retention finger 160 through rotation in the second rotational direction W2. Alternately, retainer 140 and/or IMD 102 may be configured such that retainer 140 is substantially prevented from disengaging from IMD 102 when IMD 102 is rotated in the first rotational direction W1, and/or such that IMD 102 must be rotated in the second rotational direction W2 to substantially align notch 166 and retention finger 160. [0075] Retainer 140 and/or IMD 102 may be configured to limit the rotational direction of IMD 102 wherein IMD 102 may cause substantial alignment of notch 166 and retention finger 160 to, for example, allow for consistency in a direction of rotation (e.g., rotation by a clinician) required to implant IMD 102 within a patient. For example, when attachment member 132 is configured to engage tissue when IMD 102 is rotated about device axis LD in the primary rotational direction (e.g., the first rotational direction W1), retainer 140 and/or IMD 102 may be configured such that alignment of notch 166 and retention finger 160 from the first rotational position of FIG.5 requires rotation of IMD 102 in the primary rotational direction (e.g., the first rotational direction W1). Such a configuration may avoid a requirement for rotations in different directions during different portions of an implantation procedure. For example, such a configuration may avoid a Atty Ref. No: A0010569WO01 need for a clinician to cause rotation of IMD 102 in one rotational direction to cause substantial alignment between notch 166 and retention finger 160, then cause rotation of IMD 102 in a second rotational direction opposite the first rotational direction to cause implantation of IMD 102 using attachment member 132. [0076] In some examples, retainer 140 (e.g., bearing surface 162 and/or another portion of retainer 140) is configured to establish an interference fit with IMD 102 as IMD 102 is rotated through groove 174 in the secondary rotational direction. Retainer 140 (e.g., bearing surface 162 and/or another portion of retainer 140) may be configured to establish and/or substantially maintain a slip fit with IMD 102 as IMD 102 is rotated through groove 174 in the primary rotational direction. Retainer 140 may be configured such that the interference fit substantially prevents IMD 102 from rotating in the secondary rotational direction to cause alignment between notch 166 and retention finger 160. [0077] In examples, IMD 102 is configured to establish the interference fit with retainer 140 (e.g., bearing surface 162 and/or another portion of retainer 140) as IMD 102 is rotated through groove 174 in the secondary rotational direction. For example, FIG.7 and FIG.8 are perspective illustrations of IMD 102 defining an interference portion 194. [0078] Interference portion 194 may be configured to establish the interference fit with bearing surface and/or another portion of retainer 140 when rotation of IMD 102 (e.g., in the secondary rotational direction) causes interference portion 194 to enter groove 174. IMD 102 may define a slip portion 196 configured to establish and/or substantially maintain the slip fit with retainer 140 (e.g., bearing surface 162 and/or another portion of retainer 140) as IMD 102 is rotated through groove 174 in the primary rotational direction. IMD 102 may be configured such that a rotation of IMD 102 about device axis LD (e.g., in the primary rotational direction and/or the secondary rotational direction) causes both slip portion 196 and interference portion 194 to enter groove 174. Hence, IMD 102 may be configured such that rotation of IMD 102 about device axis 102 may occur when slip portion 196 rotates through groove 174 and substantially limited when interference portion 194 enters and/or positions within groove 174. In examples, crown portion 172 defines interference portion 194 and slip portion 196. In examples, device perimeter PC extends through interference portion 194 and slip portion 196. [0079] In examples, a first portion PC1 of device perimeter PC intersects interference portion 194 and a second portion PC2 of device perimeter PC different from the first Atty Ref. No: A0010569WO01 portion intersects slip portion 196. In some example, first portion PC1 and second portion PC2 form a continuous portion of device perimeter PC. For example, first portion PC1 and second portion PC2 define substantially all of device perimeter PC. IMD 102 (e.g., crown portion 172) may be configured such that IMD 102 transitions from the slip fit with retainer 140 provided by slip portion 196 to the interference fit with retainer 140 provided by interference portion 194 as device perimeter PC rotates through retainer 140 (e.g., rotates through groove 174). [0080] In examples, interference portion 194 defines a first thickness T1 configured to establish the interference fit with retainer 140. First thickness T1 may be substantially parallel to device axis LD. Slip portion 196 may define a second thickness T2 substantially parallel to first thickness T2. Second thickness T2 may be configured to establish and/or substantially maintain the slip fit with retainer 140. In examples, first thickness T1 is greater than second thickness T2. IMD 102 (e.g., crown portion 172) may be configured such that second thickness T2 allows some portion of IMD 102 (e.g., a portion of crown portion 172) to pass through groove 174 as IMD 102 rotates about device axis LD. IMD 102 (e.g., crown portion 172) may be configured such that first thickness T1 substantially limits and/or prevents the portion of IMD 102 from passing through groove 174, such that IMD 102 is substantially prevented from rotating about device axis LD. [0081] For example, FIG.9 is a cross-sectional schematic illustration of a first section of slip portion 196 rotating through groove 174 as IMD 102 rotates about device axis LD. FIG.10 is a cross-sectional schematic illustration of a second section of slip portion 196 rotating through groove 174 as IMD 102 rotates about device axis LD. FIG.11 is a cross- sectional schematic illustration of interference portion 194 having entered and positioned within groove 174 as a result of the rotation of IMD 102 about device axis LD. In FIG.9, FIG.10, and FIG.11, first rotational direction W1 extends out of the page and second rotational direction W2 extends into the page. In FIG.9, FIG.10, and FIG.11, the cutting plane extends through device axis LD and is parallel to the page. [0082] FIG.9 illustrates a first section of slip portion 196 (“first slip portion”) defining second thickness T2. FIG.9 depicts the first slip portion moving through groove 174 as IMD 102 rotates about device axis LD in the primary rotational direction. The first slip portion may be in contact (e.g., sliding contact) with bearing surface 162 and/or backing surface 176 of retention finger 160. The first slip portion establishes and/or substantially Atty Ref. No: A0010569WO01 maintains a slip fit with retainer 140 as IMD 102 rotates about device axis LD and the first slip portion moves through groove 174. For example, the first slip portion may maintain a clearance C2 (e.g., a displacement) between bearing surface 162 and the first slip portion as IMD 102 rotates about device axis LD. In examples, clearance C2 is parallel to device axis LD. The first slip portion may be configured such that second thickness T2 substantially establishes clearance C2. Clearance C2 may substantially establish and/or maintain the slip fit between IMD 102 (e.g., crown portion 172) and retainer 140, such that IMD 102 may continue to rotate in the primary rotational direction. [0083] FIG.10 illustrates a second section of slip portion 196 (“second slip portion”) defining an intermediate thickness TN. FIG.10 depicts the second slip portion moving through groove 174 as IMD 102 rotates about device axis LD in the primary rotational direction. Intermediate thickness TN is greater than second thickness T2 and less than first thickness T1. In examples, intermediate thickness TN is substantially parallel to first thickness T1 and/or second thickness T2. The second slip portion is in contact (e.g., sliding contact) with bearing surface 162 and/or backing surface 176. The second slip portion may substantially maintain the slip fit between IMD 102 and retainer 140 as IMD 102 rotates about device axis LD and the second slip portion moves through groove 174. For example, the second slip portion may maintain a clearance CN (e.g., a displacement) between bearing surface 162 and the second slip portion as IMD 102 rotates about device axis LD. Clearance CN may be parallel to device axis LD. In examples, clearance CN is less than clearance C2. The clearance CN may substantially maintain the clearance fit between IMD 102 (e.g., crown portion 172) and retainer 140 such that IMD 102 may continue to rotate in the primary rotational direction as slip portion 196 transitions from second thickness T2 to intermediate thickness TN. [0084] FIG.11 illustrates interference portion 194 defining first thickness T1. FIG.11 depicts interference portion 194 having entered and positioned within groove 174 as IMD 102 rotated about device axis LD in the primary rotational direction. Interference portion 194 is in contact with bearing surface 162 of retention finger 160, and may be in contact with backing surface 176. Interference portion 194 substantially establishes and/or maintains an interference fit with at least one of bearing surface 162 or backing surface 176 as IMD 102 rotates about device axis LD to cause interference portion 194 to enters and/or position within groove 174. The interference fit limits and or substantially prevents Atty Ref. No: A0010569WO01 further rotation of IMD 102 about device axis LD in the primary rotational direction. [0085] For example, the interference portion 194 may establish a clearance C1 (e.g., a displacement) from bearing surface 162 and/or may substantially eliminate any clearance from bearing surface 162 as the rotation of IMD 102 causes interference portion 194 to enter and/or position within groove 174. In examples, when clearance C1 is present, clearance C1 may be parallel to device axis LD and less than clearance CN. Interference portion 194 may substantially maintain the interference fit between IMD 102 (e.g., crown portion 172) and retainer 140, such that additional rotation of IMD 102 in the primary rotational direction is limited and/or substantially prevented. [0086] In examples, IMD 102 (e.g., crown portion 172) comprises one or more surfaces which provide a smooth transition (e.g., a substantially continuous transition) from second thickness T2 to intermediate thickness TN and/or from intermediate thickness TN to first thickness T1. In some examples, IMD 102 (e.g., crown portion 172) comprises one or more surfaces which provide a substantially abrupt transition (e.g., a substantially discontinuous transition) from second thickness T2 to intermediate thickness TN and/or from intermediate thickness TN to first thickness T1. IMD 102 (e.g., crown portion 172) may define any number of additional thicknesses for IMD portions intended to rotate through groove 174 when IMD 102 rotates about device axis LD. For example, IMD 102 (e.g., crown portion 172) may include IMD portions defining one or more additional thickness greater than second thickness T2 and less than intermediate thickness TN and/or one or more additional thickness greater than intermediate thickness TN and less than first thickness T1. [0087] In some examples, instead of or in addition to interference portion 194, IMD 102 may include a stop 198 (FIG.8) configured to limit the rotational direction of IMD 102 wherein IMD 102 may cause substantial alignment of notch 166 and retention finger 160. Stop 198 may be configured such that IMD 102 is limited to rotation in the primary rotational direction to substantially align notch 166 and retention finger 160. Stop 198 may be configured to limit and/or substantially prevent IMD 102 from rotating in the secondary rotational direction to cause alignment between notch 166 and retention finger 160. For example, stop 198 may be configured such that, when IMD 102 is in the first rotational position of FIG.5, IMD 102 must be rotated in the primary rotational direction (e.g., first rotational direction W1) to achieve the second rotational position of FIG.6. Stop 198 may Atty Ref. No: A0010569WO01 be configured to limit the rotation of IMD 102 in the secondary rotational direction (e.g., second rotational direction W2) when IMD 102 is in the first rotational position of FIG.5, such that IMD 102 is substantially prevented from aligning notch 166 and retention finger 160 through rotation in the secondary rotational direction. [0088] Stop 198 may be configured to contact retainer 140 and/or another structure of receptacle device 108 (e.g., another structure configured to remain stationary with respect to IMD 102) when IMD 102 rotates about device axis LD at least in the secondary rotational direction. Stop 198 may be configured such that, when stop 198 contacts retainer 140 and/or the other structure of receptacle device 108 due to a torque on IMD 102 causing rotation in the secondary rotational direction, retainer 140 and/or the other structure exerts a reaction force on stop 198 counteracting the torque, such that additional rotation in the secondary rotational direction is limited and/or substantially prevented. [0089] In examples, IMD 102 is configured such that rotation of IMD housing 156, device structure 168, and/or another portion of IMD 102 around device axis LD causes rotation of stop 198 around device axis LD. Stop 198 may be configured to rotate around device axis LD synchronously with IMD housing 156, device structure 168, and/or the other portion of IMD 102. In examples, stop 198 is attached to, affixed, or unitary with a portion of IMD housing 156, such as a portion of IMD housing 156 comprising IMD proximal portion 143. In some examples, stop 198 is attached to, affixed, or unitary with a portion of device structure 168, such as crown portion 172 and/or stem portion 170. In some examples, stop 198 extends into gap G (e.g., from IMD proximal portion 143, stem portion 170, and/or crown portion 172. Stop 198 may extend substantially across gap G from, for example, crown portion 172 to IMD proximal portion 143, from crown portion 172 to stem portion 170, and/or from stem portion 170 to IMD proximal portion 143. [0090] In examples, an area of interference portion 194 may substantially define a step which causes first thickness T1 to be sufficiently thick (or causes displacement D1 of gap G to be sufficiently narrow) that interference portion 194 acts like a stop. This may signal to a clinician that IMD 102 is substantially secured within retainer 140. In examples, the step may act as a stop from when IMD 102 is rotated bout device axis LD in the first rotational direction W1 and/or the second rotational direction W2. For example, one side of the stop might signal securement of IMD 102 with retainer 140. Another side of the stop might signal alignment with notch 166 (e.g. signaling that IMD 102 can be moved in Atty Ref. No: A0010569WO01 the distal direction D). [0091] In some examples, IMD 102 defines a proximal surface 171 (“IMD proximal surface 171”) configured to face substantially in the proximal direction P when IMD 102 is positioned within receptacle volume 112. In examples, IMD proximal surface 171 is a surface defined by IMD proximal portion 143. IMD proximal surface 171 may be a surface of IMD housing 156. Device structure 168 may extend (e.g., extend substantially in the proximal direction P) from IMD proximal surface 171. In examples, stem portion 170 extends (e.g., extends substantially in the proximal direction P) from IMD proximal surface 171. In examples, gap G extends between IMD proximal surface 171 and crown portion 172. [0092] In some examples, gap G is substantially defined by a displacement between IMD proximal surface 171 and crown portion 172 (e.g., second IMD surface 164). IMD 102 may be configured such that the displacement between IMD proximal surface 171 and crown portion 172 varies in a circumferential direction around device axis LD. For example, a first portion of gap G may be substantially defined by a displacement D1 between IMD proximal surface 171 and crown portion 172, while a second portion of gap G may be substantially defined by a displacement D2 between IMD proximal surface 171 and crown portion 172. Displacement D1 may be less than displacement D2. In some examples, the first portion of gap G is substantially between interference portion 194 and IMD proximal surface 171. The second portion of gap G may be substantially between slip portion 196 and IMD proximal surface 171. In examples, IMD 102 defines gap G such that the displacement between IMD proximal surface 171 and crown portion 172 varies substantially continuously in the circumferential direction around device axis LD. For example, IMD 102 may be configured such that the displacement defined by gap G increases smoothly from displacement D1 to displacement D2, and/or decreases smoothly from displacement D2 to displacement D1. [0093] In some examples, instead of or in addition to interference portion 194, retention finger 160 may be configured to establish the interference fit with IMD 102. For example, retention finger 160 may be configured to establish the interference fit as retention finger 160 passes through a portion of gap G defining displacement D2 during the rotation of IMD 102 about device axis LD (e.g., as . as IMD 102 is rotated in the secondary rotational direction). For example, FIG.12 is a cross-sectional schematic Atty Ref. No: A0010569WO01 illustration of a retention finger 160 passing through gap G defining displacement D1 as IMD 102 rotates about device axis LD. FIG.13 is a cross-sectional schematic illustration of retention finger 160 having entered and positioned within a portion of gap G defining displacement D2 as a result of the rotation of IMD 102 about device axis LD. In FIG.12 and FIG.13, first rotational direction W1 extends out of the page and second rotational direction W2 extends into the page. In FIG.12 and FIG.13, the cutting plane extends through device axis LD and is parallel to the page. [0094] Gap G and/or retainer 140 may be configured such that retainer 140 (e.g., retention finger 160) establishes the interference fit with IMD proximal surface 171 and/or another portion of IMD 102. IMD 102 (e.g., gap G) may be configured such that a rotation of IMD 102 about device axis LD (e.g., in the primary rotational direction and/or the secondary rotational direction) causes retention finger 160 to pass through the portion of gap G defining displacement D1 and position within the portion of gap G defining displacement D2. Hence, IMD 102 may be configured such that rotation of IMD 102 about device axis 102 may occur when retention finger 160 passes through the portion of gap G defining displacement D2 and substantially limited when retention finger 160 positions within the portion of gap G defining displacement D1. Device perimeter PC may be configured such that that IMD 102 (e.g., IMD proximal surface 171) transitions to the interference fit with retention finger 160 as device perimeter PC rotates through retainer 140 (e.g., rotates through groove 174). [0095] For example, FIG.12 illustrates retention finger 160 passing through the portion of gap G defining displacement D2 as IMD 102 rotates about device axis LD in the primary rotational direction. Retention finger 160 establishes and/or substantially maintains a clearance C3 from IMD proximal surface 171 as IMD 102 rotates about device axis LD and retention finger 160 passes through the portion of gap G defining displacement D2. IMD 102 and/or retention finger 160 is configured such that the displacement D2 allows further rotation of IMD 102 about device axis LD at least in the primary rotational direction as retention finger 160 passes through gap G. In examples, clearance C3 is parallel to device axis LD. [0096] FIG.13 illustrates retention finger 160 having entered and positioned within the portion of gap G defining displacement D1 as IMD 102 rotates about device axis LD in the primary rotational direction. Retention finger 160 is in contact with IMD proximal Atty Ref. No: A0010569WO01 surface 171, and may be in contact with stem portion 170 and/or crown portion 172. Retention finger 160 substantially establishes and/or maintains an interference fit with at least one of IMD proximal surface 171, stem portion 170, and/or crown portion 172 as IMD 102 rotates about device axis LD to cause retention finger 160 to enter and positioned within the portion of gap G defining displacement D1. The interference fit limits and or substantially prevents further rotation of IMD 102 about device axis LD in the primary rotational direction. For example, the interference portion 194 may establish a clearance C4 (e.g., a displacement) between retention finger 160 and IMD 102 (e.g., IMD proximal surface 171, stem portion 170, and/or crown portion 172), and/or may substantially eliminate any clearance from some portion of IMD 102 (e.g., IMD proximal surface 171, stem portion 170, and/or crown portion 172) as the rotation of IMD 102 causes retention finger 160 to enter and/or position within the portion of gap G defining displacement D1. Retention finger 160 and/or IMD 102 may be configured to substantially maintain the interference fit between retention finger 160 and IMD 102 (e.g., IMD proximal surface 171, stem portion 170, and/or crown portion 172), such that additional rotation of IMD 102 in the primary rotational direction is limited and/or substantially prevented. [0097] In examples, crown portion 172 defines a greater radial extension from device axis LD than stem portion 170. For example, stem portion 170 may define a first maximum cross-sectional dimension such as stem radial displacement RN (FIG.4) from device axis LD. Crown portion 172 may define a second maximum cross-sectional dimension such as crown radial displacement RC (FIG.4) from device axis LD. The crown radial displacement (e.g., RC) may be greater than the stem radial displacement (e.g., RN). In examples, the crown radial displacement and/or the stem radial displacement are substantially perpendicular to device axis LD. In examples, crown portion 172 may be configured to limit and/or eliminate a tendency of a snare surrounding stem portion 170 to slip off of device structure 168 when the snare (e.g., a snare loop of the snare) constricts around stem portion 170 and a proximally directed force is exerted (e.g., by a clinician) on the snare. In examples, stem portion 170 defines a stem perimeter PS (FIG.7, FIG.8) extending at least partially around device axis LD. In examples, stem perimeter PS is substantially perpendicular to device axis LD. Stem perimeter PS may define a curved, curvilinear, or polygonal shape extending at least partially around device axis LD. In Atty Ref. No: A0010569WO01 examples, stem perimeter PS completely surrounds device axis LD. [0098] Retainer 140 may be affixed to receptacle wall 110 such that retainer 140 is limited and/or constrained from movement relative to receptacle wall 110. In examples, retainer 140 is substantially embedded within receptacle wall 110. In some examples, retainer 140 is affixed to receptacle wall 110 by overmolding. For example, receptacle wall 110 may be fabricated by injection molding a molten polymer and/or other molten material substantially around retainer 140. In some examples, retainer 140 may be affixed to receptacle wall 110 using adhesives, soldering, boundary interlocking, and/or other methods. Receptacle wall 110 may be fabricated by any method, including molding, machining, forging, or other methods. In examples, receptacle wall 110 comprises a polymer. In examples, receptacle wall 110 comprises a metal or a conductive polymer. [0099] IMD 102, may comprise a pacemaker such as a leadless and/or wholly intracardiac pacemaker. One or more of electrodes 138, 152, 154, 158 may be electrically connected to processing circuitry 134. Processing circuitry 134 may be operably connected to operating circuitry configured to deliver therapy to a patient and/or sense physiological signals of the patient using electrodes 138, 152, 154, 158. [0100] Processing circuitry 134 may include fixed function circuitry and/or programmable operating circuitry. In examples, processing circuitry 134 includes circuitry configured to perform one or more functions of operating circuitry, such as therapy delivery circuitry, sensing circuitry, processing circuitry, switching circuitry, communication circuitry, and/or other circuitries. Processing circuitry 134, as well as other processors, operating circuitry, controllers, control circuitry, and the like, described herein, may include any combination of integrated circuitry, discrete logic circuity, analog circuitry, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), or field-programmable gate arrays (FPGAs). In some examples, processing circuitry 134 includes multiple components, such as any combination of one or more microprocessors, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry, and/or analog circuitry. [0101] Functions attributed to processing circuitry 134 may be embodied as software, firmware, hardware or any combination thereof. Processing circuitry 134 may include, for instance, a variety of capacitors, transformers, switches, and the like configured to perform Atty Ref. No: A0010569WO01 the functions of processing circuitry 134. In examples, processing circuitry 134 may be configured to communicate with another device, such as a patient input/output device, a clinician input/output device, and/or others. Processing circuitry 134 may include any suitable hardware, firmware, software or any combination thereof for communicating with another device. In addition, processing circuitry 134 may communicate with a networked computing device and a computer network. In examples, processing circuitry 134 and/or other circuitry of medical system 100 is configured to deliver stimulation signals to and/or receive sensing signals from electrodes 138, 152, 154, 158, and/or other electrodes and/or sensors within medical system 100 or external to medical system 100. Processing circuitry 134 may be configured to provide electrical signals, e.g., pacing therapy, to electrodes 138, 152, 154, 158, and/or other electrodes within medical system 100. Processing circuitry 134 may be configured to receive electrical signals, e.g., sensed cardiac electrical signals, from electrodes 138, 152, 154, 158, and/or other electrodes within medical system 100. [0102] Medical system 100 (e.g., processing circuitry 134) can also include memory configured to store program instructions, such as software, which may include one or more program modules, which are executable by processing circuitry 134. The program instructions may be embodied in software and/or firmware. The memory can include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically- erasable programmable ROM (EEPROM), ferroelectric RAM (FRAM), flash memory, or any other digital media. In some examples, the memory includes computer-readable instructions that, when executed by processing circuitry 134 cause processing circuitry 134 to perform various functions described herein and/or other functions of processing circuitry 134. [0103] IMD housing 156 may enclose processing circuitry 134 and/or other circuitry within medical system 100. IMD housing 156 may be configured to fluidly isolate processing circuitry 134 and/or other circuitry from an environment in contact with an exterior surface of IMD housing 156. In examples, IMD housing 156 is configured to hermetically seal an enclosure defined by IMD 102 and holding processing circuitry 134 and/or other circuitry. IMD housing 156 may be configured to define shapes that are easily accepted by the patient's body while minimizing patient discomfort. For example, Atty Ref. No: A0010569WO01 IMD housing 156 may define a substantially cylindrical shape with cylindrical sidewalls. In other examples, IMD housing 156 may define substantially rectangular or other non- cylindrical shapes. IMD housing 156 may define shapes in which corners and edges are designed with relatively large radii, in order to present a housing having smoothly contoured exterior surfaces. In examples, attachment member 132 is coupled to IMD housing 156. [0104] Communication links 135, 137 may be hard-line and/or wireless communications links. In some examples, communication links 135, 137 may comprise some portion of processing circuitry 134. In some examples, communication links 135, 137 comprise a wired connection, a wireless Internet connection, a direct wireless connection such as wireless LAN, Bluetooth^TM, Wi-Fi^TM, and/or an infrared connection. Communication links 135, 137 may utilize any wireless or remote communication protocol. [0105] As used here, when a first portion of a system (e.g., medical system 100) is substantially parallel to a second portion of or an axis defined by the system, this may mean the first portion is parallel or nearly parallel to the second portion or the axis to the extent permitted by manufacturing tolerances. In some examples, when the first portion is substantially parallel to the second portion or the axis, this may mean a first vector defined by the first component of the system defines an angle of less than 10 degrees, in some examples less than 5 degrees, and in some examples less than 1 degree, with a second vector defined by the second component or the axis. When a first portion of the system is substantially perpendicular to a second portion of or an axis defined by the system, this may mean the first portion is perpendicular or nearly perpendicular to the second portion or the axis to the extent permitted by manufacturing tolerances. In some examples, when the first portion is substantially perpendicular to the second portion or the axis, this may mean that the first vector defined by the first component of the system defines an angle of at least 80 degrees, in some examples at least 85 degrees, and in some examples at least 89 degrees, with the second vector defined by the second component. [0106] As used here, when a first portion of a system (e.g., medical system 100) supports a second portion of the system, this means that when the second portion causes a first force to be exerted on the first portion, the first portion causes a second force to be exerted on the second portion in response to the first force. The first force and/or second Atty Ref. No: A0010569WO01 force may be a contact force and/or an action-at-a-distance force. For example, first force and/or second force may be mechanical force, a magnetic force, a gravitational force, or some other type of force. The first portion of the system may be a portion of the system or a portion of a component of the system. The second portion of the system may be another portion of the system or another portion of the same component or a different component. In some examples, when the first portion of the system supports the second portion of the system, this may mean the second portion is mechanically supported by and/or mechanically connected to the first portion. [0107] A technique for holding an implantable medical device within a receptacle device is illustrated in FIG.14. Although the technique is described mainly with reference to medical system 100 of FIGS.1–13, the technique may be applied to other medical systems in other examples. [0108] The technique includes engaging an implantable medical device 102 (IMD 102) using a retention finger 160 of a retainer 140 of a receptacle device 108 (1402). IMD 102 may be positioned within a receptacle volume 112 of receptacle device 108. Retention finger 160 may be substantially stationary with respect to a receptacle wall 110 of receptacle device 108. In examples, receptacle wall 110 defines and/or forms a portion of a boundary of receptacle volume 112. [0109] In examples, retention finger 160 engages a device structure 168 of IMD 102. Device structure 168 may be supported by an IMD proximal portion 143. In examples, device structure 168 is a retrieval structure. In examples, retention finger 160 engages a crown portion 172 of device structure 168. In examples, retention finger 160 inserts within a gap G defined by IMD 102 when retention finger 160 engages IMD 102. IMD 102 may define gap G substantially between IMD proximal portion 143 (e.g., IMD proximal surface 171) and crown portion 172. In examples, retention finger 160 is positioned within a receptacle proximal portion 184 of receptacle device 108. [0110] Retention finger 160 may limit and/or substantially prevent translation of IMD 102 relative to receptacle wall 110 at least in a distal direction D when retention finger 160 engages IMD 102. Retention finger 160 may limit and/or substantially prevent translation of IMD 102 toward a receptacle opening 109 defined by receptacle wall 110 at a receptacle distal end 150 of receptacle device 108. In examples, IMD 102 inserts within a groove 174 of retainer 140 when retention finger 160 engages IMD 102. Groove 174 may Atty Ref. No: A0010569WO01 be proximal to retention finger 160. In examples, IMD 102 (e.g., crown portion 172) contacts a bearing surface 162 when retention finger 160 engages IMD 102. IMD 102 (e.g., crown portion 172) may contact a backing surface 176 when retention finger 160 engages IMD 102. In examples, backing surface 176 substantially urges IMD 102 to position within groove 174 and/or in contact with retention finger 160. [0111] Receptacle device 108 may be supported by a delivery catheter 106 defining a delivery lumen 118. The technique may include coupling IMD 102 (e.g., device structure 168) and a delivery system 120 extending through delivery lumen 118. In examples, a delivery catheter distal portion 114 supports receptacle proximal portion 184 when retention finger 160 engages IMD 102. In examples, delivery lumen 118 opens to receptacle volume 112 when delivery catheter 106 supports receptacle device 108. In some examples, the technique includes positioning an interference portion 194 within groove 174. In some examples, the technique includes contacting a stop 198 with retainer 140 and/or another structure of receptacle device 108. In some examples, the technique includes establishing an interference fit between retainer 140 (e.g., retention finger 160) and some portion of IMD 102 (e.g., IMD proximal surface 171). [0112] In examples, retention finger 160 engages IMD 102 as delivery catheter 106 and receptacle device 108 transit through vasculature of a patient. In examples, the technique includes flowing a fluid from delivery lumen 118 to receptacle volume 112 when retention finger 160 engages IMD 102. The fluid may flow at least partially through a flow channel 186, 187, 188 when retention finger 160 engages IMD 102. In examples, the fluid substantially bypasses at least some portion of retainer 140 and/or IMD 102 when the fluid flows through flow channel 186, 187, 188. [0113] The technique includes disengaging retention finger 160 and IMD 102 (e.g., crown portion 172) using a rotation of IMD 102 about a device axis LD (1404). IMD 102 may rotate relative to receptacle wall 110 when IMD 102 rotates about device axis LD. The technique may include imparting a torque (e.g., by a clinician) on IMD 102 using delivery system 120 to cause the rotation of IMD 102. In examples, a portion of IMD 102 rotates through groove 174 when IMD 102 rotates about device axis LD. In examples, slip portion 196 of IMD 102 rotates through groove 174 when IMD 102 rotates about device axis LD. [0114] IMD 102 may cause an alignment between a notch 166 defined by IMD 102 Atty Ref. No: A0010569WO01 and retention finger 160 when IMD 102 rotates about device axis LD. IMD 102 may rotate in a primary rotational direction about device axis LD to cause the alignment. The technique may include limiting, using retainer 140 and/or IMD 102, rotation of IMD 102 in a second rotational direction opposite the first rotational direction. In examples, IMD 102 (e.g., IMD surface 164) slidably engages bearing surface 162 when IMD 102 rotates about device axis LD. IMD 102 (e.g., second IMD surface 178) may slidably engage backing surface 176 when IMD 102 rotates about device axis LD. In examples, delivery system 120 (e.g., driver 122 and/or head portion 128) transfers a force in the distal direction D to IMD 102 when notch 166 aligns with retention finger 160. Retention finger 160 may move through notch 166 as the force in the distal direction D causes IMD 102 to translate distally relative to receptacle wall 110. [0115] At least an IMD distal portion 142 may protrude and/or exit receptacle volume 112 via receptacle opening 109 when IMD 102 translates distally relative to receptacle wall 110. In examples, a rotation of IMD 102 about device axis LD causes an attachment member 132 to engage tissues of a patient. In examples, a torque imparted to IMD 102 (e.g., by a clinician) using delivery system 120 causes attachment member 132 to engage tissues. In examples, attachment member 132 engages the tissues when IMD 102 is rotated in the primary rotational direction. [0116] Various examples of the disclosure have been described. Any combination of the described systems, operations, or functions is contemplated. These and other examples are within the scope of the following claims. [0117] Example 1. A medical system comprising a receptacle device configured to be positioned within an anatomical volume defined by a body of a patient, wherein the receptacle device includes a receptacle wall defining a receptacle volume configured to receive at least a portion of an implantable medical device, the receptacle wall at least partially surrounding a longitudinal axis extending through the receptacle volume, wherein the receptacle device defines a receptacle opening which opens to the receptacle volume at a distal end of the receptacle device, the receptacle opening configured to allow the implantable medical device to pass from the receptacle volume and through the receptacle opening, wherein the receptacle device includes a retainer positioned within the receptacle volume, the retainer defining a retention finger extending in a direction from an inner surface of the receptacle wall toward the longitudinal axis, wherein the retention finger is Atty Ref. No: A0010569WO01 configured to engage the implantable medical device within the receptacle volume to limit a translation of the implantable medical device relative to the receptacle wall in at least a distal direction of the receptacle device, and wherein the retention finger is configured to disengage from the implantable medical device when the implantable medical device is rotated relative to the receptacle wall. [0118] Example 2. The medical system of Example 1, wherein the retainer is configured to be stationary with respect to the receptacle wall when the implantable medical device is rotated relative to the receptacle wall. [0119] Example 3. The medical system of Example 1 or Example 2, wherein the inner surface defines a wall perimeter at least partially surrounding the longitudinal axis, and wherein the retention finger extends around a portion of the wall perimeter. [0120] Example 4. The medical system of any of Examples 1–3, wherein the receptacle device defines a delivery opening which opens to the receptacle volume, the delivery opening configured to open to a lumen of a catheter supporting the receptacle device, wherein the receptacle volume extends in a proximal direction of the receptacle device from the receptacle opening to the delivery opening, wherein the receptacle device defines a midpoint halfway between the delivery opening and the receptacle opening, wherein the receptacle device includes a proximal portion proximal to the midpoint and a distal portion distal to the midpoint, and wherein the retainer is within the proximal portion. [0121] Example 5. The medical system of any of Examples 1–4, wherein the retainer defines a groove proximal to the retention finger, the groove configured to receive a portion of the implantable medical device when the receptacle volume receives the implantable medical device. [0122] Example 6. The medical system of Example 5, wherein the retainer defines a backing surface, wherein the retainer defines the groove between the retention finger and the backing surface, and wherein the backing surface is configured to slidably engage the implantable medical device when the implantable medical device is rotated relative to the receptacle wall. [0123] Example 7. The medical system of Examples 1–6, wherein the retention finger defines a bearing surface configured to engage the implantable medical device to limit the translation of the implantable medical device relative to the receptacle device, Atty Ref. No: A0010569WO01 and wherein the receptacle wall defines a flow channel defining a flow path for a fluid from a point proximal to the bearing surface to a point distal to the bearing surface. [0124] Example 8. The medical system of Example 7, wherein the flow channel is an open channel recessed into the inner wall. [0125] Example 9. The medical system of Example 7 or Example 8, wherein the retention finger extends around a first portion of a perimeter of the inner wall at least partially surrounding the longitudinal axis, and wherein the flow channel extends around a second portion of the perimeter of the inner wall. [0126] Example 10. The medical system of any of Examples 7–9, wherein the receptacle wall defines a plurality of flow channels, each channel defining an individual flow path from a first point proximal to the bearing surface to a second point distal to the bearing surface. [0127] Example 11. The medical system of any of Examples 1–10 further comprising a delivery catheter supporting the receptacle device, the delivery catheter defining a delivery lumen opening into the receptacle volume. [0128] Example 12. The medical system of any of Examples 1–11, further comprising the implantable medical device, wherein the implantable medical device defines a device structure configured to engage the retention finger to resist at least a movement of the implantable medical device in the distal direction relative to the receptacle wall. [0129] Example 13. The medical system of Example 12, wherein the device structure defines a notch configured to align with the retention finger when the implantable medical device is rotated about a device axis relative to the receptacle wall, and wherein the retention finger is configured to pass through the notch when the notch aligns with the retention finger and the implantable medical device moves in the distal direction relative to the receptacle wall. [0130] Example 14. The medical system of any of Examples 11–13, wherein the device structure is configured to insert within the groove of Example 5 when the receptacle volume receives the implantable medical device, and wherein the device structure is configured to rotate relative to the retention finger when the implantable medical device is rotated relative to the receptacle wall. [0131] Example 15. The medical system of Example 14, wherein the device Atty Ref. No: A0010569WO01 structure is configured to slidably engage the backing surface of Example 6 when the device structure is inserted within the groove and the implantable medical device is rotated relative to the receptacle wall. [0132] Example 16. The medical system of Examples 11–15, wherein the device structure includes an interference portion configured to enter the groove as the implantable medical device rotates relative to the receptacle wall, wherein the retention finger is configured to engage the interference portion to resist the rotation of the implantable medical device. [0133] Example 17. The medical system of Example 16, wherein the interference portion is configured to establish an interference fit with the retention finger and the backing surface of claim 6 when the retention finger resists the rotation of the implantable medical device. [0134] Example 18. The medical system of Example 16 or Example 17, wherein the interference portion is configured to enter the groove as the implantable medical device is rotated relative to the receptacle wall in a first rotational direction about the device axis, and wherein the interference portion is configured to exit the groove as the implantable medical device is rotated relative to the receptacle wall in a second rotational direction opposite the first rotational direction. [0135] Example 19. The medical system of Examples 11–18, wherein the implantable medical device includes a stop configured to engage at least one of the retainer or a portion of the receptacle device as the implantable medical device rotates relative to the retention finger, wherein the stop is configured to resist the rotation of the implantable medical device relative to the retention finger when the stop engages the at least one of the retainer or the portion of the receptacle device. [0136] Example 20. The medical system of Example 19, wherein the stop is configured to engage the at least one of the retainer or the portion of the receptacle device as the implantable medical device is rotated relative to the receptacle wall in a first rotational direction about the device axis, and wherein the stop is configured to disengage from the at least one of the retainer or the portion of the receptacle device as the implantable medical device is rotated relative to the receptacle wall in a second rotational direction opposite the first rotational direction. [0137] Example 21. The medical system of Examples 1–20, wherein the Atty Ref. No: A0010569WO01 implantable medical device includes an attachment member configured to engage tissue when the implantable medical device is rotated about the device axis. [0138] Example 22. The medical system of Example 21, wherein the attachment member is configured to engage tissue when the implantable medical device is rotated in the second rotational direction of Example 18 or Example 20. [0139] Example 23. The medical system of Example 21 or Example 22, wherein a proximal portion of the implantable medical device supports the device structure and a distal portion of the implantable medical device supports the attachment member. [0140] Example 24. The medical system of any of Examples 1 – 23, further comprising a driver including a driver body, wherein: a distal portion of the driver body is configured to extend through the delivery lumen of Example 10 and into the receptacle volume, the distal portion of the driver is configured to transfer a torque to the implantable medical device to cause the implantable medical device to rotate relative to the receptacle wall. [0141] Example 25. The medical system of Example 24, wherein the driver body is configured to slidably translate within the delivery lumen to cause the implantable medical device to translate within the receptacle volume in a distal direction or a proximal direction relative to the receptacle wall when the notch of claim 12 aligns with the retention finger. [0142] Example 26. The medical system of any of Examples 1–25, wherein the anatomical volume is a chamber of a heart of the patient. [0143] Example 27. The medical system of any of Examples 1–26, wherein the delivery catheter of claim 10 is configured transport the receptacle device through vasculature of the patient. [0144] Example 28. A method, comprising: engaging, using a retention finger of a retainer, an implantable medical device to limit a translation of the implantable medical device relative to a receptacle device in at least a distal direction of the receptacle device, the implantable medical device within a receptacle volume defined by a receptacle wall surrounding a longitudinal axis extending through the receptacle volume, wherein the retention finger extending in a direction from an inner surface of the receptacle wall toward the longitudinal axis, and wherein the receptacle device defining a receptacle opening which opens to the receptacle volume at a distal end of the receptacle device, the Atty Ref. No: A0010569WO01 receptacle opening configured to allow the implantable medical device to pass from the receptacle volume and through the receptacle opening; and disengaging the retention finger and the implantable medical device using a rotation of the implantable medical device relative to the receptacle wall. [0145] Example 29. The method of Example 28, further comprising supporting the receptacle device using a delivery catheter, wherein the receptacle device defines a delivery opening which opens to a delivery lumen of the delivery catheter at a proximal end of the receptacle device. [0146] Example 30. The method of Example 29, further comprising engaging the retention finger and the implantable medical device in a proximal portion of the receptacle device, wherein the receptacle device defines a midpoint halfway between the delivery opening and the receptacle opening, and wherein the proximal portion is proximal to the midpoint. [0147] Example 31. The method of any of Examples 28 – 30, further comprising receiving, within a groove of the retainer proximal to the retention finger, a portion of the implantable medical device when the receptacle volume receives the implantable medical device. [0148] Example 32. The method of any of Examples 28 – 31, further comprising slidably engaging, using a backing surface of the retainer, the implantable medical device when the implantable medical device is rotated relative to the receptacle wall, wherein the retainer defines the groove between the retention finger and the backing surface. [0149] Example 33. The method of any of Examples 28–32, further comprising flowing a fluid, using at least one flow channel defined by the receptacle wall, from a point proximal to a bearing surface of the retention finger to a point distal to the bearing surface, wherein the bearing surface is configured to engage the implantable medical device to limit the translation of the implantable medical device relative to the receptacle device. [0150] Example 34. The method of any of Examples 28–33, further comprising rotating the implantable medical device relative to the retention finger when the implantable medical device rotates relative to the receptacle wall. [0151] Example 35. The method of any of Examples 28–34, further comprising: inserting a device structure of the implantable medical device into the groove of Example Atty Ref. No: A0010569WO01 32; and engaging the device structure using the retention finger. [0152] Example 36. The method of Example 35, further comprising: aligning, using the device structure, a notch with the retention finger when the implantable medical device is rotated relative to the receptacle wall, and causing, using movement of the implantable medical device, the retention finger to pass through the notch when the notch is aligned with the retention finger. [0153] Example 37. The method of any of Examples 28–36, further comprising: positioning, using rotation of the implantable medical device, an interference portion within the groove of Example 32; and engaging, using the retention finger, the interference portion to resist the rotation of the implantable medical device. [0154] Example 38. The method of any of Examples 28–37, further comprising: engaging, using a stop of the implantable medical device, at least one of the retainer or a portion of the receptacle device as the implantable medical device rotates relative to the retention finger; and resisting, using the stop, the rotation of the implantable medical device relative to the retention finger when the stop engages the at least one of the retainer or the portion of the receptacle device. [0155] Example 39. The method of any of Examples 28–38, further comprising engaging tissue using an attachment member by rotating the implantable medical device relative to the receptacle wall. [0156] Example 40. The method of Example 39, further comprising: disengaging the retention finger and the implantable medical device by rotating the implantable medical device in a particular rotational direction relative to the receptacle wall; and engaging tissue using the attachment member by rotating the implantable medical device in the particular rotational direction relative to the receptacle wall. [0157] Example 41. The method of any of Examples 28–40, further comprising transferring a torque, using a driver extending through the delivery lumen of claim 29, to the implantable medical device to cause the implantable medical device to rotate relative to the receptacle wall. [0158] Example 42. The method of Examples 41, further comprising translating, using the driver, the implantable medical device to translate within the receptacle volume when the notch of claim 36 aligns with the retention finger.

Claims

Atty Ref. No: A0010569WO01 WHAT IS CLAIMED IS: 1. A medical system comprising a receptacle device configured to be positioned within an anatomical volume defined by a body of a patient, wherein the receptacle device includes a receptacle wall defining a receptacle volume configured to receive at least a portion of an implantable medical device, the receptacle wall at least partially surrounding a longitudinal axis extending through the receptacle volume, wherein the receptacle device defines a receptacle opening which opens to the receptacle volume at a distal end of the receptacle device, the receptacle opening configured to allow the implantable medical device to pass from the receptacle volume and through the receptacle opening, wherein the receptacle device includes a retainer positioned within the receptacle volume, the retainer defining a retention finger extending in a direction from an inner surface of the receptacle wall toward the longitudinal axis, wherein the retention finger is configured to engage the implantable medical device within the receptacle volume to limit a translation of the implantable medical device relative to the receptacle wall in at least a distal direction of the receptacle device, and wherein the retention finger is configured to disengage from the implantable medical device when the implantable medical device is rotated relative to the receptacle wall. 2. The medical system of claim 1, wherein the retainer is configured to be stationary with respect to the receptacle wall when the implantable medical device is rotated relative to the receptacle wall. 3. The medical system of claim 1 or claim 2, wherein the inner surface defines a wall perimeter at least partially surrounding the longitudinal axis, and wherein the retention finger extends around a portion of the wall perimeter. Atty Ref. No: A0010569WO01 4. The medical system of any of claims 1–3, wherein the receptacle device defines a delivery opening which opens to the receptacle volume, the delivery opening configured to open to a lumen of a catheter supporting the receptacle device, wherein the receptacle volume extends in a proximal direction of the receptacle device from the receptacle opening to the delivery opening, wherein the receptacle device defines a midpoint halfway between the delivery opening and the receptacle opening, wherein the receptacle device includes a proximal portion proximal to the midpoint and a distal portion distal to the midpoint, and wherein the retainer is within the proximal portion. 5. The medical system of any of claims 1–4, wherein the retainer defines a groove proximal to the retention finger, the groove configured to receive a portion of the implantable medical device when the receptacle volume receives the implantable medical device. 6. The medical system of claims 1–5, wherein the retention finger defines a bearing surface configured to engage the implantable medical device to limit the translation of the implantable medical device relative to the receptacle device, and wherein the receptacle wall defines a flow channel defining a flow path for a fluid from a point proximal to the bearing surface to a point distal to the bearing surface. 7. The medical system of any of claims 1–6 further comprising a delivery catheter supporting the receptacle device, the delivery catheter defining a delivery lumen opening into the receptacle volume. 8. The medical system of any of claims 1–7, further comprising the implantable medical device, wherein the implantable medical device defines a device structure Atty Ref. No: A0010569WO01 configured to engage the retention finger to resist at least a movement of the implantable medical device in the distal direction relative to the receptacle wall. 9. The medical system of claim 8, wherein the device structure defines a notch configured to align with the retention finger when the implantable medical device is rotated about a device axis relative to the receptacle wall, and wherein the retention finger is configured to pass through the notch when the notch aligns with the retention finger and the implantable medical device moves in the distal direction relative to the receptacle wall. 10. The medical system of any of claim 8 or claim 9, wherein the device structure is configured to insert within the groove of claim 5 when the receptacle volume receives the implantable medical device, and wherein the device structure is configured to rotate relative to the retention finger when the implantable medical device is rotated relative to the receptacle wall. 11. The medical system of any of claims 8–10, wherein the device structure includes an interference portion configured to enter the groove as the implantable medical device rotates relative to the receptacle wall, wherein the retention finger is configured to engage the interference portion to resist the rotation of the implantable medical device. 12. The medical system of claim 11, wherein the interference portion is configured to enter the groove as the implantable medical device is rotated relative to the receptacle wall in a first rotational direction about the device axis, and wherein the interference portion is configured to exit the groove as the implantable medical device is rotated relative to the receptacle wall in a second rotational direction opposite the first rotational direction. 13. The medical system of claims 1–12, wherein the implantable medical device includes an attachment member configured to engage tissue when the implantable medical device is rotated about the device axis. Atty Ref. No: A0010569WO01 14. A method, comprising: engaging, using a retention finger of a retainer, an implantable medical device to limit a translation of the implantable medical device relative to a receptacle device in at least a distal direction of the receptacle device, the implantable medical device within a receptacle volume defined by a receptacle wall surrounding a longitudinal axis extending through the receptacle volume, wherein the retention finger extending in a direction from an inner surface of the receptacle wall toward the longitudinal axis, and wherein the receptacle device defining a receptacle opening which opens to the receptacle volume at a distal end of the receptacle device, the receptacle opening configured to allow the implantable medical device to pass from the receptacle volume and through the receptacle opening; and disengaging the retention finger and the implantable medical device using a rotation of the implantable medical device relative to the receptacle wall. 15. The method of claim 14, further comprising supporting the receptacle device using a delivery catheter, wherein the receptacle device defines a delivery opening which opens to a delivery lumen of the delivery catheter at a proximal end of the receptacle device.