Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
  • A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
  • A61M60/10—Location thereof with respect to the patient's body
  • A61M60/122—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body
  • A61M60/126—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body implantable via, into, inside, in line, branching on, or around a blood vessel
  • A61M60/161—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body implantable via, into, inside, in line, branching on, or around a blood vessel mechanically acting upon the outside of the patient's blood vessel structure, e.g. compressive structures placed around a vessel
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
  • A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
  • A61M60/10—Location thereof with respect to the patient's body
  • A61M60/122—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body
  • A61M60/126—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body implantable via, into, inside, in line, branching on, or around a blood vessel
  • A61M60/148—Implantable pumps or pumping devices, i.e. the blood being pumped inside the patient's body implantable via, into, inside, in line, branching on, or around a blood vessel in line with a blood vessel using resection or like techniques, e.g. permanent endovascular heart assist devices
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
  • A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
  • A61M60/20—Type thereof
  • A61M60/289—Devices for mechanical circulatory actuation assisting the residual heart function by means mechanically acting upon the patient's native heart or blood vessel structure, e.g. direct cardiac compression [DCC] devices
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
  • A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
  • A61M60/40—Details relating to driving
  • A61M60/465—Details relating to driving for devices for mechanical circulatory actuation
  • A61M60/468—Details relating to driving for devices for mechanical circulatory actuation the force acting on the actuation means being hydraulic or pneumatic
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
  • A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
  • A61M60/50—Details relating to control
  • A61M60/508—Electronic control means, e.g. for feedback regulation
  • A61M60/538—Regulation using real-time blood pump operational parameter data, e.g. motor current
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
  • A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
  • A61M60/80—Constructional details other than related to driving
  • A61M60/855—Constructional details other than related to driving of implantable pumps or pumping devices
  • A61M60/871—Energy supply devices; Converters therefor
  • A61M60/873—Energy supply devices; Converters therefor specially adapted for wireless or transcutaneous energy transfer [TET], e.g. inductive charging
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
  • A61M60/00—Blood pumps; Devices for mechanical circulatory actuation; Balloon pumps for circulatory assistance
  • A61M60/80—Constructional details other than related to driving
  • A61M60/855—Constructional details other than related to driving of implantable pumps or pumping devices
  • A61M60/871—Energy supply devices; Converters therefor
  • A61M60/876—Implantable batteries
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61M—DEVICES FOR INTRODUCING MEDIA INTO, OR ONTO, THE BODY; DEVICES FOR TRANSDUCING BODY MEDIA OR FOR TAKING MEDIA FROM THE BODY; DEVICES FOR PRODUCING OR ENDING SLEEP OR STUPOR
  • A61M2205/00—General characteristics of the apparatus
  • A61M2205/82—Internal energy supply devices
  • A61M2205/8237—Charging means
  • A61M2205/8243—Charging means by induction

Definitions

• the present invention relates to a blood circulation assistance device and a method of effecting counterpulsation, especially diastolic counterpulsation.
• VADs ventricular assist devices
• bridge to transplantation a heart transplantation
• bridge to recovery a heart transplantation
• VADs have been considered as an alternative to heart transplantation (destination therapy, permanent use) 3 .
• Total artificial hearts 4 which are less widely used than VADs, entail removal of the natural heart and have been designed as a bridge to transplantation and for destination therapy. As VADs and total artificial hearts entail blood contact, continuous anticoagulation of the patient is mandatory to minimise the risk of blood clotting (thrombogenesis). ⁇ *
• Intra-aortic balloon counterpulsation (IABC) 5 is a widely applied treatment which is predominantly used for acute heart failure. Insertion of an intra-aortic balloon (IAB) is relatively non-invasive with respect to a VAD but support duration is typically limited to less than a fe ⁇ 5V weeks because the patient is non-ambulant, and there is a significant risk of occlusion of the artery through which the IAB catheter is inserted leading to lower limb ischaemia 6 . Like VADs and total artificial hearts, the IAB catheter is in contact with the blood as a result of which the patient is required to receive continuous anticoagulation.
• WO-A-02/24254 discloses a novel, totally implantable, extravascular, (non blood contacting) counterpulsator suitable for chronic use in ambulatory patients, comprising a sophisticated electrohydraulic energy converter.
• the energy converter comprises an impeller which rotates about an axis in order to drive a fluid and which is moved axially, in reciprocating fashion, in order to change the alignment of the impeller.
• the operating principle is as follows.
• the energy converter pumps hydraulic fluid from an integral intracorporeal reservoir (as described in US-A-5,346,458) into a peri-aortic jacket, thereby compressing the aorta and displacing blood proximally and distally (towards and away from the heart, respectively).
• This has the effect of raising the diastolic blood pressure thereby improving organ perfusion, particularly that of the heart muscle, which receives the majority of its blood supply during diastole.
• the direction in which the energy converter pumps the hydraulic drive fluid is rapidly reversed, with the result that the peri-aortic jacket deflates leading to a fall in the end-diastolic arterial blood pressure (the pre-systolic dip). This reduces the amount of work that the heart is required to perform during the ejection phase (systole) of the subsequent heartbeat.
• WO-A-02/24254 also discloses an alternative means of achieving extra-aortic counterpulsation by means of a solid-state actuator coupled directly to the aorta.
• the aorta within the peri-aortic jacket can withstand repetitive deformation for prolonged periods without inducing mechanical failure of the aorta or deleterious tissue remodelling.
• the configuration of the implantable system is compatible with a minimally traumatic surgical procedure.
• the current invention seeks to meet one or more of these requirements and it has been found that the performance of embodiments of the present invention exceeds that of the most sophisticated IAB systems currently available in crucial aspects (see Figure 19).
• a blood circulation assistance device comprising: one or more cuff elements defining a chamber for receiving a blood conduit, the chamber being open at both ends of the one or more cuff elements for the blood conduit to extend therethrough, the one or more cuff elements bearing at least one inwardly expanding inflatable element for compressing the blood conduit.
• a set of (e.g. two) inflatable elements act in conjunction to work, in effect, as a single inflatable element.
• inflatable element includes a set of inflatable elements working in conjunction.
• the inflatable elements are disposed diametrically opposite each other across the chamber.
• the terms "diametrically opposite” and “diametrically opposed” mean that, if the points of the inflatable elements that can come closest to each other, on inflation, are considered, then the respective normals at each point have between. them an angle of at least 135°, preferably 160°, more preferably 170°, more preferably 175° and most preferably 180°.
• two cuff elements define the chamber, the aperture being between the two cuff elements.
• the two cuff elements, the inflatable elements and the chamber that they define are elongate, such that the chamber can receive the blood conduit longitudinally therethrough, with the cuff elements and the inflatable elements being parallel to the blood conduit.
• the blood circulation assistance device further comprises an inlet, leading to the two inwardly expanding inflatable elements.
• the inlet leads to the centre of each inflatable element, along the longitudinal axis.
• the inlet leads to one end of each inflatable element.
• the inlet leads to both ends of each inflatable element.
• the inlet leads to the side of each inflatable element, along their full length.
• the inlet is substantially parallel to the longitudinal axis of the inflatable elements.
• the inlet is at an acute angle to the longitudinal axis of the inflatable elements.
• the inlet leads to the side of each inflatable element, at a point between the centre and the end of the longitudinal axis thereof.
• the two inflatable elements are inflatable simultaneously.
• the blood circulation assistance device further comprises a manifold leading to the inflatable elements, the cross-section of the manifold leading to each inflatable element being different,.. ..
• opposing sides of a blood conduit received in the chamber do not contact one another.
• the minimum trans-sectional curvature of a blood conduit received in the chamber is maximised, preferably by having a minimal trans-sectional radius of curvature of at least 30% of the original value, such as for example at least 36.3% of the original value or exactly 36.3% of the original value. It is particularly preferred that the minimal trans-sectional radius of curvature is at least or exactly 40.4% of the original value.
• a blood conduit received in the chamber has a reduction in its lumenal cross-section to more than 50% of its original value, more preferably to more than 51.5% of the original value.
• the Iumenal clearance of the blood vessel when compressed is at least 10%, more preferably at least 15% and more preferably at least 20% of the resting diameter.
• a blood circulation assistance device may be set to result in a Iumenal clearance, on compression of the blood vessel, of 20% when initially fitted but this may change to, for example a 15% or 10% clearance over time, as the inflatable elements stretch slightly.
• the inflatable elements are made from a material which resists elastic deformation.
• the ends of the inflatable elements are rounded.
• the ends of the inflatable, elements protrude longitudinally from the one or more cuff elements, along the axis of a blood conduit received in the chamber.
• the blood circulation assistance device further comprises a pump in fluid communication with the inflatable elements.
• the fluid path from the pump to the inflatable elements has an increasing cross-section.
• the fluid which communicates between the pump and the inflatable elements has a viscosity of between 8 x 10 "4 and 1.2x10 "3 Pa. s (0.8 and 1.2cP), more preferably 1x10 "3 Pa.s (1.0cP).
• the fluid which communicates between the pump and the inflatable elements is a fluorocarbon.
• connection provided between the pump and the inflatable elements in order to effect fluid communication therebetween is flexible.
• the flexible connection comprises a joint having a ball with a passage extending therethrough, connected rotatably at either end to a socket.
• the flexible connection is concertinaed in order to provide flexibility.
• the flexible connection is lockable in a particular configuration.
• the pump is adjacent the one or more cuff elements and the aperture is located on the opposite side of the chamber from the cuff.
• the pump is adjacent to the one or more cuff elements and the aperture is located on the chamber at 90° with respect to the pump.
• the pump comprises an impeller rotatable about an axis to effect pumping, the impeller being axially moveable from a first position to a second position to effect reversal of the direction of pumping.
• the blood circulation assistance device further comprises a counterbalance, moveable in synchronicity with the impeller but in an opposing direction from the axial movement of the impeller so as to counteract the reaction of the -movement of the impeller.
• a counterbalance moveable in synchronicity with the impeller but in an opposing direction from the axial movement of the impeller so as to counteract the reaction of the -movement of the impeller.
• the counterbalance is movable parallel to the impeller.
• the counterbalance comprises a second rotatable impeller, both impellers being rotatable about an axis to effect pumping.
• the blood circulation assistance device further comprises a sensor capable of detecting whether the impeller is in the first or second position; and a control mechanism for shutting down the impeller in response to the sensor detecting that the impeller is locked in a position causing inflation of the inflatable elements.
• the pump comprises a rotatable impeller; an inlet port for drawing in fluid; an outlet port for ejecting fluid; and a valve assembly interposed between the rotatable impeller and the inflatable element, the valve assembly being slidable or rotatable from a first position in which the inlet port is in fluid communication with the inflatable element and a second position in which the outlet port is in fluid communication with the inflatable element, such that movement of the valve assembly between the first and second positions effects deflation and inflation of the inflatable elements, respectively.
• the pump is beatable in the pleural cavity.
• the pump is locatable within the pre-peritoneal or intra-peritoneal cavity and is connected to the inflatable elements via a hydraulic tube.
• the pre-peritoneal cavity would be fashioned by a surgeon.
• the pump is locatable extra-corporeally.
• flow guides are provided between the pump and the inflatable elements.
• the flow guides are preferably vanes, swivels and or deswirlers.
• the flow guides preferably minimise undesirable secondary flow behaviour or create desirable secondary flow features.
• the blood circulation assistance device further comprises a sleeve provided around the outer circumference of the one or more cuff elements.
• the blood circulation assistance device further comprises at least one band about the one or more cuff elements.
• one or more cuff elements are movable so as to increase or decrease the size of the chamber.
• cuff elements there are provided two or more cuff elements, connected by a lockable hinge.
• the blood circulation assistance device further comprises an inner sleeve located between the inflatable elements and the chamber.
• the blood circulation assistance device further comprises one or more eyelets for attaching the blood circulation assistance device to a structure.
• the inflatable elements are between 3 and 15 cm long, more preferably between 5 and 9 cm long.
• the inflatable elements are inflatable once in each cardiac cycle of a patient fitted with the device.
• the inflatable elements are inflatable in the diastolic phase of each cardiac cycle of a patient fitted with the device or less frequently, such as in alternative cardiac cycles.
• the device is locatable in the left paravertebral gutter of a human.
• the blood circulation assistance device further comprises an integral ECG electrode or electrodes for detecting the heartbeat of an individual.
• the blood circulation assistance device further comprises a position sensor, a pressure sensor or an accelerometer for detecting the heartbeat of an individual.
• a blood circulation assistance device comprising : one or more cuff elements defining a chamber for receiving a blood conduit, the one or more cuff elements bearing an inwardly expanding inflatable element for compressing the blood conduit received in the chamber, the inflatable element being expandable such that, at its maximum expansion, the minimal trans-sectional radius of curvature of the blood conduit received in the chamber is maximised.
• the minimum trans-sectional radius of curvature of the blood conduit is at least 30% of the original value, preferably at least or exactly 36.3%, more preferably at least or exactly 40.4% of the original value.
• the reduction of the lumenal cross-section of the blood conduit is to more than 50%, preferably more than 51.5% of the original value.
• the lumenal clearance of the blood vessel when compressed is at least 10%, preferably at least 15% and more preferably at least 20% of the resting diameter.
• a blood circulation assistance device comprising: one or more cuff elements defining a chamber for receiving a blood conduit, the one or more cuff elements bearing an inwardly expanding inflatable element for compressing the blood conduit received in the chamber; and a pump, the pump comprising: a rotatable impeller; an inlet port for drawing in fluid; an outlet port for ejecting fluid; and a valve assembly interposed between the rotatable impeller and the inflatable element, the valve assembly being slidable or rotatable from a first position in which the inlet port is in fluid communication with the inflatable element and a second position in which the outlet port is communication with the inflatable element, such that movement of the valve assembly between the first and second positions effects deflation and inflation of the inflatable element respectively.
• a blood circulation assistance device comprising: one or more cuff elements defining a chamber for receiving a blood conduit, the one or more cuff elements bearing an inwardly expanding inflatable element for compressing the blood conduit received in the chamber; and a pump in fluid communication with the inflatable element, the pump comprising an impeller rotatable about an axis to effect pumping, the impeller being axially movable from a first position to a second position to effect reversal of the direction of pumping, there being a counterbalance provided, movable in synchronicity with but in an oppositing direction from the axial movement of the impeller so as to counteract the reaction of the movement of the impeller.
• the blood circulation assistance device further comprises a second inwardly expanding inflatable element for compressing a blood conduit received in the chamber.
• a method of effecting counterpulsation of a blood vessel comprising: introducing an annular stent into the lumen of the blood vessel, at each end of a section of the blood vessel; providing an external band, around the blood vessel at each end of the section of the blood vessel such that a portion of the blood vessel is trapped between each external band and its respective annular stent; and effecting counterpulsation on the blood vessel between the two annular stents.
• compression of the blood conduit is carried out using a blood circulation assistance device as described above.
• each annular stent comprises a circumferential groove about its outer surface, for receiving its respective external band.
• the word “comprising” means “including” or “consisting of and the word “comprises” means “includes” or “consists of.
• blood conduit means a natural blood vessel; a synthetic or artificial blood vessel; or other tubular structure for carrying blood.
• Figure 1 is a perspective view of a blood circulation assistance device in accordance with one embodiment of the present invention.
• Figure 2 is a cross-sectional view of a peri-aortic cuff, in a deflated state (left), and an inflated state (right) located about an aorta, in accordance with an embodiment of the present invention.
• Figure 3 is a longitudinal view of the peri-aortic cuff and aorta of the embodiment of Figure 2 along the lines A-A(left) and B-B (right) respectively, with the inflatable elements being deflated (left) and inflated (right).
• Figure 4 is an axial cross-sectional view of a blood circulation assistance device according to another embodiment of the present invention.
• Figure 5 is a perspective view of a blood circulation assistance device in accordance with one embodiment of the present invention, superimposed on a computer tomography-derived illustration of a human chest.
• Figure 6 is a perspective view of the manifold and inflatable elements of a blood circulation assistance device in accordance with another embodiment of the present invention.
• Figure 7 is a perspective view of the manifold and inflatable elements of a blood circulation assistance device in accordance with another embodiment of the present invention.
• Figure 8 is a perspective view of the manifold and inflatable elements of a blood circulation assistance device in accordance with another embodiment of the present invention.
• Figure 9 is a perspective view of the manifold and inflatable elements of a blood circulation assistance device in accordance with another embodiment of the present invention.
• Figure 10 is a perspective view of the manifold and inflatable elements of a blood circulation assistance device in accordance with another embodiment of the present invention.
• Figure 11 is a perspective view of the manifold and inflatable elements of a blood circulation assistance device in accordance with another embodiment of the present invention.
• Figure 12 is a perspective view of a universal coupling, in a first position, between an energy converter and the inflatable elements of a blood circulation assistance device in accordance with another embodiment of the present invention.
• Figure 13 is a perspective view of the universal coupling shown in Figure 12, in a second position.
• Figure 14 is a perspective view of another universal coupling, in a first position, between an energy converter and the inflatable elements of a blood circulation assistance device in accordance with a further embodiment of the present invention.
• Figure 15 is a perspective view of the universal coupling shown in Figure 14, in a second position.
• Figure 16 is a perspective view of a blood circulation assistance device in accordance with yet another embodiment of the present invention.
• Figure 17 is a perspective view of a blood circulation assistance device in accordance with another embodiment of the present invention.
• Figure 18 is a perspective view of the peri-aortic cuff and manifold of a blood circulation assistance device used in the Example 1 of the present invention.
• Figure 20 is a graph of : i) an electrocardiogram (ECG), ii) instantaneous coronary artery blood flow (proximal left anterior descending artery), iii) arterial blood pressure with respect to time in a model using a 50 mm length peri-aortic jacket. These traces were obtained simultaneously at an assist ratio of 1 :2. Assisted beats are depicted with an "A" and unassisted with a "U".
• Figure 21 is a perspective view of a blood circulation assistance device in accordance with a further embodiment of the present invention, as shown in position within a human torso.
• Figure 22 is a schematic representation of a longitudinal sectional view of an energy converter according to one embodiment of the present invention comprising an axially reciprocating valve assembly in the "cuff deflation" state.
• Figure 23 is a schematic representation of a longitudinal sectional view of an energy converter according to the embodiment shown in Figure 22 in the "cuff inflation" state.
• Figure 24 is a schematic representation of a longitudinal sectional view of an energy converter according to another embodiment of the present invention comprising a rotating valve assembly in the "cuff deflation" state.
• Figure 25 is a schematic representation of a cross-sectional view along the line A-A 1 of Figure 24.
• Figure 26 is a schematic representation of a cross-sectional view along the line B-B' of Figure 24.
• Figure 27 is a schematic representation of a longitudinal sectional view of an energy converter according to the embodiment shown in Figure 24 in the "cuff inflation" state.
• Figure 28 is a schematic representation of a cross-sectional view of the energy converter shown in Figure 27 along the line A-A'.
• Figure 29 is a schematic representation of a cross-sectional view of the energy converter shown in Figure 27 along the line B-B'.
• Figure 30 is the result of an investigation, using an experimental model, into the curvature of an aorta when under compression by two inflatable elements.
• Figure 31 is the result of an investigation, using an experimental model, into the curvature of an aorta when under compression by two inflatable elements and additional constraints to model the effect of lumenal pressure.
• Figure 32 is the result of an investigation, using an experimental model, into the curvature of an aorta when under compression by-three inflatable elements.
• Figure 33 is the result of an investigation, using an experimental model, into the curvature of an aorta when under compression by three inflatable elements.
• a blood circulation assistance device 1 comprises a pump or energy converter 2 connected to a hollow peri-aortic jacket or cuff 3.
• the peri-aortic cuff 3 is rigid or semi-rigid and is curved so as to define a substantially cylindrical chamber within it.
• two elongate inflatable elements 4, 5 In contact with the inner surface of the peri-aortic cuff 3- are provided two elongate inflatable elements 4, 5 which are held in place by the peri-aortic cuff 3, diametrically opposite one another across the chamber.
• the chamber is sized and shaped to receive (preferentially) the descending aorta 6 (although in alternative embodiments, it is sized and shaped to receive the ascending aorta) and the inflatable elements 4,5 are positioned parallel to the descending aorta 6 and contra-lateral Iy with respect to it.
• Each end of the chamber is open to allow the aorta 6 to extend through it.
• a slit (or aperture) 22 is provided in the peri-aortic cuff or jacket 3 which leads between the two inflatable elements into the chamber and which extends from one end of the chamber to the other so that the peri-aortic cuff 3 can be slipped over the aorta 6 in a direction perpendicular to the longitudinal axis of the aorta 6.
• the inflatable elements 4, 5 are filled with a hydraulic drive medium contained within a common manifold 7 of low compliance, in fluid communication with the pump or energy converter.
• the hydraulic fluid flow path between the pump and the inflatable elements 4, 5 is designed to allow rapid filling and emptying of the inflatable elements 4, 5 by minimising both flow resistance due to cross sectional changes, as far as is practicable, and undesirable secondary flow features of the hydraulic drive fluid (e.g. contra-rotations associated with abrupt cross-sectional change which may impair filling and emptying of the inflatable elements 4, 5).
• secondary flow characteristics are not invariably detrimental; indeed, it may be desirable to potentiate them by the inclusion of flow guides in order, for example, to create fluid mixing to avoid simultaneous bi-directional flow within the hydraulic fluid path.
• the entry of the hydraulic drive medium from the energy converter 2 into the manifold 7 is associated with inflation of the inflatable elements 4, 5.
• the outer surfaces of the inflatable elements 4, 5 is constrained by the peri-aortic cuff 3, the inner surfaces of the inflatable elements 4, 5 move together compressing the aorta within the jacket 3.
• the length of the peri-aortic jacket 3 is preferentially in the range of 3 to 15 cm.
• the inflatable elements 4, 5 associated with the peri-aortic cuff or jacket 3 are of relatively low compliance and so do not undergo significant elastomeric deformation during inflation in order to maximise their mechanical fatigue life. Nevertheless, a small degree of creep is expected in the inflatable elements as a result of repeated inflation/deflation cycling which is accommodated by the generous lumenal clearance, as described above. An acceptable mechanical fatigue life is achieved by appropriate material and wall thickness selection. Appropriate physical characteristics of the inflatable elements could be achieved by manufacturing them from a reinforced or filled polymer.
• the drive fluid which is provided is not associated with an adverse change in the physical characteristics of the membrane of inflatable elements with respect to time.
• Suitable drive fluids include aqueous and non-aqueous liquids with a viscosity of 0.8 and 1.2cP, preferably 1.OcP (8x10 "4 and 1.2x10 "3 Pa.s, preferably 1x10 "3 Pa.s).
• the drive medium is a fluorocarbon in some embodiments.
• the ends of the inflatable elements 4, 5 have a rounded geometry and they protrude slightly beyond the rigid peri-aortic cuff or jacket 3, in a longitudinal direction, in order to avoid an abrupt change in aortic section at the ends of the peri-aortic jacket (see Figures 1 and 3).
• the present inventors' in vitro experiments have revealed that the use of two elongate inflatable elements 4, 5, parallel to the longitudinal axis of the aorta 6, is associated with more predictable deformation characteristics of the aorta than a circumferential cuff, which the inventors' studies have shown is usually associated with marked aortic creasing and localised stress concentration. Consequently, the preferred embodiment is inherently better-suited to chronic counterpulsation, with respect to aortic durability, than a transverse, circumferential peri-aortic cuff.
• the distance between the energy converter 2 and periaortic jacket 3 is minimised thereby minimising the fluid dead space.
• a particularly preferred embodiment of the blood circulation assistance device is shown in transverse cross-section at the level of the impeller ports 8 which are in fluid communication with the inflatable elements 4, 5, in Figure 4. It can be seen that the hydraulic fluid path has a non uniform cross section in order to avoid excessive flow resistance or changes in flow resistance.
• flow guides are introduced into the hydraulic fluid path to minimise undesirable secondary flow behaviour which could otherwise compromise flow resistance and thus dQ/dt.
• undesirable secondary flow is the formation of counter-rotation features which can occur if there is an abrupt change in cross section.
• flow guides are included to create desirable secondary flow features; for example, guides that potentiate mixing to reduce simultaneous bi-directional flow may improve the performance characteristics of the device (dQ/dt).
• the flow guides would be in the form of vanes or swirlers or deswirlers in the fluid path.
• the energy converter 2 and peri-aortic cuff 3 shown are sized and shaped to fit in the left pleural cavity.
• the device is designed to be located by a surgeon in the left paravertebral gutter after the cuff has been placed around the aorta.
• the feasibility of this approach has been validated by computer-based fitting studies which entailed superimposition of the boundaries of the preferred embodiment within the Computed Tomography (CT)-derived left pleural cavity boundaries of normal humans (including those of the descending aorta) as illustrated in Figure 5.
• CT Computed Tomography
• the fluid connection between the pump or energy converter 2 and the inflatable elements 4, 5 is varied from embodiment to embodiment.
• the manifold 7 connects the pump 2 to the inflatable elements 4, 5 at the centre of their longitudinal axis.
• the manifold 7 is perpendicular to the longitudinal axis of the inflatable elements 4, 5.
• the manifold 7 connects to the inflatable elements 4, 5 at one of their respective ends 9, 10.
• the manifold 7 is directed at an acute angle from the elongate inflatable elements 4, 5 so that fluid is reflected back off the far wall 11 of the manifold 7 when it moves from the pump to the inflatable elements 4, 5 or vice versa.
• the inlet 7 is positioned at the centre of the inflatable elements 4, 5 along their longitudinal axis and perpendicular thereto.
• the manifold 7 bifurcates into first and second ducts 12, 13.
• the first duct connects the manifold 7 to the respective first ends 9, 10 of the inflatable elements 4, 5 and the second duct 13 connects the manifold 7 to the respective second ends 13, 14 of the inflatable elements 4, 5.
• the pump 2 is in fluid communication with both ends of each inflatable element 4, 5.
• the manifold 7 is located at the centre of the longitudinal axis of the inflatable elements 4, 5 and perpendicular thereto. However, in this embodiment, the manifold 7 connects to the sides of the elongate inflatable elements 4, 5, along their full length.
• the manifold 7 is provided at the first ends 9, 10, respectively of the inflatable elements 4, 5, as in the embodiment depicted in Figure 7.
• the manifold 7 is substantially parallel to the longitudinal axis of the inflatable elements 4, 5 but is offset so that there is a "dog leg" section 15 of the manifold 7 between the main part of the manifold 7 and the respective first ends 9, 10 of the inflatable elements 4, 5.
• the manifold 7 is located equidistantly between the two inflatable elements 4, 5.
• the manifold 7 is located between the centre of the longitudinal axis of the inflatable elements 4, 5 and the respective first ends, 9, 10 of the inflatable elements 4, 5.
• the manifold 7 is substantially perpendicular to the longitudinal axis of the inflatable elements 4, 5.
• some embodiments have a universal coupling between the energy converter 2 and the peri-aortic cuff 3, in order to ensure that misalignment of the cuff is avoided. Misalignment is undesirable for two reasons: the efficacy of counterpulsation may be reduced and the risk of aortic trauma may be increased as a result of increased local aortic wall stress.
• the universal coupling 16 comprises a "ball and double socket" joint. More specifically, the universal coupling 16 comprises first and second generally cylindrical sections 17, 18 connected by a ball 19 having a bore 20 through it. Each of the first and second sections 17, 18 is swivelable relative to the ball 19 so as to permit their relative movements. Fluid communication between the first and second sections 17, 18 is permitted by the bore 20.
• the universal coupling 16 comprises first and second generally cylindrical sections 17, 18 connected by a concertinaed section 21 which is sufficiently flexible to permit relative movement of the first and second cylindrical sections 17, 18.
• the movement of the universal couplings shown in Figures 12 to 15 are restricted, for example by the use of key-ways which only allow adjustment of the position of the cuff 3 relative to the energy converter 2 in one plane.
• the coupling must have a low compliance because any change in volume associated with the coupling will reduce the efficacy of the device by delaying filling or emptying of the cuff.
• the coupling can be locked rigidly in a particular configuration, after adjustment by the surgeon at the time of implantation, in a manner most appropriate to the patient anatomy. Locking of the universal coupling is achieved, in some embodiments, by means of a threaded compression ring or grub screw which is tightened by the surgeon during the implantation procedure to maintain the implant in an appropriate configuration.
• the pump 2 is located adjacent and parallel to the peri-aortic cuff 3.
• the slit 22 is located at one side of the plane defined by the pump 2 and the peri-aortic cuff 3. That is to say that the slit 22 is oriented postero-medially.
• the slit 22 is on the far side of the peri-aortic cuff 3 from the pump 2. That is to say that the slit 22 is oriented antero-medially.
• Positional stability of the peri-aortic cuff 3, with respect to the aorta 6, is of paramount importance if the efficacy of counterpulsation is to be maximised and the risk of aortic trauma minimised.
• a key advantage of embodiments of the invention is that the slit 22 in the peri-aortic jacket 3 permits the peri-aortic jacket to be placed around the aorta 6 rapidly with neither the need for surgical division of the aorta nor cardiopulmonary bypass.
• the device 1 is, in some embodiments, secured with respect to the aorta 6.
• an inert sleeve of strong synthetic fabric such as woven polyester, Dacron ® , PTFE, with a low distensibility is placed around the outer circumference of the peri-aortic jacket 3, after positioning around the aorta 6.
• a sheet of such a material is sutured into the form of a sleeve by a surgeon. Additionally, having fashioned this sleeve, the surgeon sutures it to the aorta 6 at the ends, in certain embodiments.
• ratchet plastic bands such as those made of nylon or a alternative material or materials with a high tensile strength are provided around the outside of the peri-aortic jacket 3.
• the bands have an adjustable buckle in order to vary their circumference.
• these bands are of a woven material and can be sewn by the surgeon.
• both an inert sleeve and plastic bands are provided.
• a key advantage of embodiments of the invention, which are preferentially sized and shaped for placement of the device on the descending aorta, is that the latter is associated with smaller variations in diameter than the ascending aorta and thus the efficacy of the device is more predictable. Nevertheless, it is important that the device can be coupled to the aorta effectively irrespective of its diameter.
• the outer edges of the peri-aortic cuff 3, which abut either side of the slit 22 are semirigid and are slightly splayed under normal conditions. This allows the surgeon to approximate the two inflatable elements 4, 5 to the desired degree by means of fashioning the cuff 3 appropriately such that it is under mild circumferential tension under resting conditions, i.e.
• the peri-aortic cuff 3 comprises one or two hinged jaws on which the inflatable elements 4, 5 are located and which can be locked to an appropriate diameter for the size of the aorta 6.
• aortic adventitia outermost layer
• the inflatable elements 4, 5 a woven polyester (Dacron ® ) or PTFE sleeve
• a tissue engineered construct, or mobilised intercostal muscle or other autologous tissue such as pericardium is provided between the inflatable elements 4, 5 and the chamber in which the aorta 6 is received.
• the adjustable peri-aortic jacket 3 allows for their incorporation in the implantation procedure.
• two annular stents are introduced into the lumen of the aorta 6, at either end of the peri-aortic jacket. External banding is applied around the aorta 6, at the position where each annular stent exists within the aorta 6. Thus the aorta 6 is sandwiched between a stent on the inside and an external band on the outside. This prevents propagation of the dissection.
• each annular stent has a circumferential groove around its rim for receiving the aorta 6 when the external band is applied, which presses the aorta 6 into the groove. This assists in maintaining the stent and external band in place.
• the energy converter 2 is stabilised by the incorporation of eyelets onto its outer surface to allow the device to be sutured, wire-tied, or screwed to rigid or semi rigid structures within the thoracic cavity, such as the vertebrae, ribs, intercostal muscle or cartilage.
• Energy converter variants are described below.
• the pump 3 comprises an impeller which rotates about an axis and which is moved axially in order to effect reversal of pumping (as described in detail in WO-A-02/24254), it is preferred that features are provided to avoid reaction forces on the aorta 6 during operation of the device.
• the energy converter 2 is associated with small axial reactions as a result of the bi-directional axial translation that the impeller assembly undergoes during normal operation.
• the energy converter 2 comprises two impeller assemblies which move towards, or away from, each other simultaneously to eliminate the axial reaction.
• reaction associated with axial movement of the impeller assembly is eliminated by the simultaneous electromagnetically-driven movement of an equivalent mass in the opposite direction.
• the movement of the equivalent mass need not be in exactly the opposite direction provided that its movement has a component in the opposite direction.
• An alternative solution to this problem is an energy converter consisting of a rotating impeller which does not undergo a linear translation where change in fluid flow direction is achieved by means of an axially reciprocating or rotating valve assembly.
• An energy converter 24 comprises a generally tubular housing 25 which has a first cylindrical connection 26 leading to a cuff (not shown) such as one depicted in Figure 1 and a second cylindrical connection 27 leading to a fluid reservoir (not shown).
• the tubular housing is sealed at either end.
• an axially slidable tubular valve assembly 28 which has a first aperture 29 aligned with the first cylindrical connection 26.
• second and third apertures 30, 31 located in the tubular valve assembly 28.
• the second and third apertures 30, 31 are axially aligned and separated such that by moving the tubular valve assembly 28 axially, either the second 30 or the third 31 , but not both (or at least, not both over their entire widths), is aligned with the second cylindrical connection 27.
• a tubular manifold 32 which is located coaxially with respect to the tubular valve assembly 28 and the tubular housing 25.
• the manifold 32 defines a substantially cylindrical interior 33.
• a deflation inlet 34 and an inflation outlet 35 which are both aligned with the first cylindrical connection 26.
• the first aperture 29 of the tubular valve assembly is sized and located such that it may be aligned either with the deflation inlet 34 or the inflation outlet 35 but not both (or at least, not both over their entire widths).
• an inflation inlet 36 and a deflation outlet 37 aligned with the second cylindrical connection 27.
• the second and third apertures 30, 31 are sized and located such that when the first aperture 29 is aligned with the deflation inlet 34 then the third aperture 31 is aligned with the deflation outlet 37 but the inflation inl ⁇ U36 is blocked by the valve assembly 28. If, on the other hand, the first aperture 29 is aligned with the inflation outlet 35 then the second aperture 30 is aligned with the inflation inlet 36 and the tubular valve assembly 28 blocks the deflation outlet 37.
• a drive fluid is provided in the enclosure defined by the cuff, the energy converter 24 and the fluid reservoir.
• centrifugal impeller 38 which draws in drive fluid at an axial inlet 39 and ejects it at the rim 40 of the impeller 38.
• the rim 40 of the impeller 38 is aligned with both the deflation outlet 37, on one side, and the inflation outlet 35, on the other side.
• the bladder of the cuff is deflated, with the tubular valve assembly in a first position which is shown in Figure 22.
• the impeller 38 draws in drive fluid via the first cylindrical connection 26, through the first aperture 29 and the deflation inlet 34 into the impeller 38 at the axial inlet 39 from which it is ejected at its rim 40 out through the deflation outlet 37, the third aperture 31 and through the second cylindrical connection 27 to the drive fluid reservoir as shown by the arrows 41.
• the drive fluid is pumped out from the bladders of the cuff and into the drive fluid reservoir.
• the tubular valve assembly 28 is moved axially by an electromagnetic actuator (not shown) to a second position shown in Figure 23, in order to reverse the direction of pumping.
• drive fluid is drawn from the second cylindrical connection 27, through the second aperture 30 and the inflation inlet 36 into the axial inlet 39 of the impeller 38. From there, it is pumped to the rim 40 of the impeller 38 and out through the inflation outlet 35, through the first aperture 29 and out through the first cylindrical connection 26, which leads to the bladder of the cuff as shown by the arrows 42. In this way, the drive fluid is pumped into the bladder of the cuff as shown by the arrows 42 and thus inflates it.
• the tubular valve assembly 28 is subsequently returned to the first position, shown in Figure 22, by the electromagnetic actuator in order to deflate the bladder of the cuff again.
• the tubular valve assembly 28 axially reciprocates in order alternately to deflate and inflate the cuff bladder.
• the energy converter 24 comprises a generally tubular housing 25 with first and second cylindrical connectors 26, 27 on opposing sides thereof. Located coaxially within it is a tubular valve assembly 28 which comprises a first aperture 29 which can be aligned with the first cylindrical connector 26 and second and third apertures 30, 31 which can be aligned with the second cylindrical connector 27.
• the second and third apertures 30, 31 are not axially aligned but are axially and circumferentially offset from one another such that by rotation of the tubular valve assembly 28, either the second 30 or the third aperture 31 is brought into alignment with the second cylindrical connector 27, but not both (or at least not both to their full extent).
• a fourth aperture 43 is provided in the tubular valve assembly 28, on the same side as the first aperture 29.
• the first aperture 29 and the fourth aperture 43 are circumferentially and axially offset from one another so that by rotation of the tubular valve assembly 28, either the first aperture 29 or the fourth aperture 43 is aligned with the first cylindrical connector 26, but not both (or at least not both to their full extent).
• the apertures 29, 30, 31, 43, in the tubular valve assembly 28 are arranged such that when the first aperture 29 is aligned with the deflation inlet 34 then the third aperture 31 is aligned with the deflation outlet 37 but the inflation outlet 35 and the inflation inlet 36 are blocked by the tubular valve assembly 28. Conversely, when the second aperture 30 is aligned with the inflation inlet 36 then the fourth aperture 43 is aligned with the inflation outlet 35 but the deflation inlet 34 and the deflation outlet 37 are blocked by the tubular valve assembly 28.
• a manifold 32 and an impeller 38 as described in the previous embodiment.
• the tubular valve assembly 28 is located in a first position as is shown in Figures 24 to 26.
• the first aperture 29 is aligned with the first cylindrical connector 26 and the deflation inlet 34 and the third aperture 31 is aligned with the deflation outlet 37 and the second cylindrical connector 27.
• the impeller 38 draws in drive fluid through the first cylindrical connector 26, via the first aperture 29 and the deflation inlet 34 into the axial inlet 39.
• the impeller 38 then drives the drive fluid to its rim 40 and thus out of the deflation outlet 37, via the third aperture 31 and out through the second cylindrical connector 27, thus following the course of the arrows 41.
• drive fluid is pumped from the bladder of the cuff which is in fluid communication with the first cylindrical connector 26 and to the reservoir which is in fluid communication with the second cylindrical connector 27.
• the tubular valve assembly 28 is rotated by an electromagnetic actuator (not shown) to a second position as shown in Figures 27 to 29.
• the fourth aperture 43 is aligned with the first cylindrical connector 26 and the inflation outlet 35.
• the second aperture 30, 31 is aligned with the inflation inlet 36 and the second cylindrical connector 27.
• the impeller 38 From there, it is pumped to the rim 40 of the impeller 38 and thus out through the inflation outlet 35, via the fourth aperture 43, as shown by the arrows 42, and out through the first cylindrical connector 26, as shown by the arrows 42, to the bladder of the cuff (not shown).
• the impeller 38 in the second position of the tubular valve assembly 28, the impeller 38 has the effect of pumping the drive fluid to the bladder and thus inflating the bladder.
• tubular valve assembly is rotated back to the first position to effect deflation and the process is repeated.
• the axial 39 inlet of the impeller 38 can be regarded as an "inlet port” and the rim 40 of the impeller 38 can be considered to be an "outlet port”.
• the embodiments of Figures 22 to 29 comprise valve assemblies 28 which are movable from a first position in which the inlet port 39 is in fluid communication with the inflatable bladder of the cuff to a second position in which the outlet port 40 is in fluid communication with the inflatable bladder.
• a potentially dangerous failure mode of a blood circulation assistance device 1 of the present invention is one where the aorta 6 remains obstructed for the entire cardiac cycle resulting in an elevated left ventricular stroke work.
• the embodiments of the present invention whose pump comprises an impeller rotatable about an axis which is axially movable to effect reversal of pumping direction (as described in detail in WO-A- 02/24254) have the advantage of an inherent failsafe characteristic. If the impeller of the energy converter 2 stops rotating then the aortic pressure will drive the hydraulic fluid from the inflatable elements 4, 5 into the energy converter reservoir.
• a sensor which detects the axial location of the impeller assembly.
• a control mechanism is provided which receives input from the sensor. In response to the sensor detecting that the axial movement of the impeller has failed with the impeller, in the position causing the inflatable elements 4, 5 to fill, the control mechanism shuts down power to the impeller.
• this sensor is a Hall-effect sensor which detects the proximity of one or more of the magnets of the impeller assembly.
• FIG. 21 an embodiment of the present invention is shown where the periaortic cuff 3 is secured around the descending aorta 6 but the energy converter 2 is located within the peritoneal or a pre-peritoneal cavity with a hydraulic tube 23 linking it to the implanted peri-aortic cuff 3.
• the energy converter 2 is located extracorporeal ⁇ .
• the peri-aortic jacket 3 is located around the descending thoracic aorta 6. However, in other embodiments the. peri-aortic jacket 3 is placed in alternative systemic arterial locations.
• this driveline comprises redundant electrical cables to ensure continuity of power delivery in the event of individual cable failure. Cables with a high tensile strength and high flexibility are provided such as multifilament cadmium/copper types.
• the driveline also has a textured outer sleeve in order to promote tissue ingrowth to minimise the infection risk.
• the percutaneous driveline in particular embodiments, includes conductors to allow the operator to adjust the operating characteristics of the blood circulation assistance device, namely the counterpulsator, and to convey the heartbeat signal, alarms and data to an external controller/monitor.
• TETS ⁇ Transcutaneous Inductive Coupling System
• the TETS system also comprises, in some embodiments, a telemetric link to allow the operator to adjust the operating 0 characteristics of the counterpulsator, and to convey the heartbeat signal, alarms and data to an external controller/monitor.
• the blood circulation assistance device 1 comprises an electronic controller which receives electrical power either from the percutaneous driveline or a TETS system.
• the controller comprises a means of detecting the heartbeat to ensure that the operation of the device is synchronised appropriately. Synchronisation of the CL operation of the device with the heartbeat may entail electrocardiogram (ECG) detection or the use of an alternative method such as a position or pressure sensor or sensors or an accelerometer.
• ECG electrocardiogram
• the arterial pulse wave may be used for triggering. If the heartbeat detection entails ECG monitoring, at least one of the ECG electrodes are, in some embodiments, integrated in the implanted components. In some embodiments the ECG 5 detector has integral defibrillation protection.
• the controller regulates the function of the energy converter 2.
• the controller may comprise alarms (either audible or vibrational) to alert the patient to a malfunction.
• the controller may have a data logging facility. If the system comprises a percutaneous 0 driveline, the controller may be intra- or extracorporeal. If the controller is intracorporeal, it may be integrated with either the secondary coil of the TETS system or the energy converter/peri-aortic jacket assembly. Implanted battery
• an implantable rechargeable battery is provided if, for example, patients wish to take a bath when they have a high dependence on the uninterrupted function of the counterpulsator.
• the peri-aortic jacket 3 comprises a single inflatable element 4.
• the aorta is compressed between the inflatable element and a rigid or semi rigid opposing surface which may be part of the peri-aortic jacket 3.
• peri-aortic jacket was used as illustrated in Fig 18. Two cuff variants with inflatable elements of 5 and 9 cm length were selected. It was found that for both 5 and 9 cm variants, the diastolic pressure augmentation was comparable to the intra-aortic balloon (see Figure 19). The afterload reduction was found to be comparable, if not better, than the intra-aortic balloon. For example, the peri-aortic jacket of 5 cm length decreased end-diastolic pressure by 10 mmHg more than the intra- aortic balloon.
• the exemplary device increased diastolic coronary flow by 50 mL/min and induced a larger flow reversal spike at the diastole/systole transition than observed during unassisted beats, providing compelling evidence of effective left ventricular afterload reduction (see Figure 20).
• the preferred embodiments of the present invention have a dual inflatable element peri-aortic jacket because it has more symmetrical longitudinal aortic stress and strain characteristics and lower peak cyclical excursion of the aortic wall induced by the counterpulsator.
• This has theoretical benefits both in terms of rate of change of aortic blood volumetric flow with respect to time (dQ/dt) which is a crucial determinant of the clinical efficacy of counterpulsation, and in terms of mechanical fatigue of the aorta.
• dQ/dt rate of change of aortic blood volumetric flow with respect to time
• a crucial question was whether there are additional benefits from increasing the number of inflatable elements from two to a greater value. Our investigations suggest otherwise.
• Figures 30-33 are cross sectional views of a model representing the aorta in the centre of a peri-aortic jacket (i.e. where aortic deformation is maximal). These were derived by aerosol-spraying a butyl rubber O-ring of 2 mm and 99.1 mm section and inner diameter, respectively, in its resting 44 (circular) and deformed 45 states on A4 size graph paper. Deformation was achieved by compression with rigid discs 46 of 76.1 mm diameter to simulate the action of the inflatable elements in their inflated state. The aortic/ inflatable element diametric ratio was selected on the basis of what appeared pragmatic both in terms of manufacture and placement of the jacket around the aorta. (Subsequent studies revealed that the aortic deformation is relatively insensitive to this diameter ratio).
• Figure 30 illustrates the simulated aorta in its resting state and compressed to a degree 5 associated with a clearance at the narrowest part of 20% of the resting internal diameter of the aorta.
• This deformation was associated with a cross-sectional area reduction to 51.5 % of the original value.
• the deformed aortic profile was associated with a minimal transsectional radius of curvature of 18 mm (i.e. 36.3% of the rest radius). It was concluded that in this experiment the minimal transsectional radius of curvature was 0 underestimated because the effect of lumenal pressure on the aortic wall profile was neglected. As a consequence, a more elliptical aortic curvature was generated in the model at the positions furthest from contact with the inflatable elements than would have been observed in vivo.
• the O-ring was constrained with bars 47 in an axis perpendicular to the action of the inflatable elements (see Figure 31) and this was associated with elevated minimum transsectional radius of curvature values of > 20 mm (i.e. >40.4% of the rest radius). _0 ⁇ ⁇ .
• Figures 32 and 33 illustrate the effect of moving to a three-inflatable element design.
• the lumenal clearance was maintained at 20% of the resting diameter, although the reduction in cross-sectional area was to 41% of the original area, this was achieved at the expense of a reduction in the minimum radius of curvature to 10 mm (i.e. 5 20.2% of the rest radius) with a resultant increase in local aortic stress and strain.
• the minimum radius of curvature was similar to that measured in Figure 31 , the reduction in cross-sectional area was less: only to 54% of the original value.
• a two-inflatable element design is preferable to one comprising three (or more) elements.
• a two-inflatable element design is also associated with the added advantage of being easier to place around the aorta than a three (or more) inflatable element system because it is associated with less interference with other anatomical structures.
• Our fitting studies using clinical computer tomography data substantiate this conclusion.

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