AU8328487A

AU8328487A - Implantable biventricular circulatory support system

Info

Publication number: AU8328487A
Application number: AU83284/87A
Authority: AU
Inventors: Michael J. Conley; Jal Sohrab Jassawalla; Phillip Jay Miller
Original assignee: Novacor Medical Corp
Current assignee: Novacor Medical Corp
Priority date: 1986-11-14
Filing date: 1987-11-12
Publication date: 1988-06-01
Also published as: BR8707532A; EP0293421A1; WO1988003394A1; JPH01501371A; EP0293421A4

Description

I PLANTABLE BIVENTRICULAR CIRCULATORY SUPPORT SYSTEM

This invention relates generally to cardiovascular circulatory systems. More particularly, the invention relates to a totally implantable circulatory system which provides both left ventricular pumping for systemic circulation and coordinated right ventricular pumping for pulmonary circulation. The system may be used as an assist system to operate along with the patients own heart, or may be used as a total replacement for the patient's ventricular functions.

Various types of prosthetic devices which may be implanted in patients to provide or assist cardiac function are well known. Total heart implants, designed to completely replace a surgically removed heart, have been employed with some success and widely publicized.

A problem with respect to currently known implanted devices which totally replace the heart is that the power requirements of such devices and the limited volume of the thoracic cavity make it impractical or impossible to provide totally implantable power support for the device. Accordingly, transcutaneous pneumatic power lines have typically been employed for operating such devices. The necessity for the patient to remain tethered to a relatively large external device, together with the problems attendant to permanent transcutaneous power lines, lend significant undesirable attributes to the employment of devices of this type. Since the left ventricle of the human heart typically supports about 85% of the total heart pumping load, it has been proposed to utilize a device which is limited to supporting left ventricular function. One type of such a device is shown and described in United States Patent No. 4,565,497, issued January 21, 1986 and assigned to the assignee of the present invention. The pump described in the foregoing patent is implanted in a patient in the abdominal cavity and is connected such that, upon contraction or systole of the left ventricle, it receives blood therefrom and thereby fills. Upon cessation of systole, the pump contracts to expel its contents into the arterial system of the patient. As such, the pump takes over the load of the ventricle from which it receives blood, either partially or completely, thereby relieving the ventricle.

A major advantage of the type of pump just described is in its relatively low power requirements. Thus, the device is powered through a totally implantable power supply containing rechargeable batteries, which can be recharged by power transmitted transcutaneously through a so-called belt-skin transformer system. Such a system is described in U.S. Patent No. 4,143,661, assigned to the assignee of the present invention. Although the left ventricular assist system approach described above has been successful in assuming up to the full load imposed on the left ventricle, the right ventricle remains only indirectly assisted (only under circumstances of normal pulmonary ventricular resistance) . This may be satisfactory in situations in which the right ventricle is not significantly impaired. However, where right ventricular function is significantly impaired along with the left ventricle function, the installation of a similar assist system for the right ventricle is very difficult or impossible due to limitations in body cavity volume.

It is an object of the present invention to provide an improved circulatory support system which is biventricular, namely, capable of supporting the load for both left and right ventricles of a human patient. Another object of the invention is to provide an improved biventricular circulatory support system, for either assisting or totally replacing the heart, which may be driven from a fully implantable power supply system.

Another object of the invention is to provide an improved biventricular circulatory support system which has relatively low power requirements.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIGURE 1 is a full section view of a biventricular circulatory system constructed in accordance with the invention at the start of left ventricular ejection and right ventricular filling; FIGURE 2 is a full section view similar to FIGURE 1, showing the system at the. start of left ventricular filling and right ventricular ejection; FIGURE 3 is a partial cross sectional view taken from the right end of FIGURE 2;

FIGURE 4 is a plot of ejection force vs. displacement for the left ventricular potion and the right ventricular portion of the device of FIGURES 1-3 without magnetic alteration of the right ventricular portion; and

FIGURE 5 is a plot similar to FIGURE 4 but illustrating the conditions with magnetic alteration of the right ventricular portion. Very generally, the biventricular circulatory support system of the invention includes first pumping means for assuming at least a part of the pumping load of the patient's left ventricle. The first pumping means is adapted for connection to the systemic arterial circulation of the patient and is responsive to electrical signals applied thereto to contract and pump blood. A second pumping means for assuming at least a part of the pumping load of the patient's right ventricle is also implanted. The second pumping means is adapted for connection to the pulmonary arterial circulation of the patient. A control system applies electrical signals to the first pumping means. The first and second pumping means are encapsulated in a common volume such that contraction of the first pumping means allows expansion of the second pumping means and such that expansion of the first pumping means allows contraction of the second pumping means.

Referring now more particularly to FIGURES 1-3, a blood pump in the form of a biventricular assist device is depicted. The device is adapted for implantation within the body (not shown) of a patient. The device is preferably of the type shown and described in U.S. Patent No. 4,565,497. The system also includes an implanted module, not shown, that includes electronic power shaping and control components and a standby storage battery.

The system is preferably powered and charged via a belt skin transformer with associated implanted and external power supplies, controllers, and other components, not shown. Such a transformer and other elements may be constructed in accordance with those described in U.S. Patent No. 4,143,661 and U.S. Patent Application Serial No. 757,786, filed July 22, 1985. In addition to providing power, the transformer can be used to communicate data and reprogram internal elements.

The system includes a pump 10, which is for the purpose of supporting left ventricular circulation (systemic) , and a pump 110 for supporting right ventricular circulation (pulmonary) , both of which are to be implanted in a human patient. The device 10 includes an enclosure or sac 11 defining a pumping chamber 13. Opposed pusher plates 15, 17 are disposed on opposites sides of the enclosure 11 and in contact therewith. Movement of the plates toward one another acts to compress the flexible enclosure and force the ⁵ contents of the chamber out through a suitable outlet duct 16. Movement of the plates away from one another acts to permit the Chamber 13 to fill through an inlet duct 18. Greater detail of this pump chamber configuration is given in the U.S. Patent No. ¹⁰ 4,557,673. An annular support 19 surrounds the flexible enclosure 11 to position it with respect to the remaining portions of the pump, including the actuator mechanism.

In the illustrated embodiment of the -¹-⁵ invention, a pair of opposed beam springs 27 and 33 are used. Opposed posts 21 and 23, extend from the plates 15 and 17 respectively. The post 21 is pivotlly connected to the end of the beam spring 27 by a pin 29. Similarly, the post 23 is pivotally connected to the end of the beam spring 33 by a pivot pin 35.

The end of beam spring 27 opposite the post 21 is secured to a support 39. Similarly, the end of the beam spring 33 opposite the post 23 is secured to a support 45. The springs 27 and 33 extend through 5 slots, not shown, formed in the solenoid assemblies.

Each of the supports 39 and 45, respectively, is provided with a pair of arms 49 and 51, respectively, extending therefrom coextensively with the corresponding beam springs 27 and 33. A projection 0 or preload stop 53 is provided on the spring 27 to engage the free end of the arm 49. Similarly a preload stop 55 is provided on the spring 33 to engage the free end of the arm 51. For reasons which will be explained subsequently, the beam springs 27 and 33 lie in a 5 plane such that the engaging points of the preload stops 53 and 55 project beyond that plane and. accordingly, preload the beam springs 27 and 33 in bending stress. The result is that each of the beam springs 27 and 33 is always stressed in bending by a minimum amount provided by the preload of the preload stops 53 and 55.

Each of the supports 39 and 45 is mounted to a frame 61 for pivotal movement about an axis through a pivot pin 57 and 59, respectively. Thus, as the support 39 pivots on the pin 57, so likewise does the adjacent section of the beam springs 27 moves pivotally about the axis. Similarly, as the support 45 pivots on the axis of the pin 59, so likewise the adjacent end of the beam spring 33 pivots about the axis of the pin 59 with the support 45. Each of the pins 57 and 59 is supported by the frame 61 which comprises a portion of the general frame (not shown) of the pump which includes the annular enclosure support 19.

For the purpose of pivoting the beam springs about the axes of the pins 57 and 59, solenoid means are provided. The solenoid means include a pair of solenoid assemblies 63 and 65 mounted, respectively, on supports 39, 45. Supports 39, 45 and attached solenoid assemblies 63, 65, respectively, each forms a unit pivotally mounted on the frame 61 via the pins 57 and 59. Energization of the coils solenoid assemblies by suitable control means, not shown, causes the ends of the solenoid assemblies to be attracted toward each other.

The operation of the actuator mechanism and is more fully described in U.S. Patent No. 4,457,673. FIGURE 1 illustrates the apparatus in a condition in which the pump chamber 13 is full and the solenoid assemblies 63 and 65 are energized. In this condition, the arms 49 and 51 are swung closed to their closest condition, whereas the beam springs 27 and 33 are in the condition of greatest energy storage. In this condition, the preload bias provided to the springs by the arms 49 and 51 engaging the preload stops 53 and 55 removed.

After closure to the position of FIGURE 1, the solenoid assemblies are held there by a relatively small latching current. If additional holding force is needed, a small permanent magnet may be used. The force of the latter may be overcome when necessary by a small reverse current in the solenoid coils. From the condition of FIGURE 1, the natural tendency for the beam springs 27 and 33 to relieve the stressed condition results in the plates 15 and 17 being moved toward each other, thus expelling the contents in the pump chamber 13. At the end of the pump stroke, shown in FIGURE 2, the beam springs have returned to their less stressed condition with the preload stops 53 and 55 abutting the arms 49 and 51. Once this has occurred, the solenoid assemblies are de-energized or unlatched. The apparatus is returned to the sac-full condition as the result of blood filling the sac via the inlet duct 18. The solenoid gap thus increases as the supports 39 and 45 pivot about the pivot pins 57 and 59, respectively, as the plates 15 and 17 push out the ends of the springs 27 and 33. A suitable detector, not shown, is provided to determine the time at which the rate of pump fill drops below a preselected threshold. At this point, a microprocessor, not shown, is programmed to resume the pumping function.

In the case of a cardiac assist application, the inlet duct 18 may be adapted to connect to the left ventricle of the patient's heart. The outlet may be adapted for connection to the patient's systemic circulation, such as by grafting onto the supraceliac aorta. Of course, the possible connecting arrangements will be apparent to those skilled in the art. The connecting conduit may be of any suitable construction such as a woven Dacron (TM) tube, properly protected from kinking or compression, which may extend through the pericardial portion of the patient's diaphragm. Actual connection of the conduit to the left ventricle may be via a suitable cannula (not shown) inserted, via the apex, into the left ventricular cavity. The outflow conduit may be of a suitable material, such as ° woven Dacron TM, and may be connected to the aorta at different locations, such as the supraceliac axis (or other systemic artery) by anastomosis routine to vascular surgery.

In operation as an assist system, the sac 11 fills during ventricular systole by offering low resistance to the outflow from the left ventricle. Synchronized with systole, at, before, or after termination thereof, the pump begins an eject cycle in which it contracts to expel its contents into the 0 patient's circulatory system. Energization signals are provided to the device from the implanted module (not shown) .

The right or pulmonary circulatory function, although requiring approximately the same pumping 5 volume as the left ventricular function, does not have the same pumping load. The present invention makes use of this fact in a way that requires only a 20% increase in the power requirements of the system from what would be required in a system designed solely for left 0 ventricular circulation. Components of the right ventricular circulatory device 110 having function similar to those of the left ventricular circulatory device 10 have been given identical reference numbers in FIGURES 1 through 3, preceded by 1. Thus, the right 5 ventricular circulatory device 110 includes an enclosure sac 111 defining a pumping chamber 113. Opposed pusher plates 115 and 117 are disposed on opposite sides of the enclosure 111 and in contact therewith. Movement of plates toward one another acts to compress the flexible enclosure or sac and force the contents of the chamber out through the outlet duct

116. An annular support 119 surrounds the periphery of the enclosure or 111 to position it with respect to the remaining portions of the pump.

Bias springs 127 and 133 are provided which, in the illustrated embodiment, comprise a single

U-shaped unit mounted on a mounting block 161. The ends of the springs are secured to the pusher plates 115 and 117 by suitable posts 121 and 123 by pivot pins 129 and 135, respectively. The bias of the springs 127 and 133 is such as to be biased toward the closed position, as shown FIGURE 1. With the sac or enclosure 111 in its maximum volume position, shown in FIGURE 2, the beams are pulled towards their open position by a small set of permanent magnet pairs, 136 and 138, respectively. For reasons which will be more fully explained below, a plurality of ejection limit stops 140 (FIGURE 3) are provided on the pusher plates 115 and 117.

The present invention enables use of only a single energy converter or pump drive to drive both left and right ventricular devices. In the illustrated configuration, the right ventricular circulatory device is "slaved" off the left ventriculatory circulatory device by encapsulating both devices in a common shell 71 which provides a common volume. As illustrated in FIGURES 1 and 2, the common shell includes two chambers 73, 75 inter-connected by a tube 77 of suitable length. This is to provide flexibility in the positioning of the two pumping devices within the patient. However, a single chamber would also be within the scope of the invention. In either case, the volume enclosed by the shell within the chamber or chambers and, if present, inter-connecting tube, is filled with a fluid. The fluid may be incompressible or may be partially compressible as will be explained below. In FIGURE 1, the system is illustrated at the start of the left ventricular device eject stroke and the right venticular device fill stroke. The solenoid in the energy converter of the left ventricular device has closed and the springs are exerting an ejecting force on the pusher plates, tending to move the pusher plates toward one another in the direction of the arrows. As blood is expelled from the interior of the sac 11, thereby decreasing the volume occupied by the sac 11 and the blood contained therein, the volume of the space outside of or surrounding the sac 11 increases, with a consequent tendency to decrease the pressure within the volume defined by the encapsulation shell 71. In the .illustrated embodiment of the invention, fluid will be drawn through the inter-connecting tube 77 from the chamber 75 surrounding the right ventricular device 110 into the chamber 73 surrounding the left ventricular device 10.

As the fluid is drawn out of the chamber 75 surrounding the right ventricular device, or as the pressure surrounding the sac 111 of the right ventricular device decreases, the force of the bias springs 127 and 133 is overcome, causing the pusher plates to move apart, allowing the sac to fill. In FIGURE 2, the energy converter has completed its ejection stroke expelling the contents of the left ventricular device sac 11 and resulting in the filling of the right ventricular device sac 111. With the solenoid de-energized, the left ventricular device sac 11 is free to fill. At the same time, the bias springs in the right ventricular device, since they are exerting an ejecting force on the pusher plates, begins -li¬

the ejection stroke of the right ventricular device. As the coupling fluid is drawn from the left ventricular device chamber 73 back to the right ventricular device chamber 75, the left ventricular device fills at the same time the right ventricular device ejects.

In order to reduce the load on the left ventricular device springs at the close of left ventricular device ejection, the small permanent magnet pairs 136 and 138 are utilized on the right ventricular αevice 110 to pull on the right ventricular device springs 127 and 133, reducing their net ejection force. This force is readily overcome by the springs of the right ventricular device once left ventricular device fill begins.

By fluid coupling of the two devices in the manner described and illustrated, the two devices opexate in synchronous counterpulsation with respect to each other, with the energy converter-driven ejection of the left ventricular device also providing the energy temporarily stored in the right ventricular device bias springs. It has been found that an increase in output of the energy converter of the left device of about twenty percent, over that necessary for the left ventricular device alone, will provide more than sufficient energy for the right ventricular device as well.

As is known to those skilled in the art, the output of the right ventricle is somewhat lower than that of the left ventricle due to differing systemic circulatory requirements. This difference is accommodated, in accordance with the invention, by designing a certain amount of compliance into the encapsulation shell 71. This may be done by providing a compliant region or accumulator communicating with the interior of the shell, or by utilizing a shell material which provides the desired degree of compliance. With compliance of the appropriate magnitude, the difference in stroke volume between the left and right devices may be made to match the anticipated differential between the pulmonary and systemic flows.

Since the right ventricular device ejection is spring driven, its ejected volume on any stroke is responsive to both preload and after load in the pulmonary circulation. As the pulmonary artery pressure increases, the residual volume of the right ventricular device will increase and the right ventricular device output will drop accordingly. In the case where the device is used as an assist device, rather than a cardiac replacement device, as the right atrial pressure rises, the compliance of the coupling between the left and right devices allows some increased filling, of the right device sac, with resultant increase in ejected volume. In this manner the output of the right ventricular device automatically adjusts to the physiologic system.

The force/displacement characteristics of the right ventricular device bias springs may be seen by comparing the effective force profiles at each pusher plate as illustrated in FIGURES 4 and 5. It may be seen by comparing the two force profiles, the upper profiles being without the permanent magnets and the lower profile being with the permanent magnets 136, 138, the overall force profile can be tailored to provide optimum pumping action, while at the same time maximizing system efficiency and minimizing spring size and stress level. The presence of the magnets moves the equilibrium point to the right, as viewed in the curves or profiles, to provide more ejection for the left ventricular device.

It should be noted that, as separation between the two ventricular devices in the system is increased, the fluid head effects due to changes in patient orientation may become more pronounced. The effect of head changes may be taken into account in designing the device as required in accordance with the placement of the device within the patient. This can be done by adjusting the bias springs to compensate for some or all of any additional loading caused by fluid head. It is preferred to design for nominal or average fluid head. Variations are readily accommodated by the device itself.

It is preferred that the encapsulation shell be a biocompatible material which is slightly compliant to allow for the slight differentiation between the flows in the left and right ventricular devices. A titanium or fiber reinforced plastic or a relatively rigid polyurethane material are all suitable.

The preferred fluid within the encapsulating shell is a low vicosity silicon oil. However, the fluid could also comprise a gas or a gas mixture designed to balance osmotic diffusion through the blood or pump sacs. Another alternative would be a rigid shell material containing compressible gas. Air, freon, and mixtures thereof are all suitable, as are liquid-gas mixtures.

As mentioned before, the device may be constructed as a ventricular assist system, or may be utilized as a total replacement for the left and right ventricles. When used as a total replacement device, springs are required on both sacs. However, in the event that the device is utilized as an assist system, bias springs may be dispensed within the right ventricular device. In such a case, the effect is that the left ventricular device assumes the entire load of the right ventricle. When used as an assist device, the invention may totally unload the patients ventricles, or may be operated to only partially unload the patients ventricles.

In order to limit inward displacement of the pusher plates on the right ventricular device, the plurality of stops 140 are provided to engage the outer support ring. Of course, other expedients may be employed as appropriate. In addition, although beam springs have been shown for the right ventricular device, other types of springs may be employed, such as coil or torsion springs.

The energy converter of the left ventricular device can function normally with little loss of efficiency while submerged in a fluid, such as silicon oil. Appropriate sizing of the inter-connecting tube between the two device chambers limits losses due to fluid vicosity. Thus, a substantial degree of implant flexibility is possible with the system of the invention as opposed to a system wherein both pumps are ^•directly coupled to a single energy converter. When used as an assist system, control of the right ventricle and left ventricle portions of the system of the invention is responsive to the cardiac cycle and outputs from the right and left ventricles. In this respect, the system as described operates as a pair of demand pumps. Nevertheless, those skilled in the art will be able to appreciate other control modes within the scope of this invention. Although described as connected to the left ventricle, it will be apparent that the input for the device 10 could be connected to the left atrium instead.

It may be seen, therefore, that the system of the invention provides an improved means for biventricular circulatory support. The system combines a pair of mechanical type pumps for left and right ventricular action from a single energy converter. The system is readily implantable within the normal adult human and is capable of being operated by a fully implanted control system.

Various modifications of the invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A biventricular circulatory support system for a human patient, comprising, first pumping means including first sac means adapted for coupling to

⁵ the circulatory system of a patient for assuming at least part of the pumping load of the patient's left ventricle, second pumping means including second sac means adapted for coupling to the circulatory system of a patient for assuming at least part of the pumping

¹⁰ load of the patient's right ventricle, said first pumping means including means for compressing said first sac means from an expanded condition to a compressed condition to expel the contents thereof and to permit said first sac means to fill from the

^■-⁵ compressed condition to the expanded condition, and fluid coupling means coupling said first and second sac means, said fluid coupling means being responsive to compression of said first sac means to effect expansion of said second sac means and being responsive to

2° expansion of said first sac means to allow compression of said second sac means.

2. A system according to Claim 1 wherein said fluid coupling means include encapsulation means enclosing said first and second sac means in a common

²⁵ volume, and including a fluid filling said common volume such that compression of said first sac means allows expansion of said second sac means and such that expansion of said first sac means allows compression of said second sac means.

3° 3. A system according to Claim 2 wherein at least a portion of said encapsulation means is comprised of a compliant material.

4. A system according to Claim 2 wherein said encapsulation means define a first chamber in

35 which first sac means are disposed, a second chamber in which said second sac means are disposed, and at least one flexible tube inter-connecting said first and second chambers.

5. A system according to Claim 2 wherein said fluid comprises a liquid.

6. A system according to Claim 2 wherein said fluid comprises a compressible gas.

7. A system according to Claim 2 wherein said fluid comprises a liquid-gas mixture.

8. A system according to Claim 1 wherein said first pumping means include first spring means arranged for biasing said first sac means to a compressed condition.

9. A system according to Claim 8 including electromagnetic means for holding said first spring means in the expanded condition of said first sac means, said electromagnetic means being energizable to release said first spring means to cause compression of said first sac means.

10. A system according to Claim 1 wherein said second pumping means include spring means arranged for biasing said second sac means to a compressed condition.

11. A system according to Claim 10 wherein said second pumping means include permanent magnet means for pulling said spring means towards the expanded condition of said second sac means.