Artificial heart transplants

John DunningCardiothoracic Surgical Unit, Papworth Hospital, Cambridge, UK

The use of a mechanical device to support a failing heart is one of the greatestchallenges in cardiothoracic practice. Many different approaches are beingconsidered, but they share the use of many advanced engineering principles. Powersupplies and the interface between artificial surfaces and the blood remain areasof difficulty.The accent is moving from console driven devices with drive lines whichmust cross the body wall to reach the pump, towards smaller control packs, withinductive coupling to fully contained pumps. More attention is focused on the use ofaxial pumps hying within the lumena of the great vessels and the ventricles. Despitethe wideheld belief that mechanical pumps must confer survival advantage to therecipients, there has been no prospective study demonstrating any advantage overmedical management of the railing heart. Economic considerations must be takeninto account if the technology is to be available to everyone with heart failure.

Introduction and history

Cardiac transplantation is now established as an effective therapy forpatients who suffer from end-stage heart failure. Equally well establishedis the discrepancy between the number of organs available fortransplantation and the number of patients who would benefit fromthe therapy if there was a limitless supply of organs. Although waitinglists for cardiac transplantation are deliberately restricted, around 25%of patients on such a list may die waiting. Strategies to expand the donorpool are being explored, and there is optimism that alternative organsources may bear fruit in the long term as xenotransplantation becomesa realistic clinical option. However, the use of temporary mechanicalcirculatory assistance has extended the indications for transplantationand such a philosophy dictates the use of left ventricular assist devices

Correspondence fo- (LVAD) or total artificial heart (TAH). As a bridge to transplantation,Mr John Dunning, the use of these devices is now well established, although their impact on

Consultant the overall management of heart failure is small. They do increaseCardiothoracic Surgeon, o v e r a u c o s t of cardiac transplantation and, in some measure, worsen the

Papworth Hospital, . . r '. . ' . .Papworth Everard, donor organ shortage, since patients survive to transplant who would

Cambridge CB3 8RE, UK previously have been considered too sick to receive a heart transplant. In

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Artificial heart transplants

this context, the concept of a totally implantable device, which would bea permanent augmentation or replacement for a failing heart, appearsattractive. Such a device would be an alternative therapy for heart failurewhilst relieving the shortage of suitable allografts.

The first total artificial heart (TAH) was implanted in a dog in 1957,with a survival of 1.5 h1. Subsequently, several centres developedpneumatic devices2 and success was measured m terms of length ofsurvival. Durmg the 1960s, calves were kept alive for a little over a day3.Development was costly and, by the end of 1989, some $264.4 millionhad been spent with only limited success4. Haemolysis was a majorproblem with the earliest devices and it was not until the introduction ofa diaphragm pumping mechamsm in the late 1960s that survival wasprolonged to 2 weeks with reduced cell fragmentation5. Subsequentchanges in the diaphragm materials permitted survival up to 4 months,and the design shapes were changed to conform to human anatomy.

Animal models reached the end of their usefulness as long termsurvival was achieved with periods of more than 200 days in sixcentres world-wide1. Not only did the animals grow substantially sothat the devices were insufficient to maintain their circulation, but theanimals did not provide sensitive enough models to assess neurologicalcomplications6. However the experience with this model did allowmany issues to be carefully evaluated including those of infection,thrombogenicity, control and durability. It became apparent that it wasnot enough to control the intended function of a pump, but it was alsonecessary to monitor the pump's function to ensure it achieved thegoal.

The first permanent TAH was implanted in a man in December 19827

and, prior to this land mark, some 200 implants had been made with thesame device (Jarvik-7) over a period of 12 years. There had been onedeath from mechanical dysfunction of the device, and in patients whohad received the device as a bridge to transplantation, 57% of patientstransplanted survived at least 1 year. Major causes of death were sepsis(38%), multi-organ failure (25%), and post transplant rejection (12%).Within this experience, there were only nine strokes8. The duration ofsuch implants was from a few hours to 620 days6.

Such apparently positive reports from the bridging experience weretempered by daily bulletins on the progress of the first permanentrecipient's clinical condition and complications which included fivestrokes.

Most investigators also reported 'the major limiting factor' as thespread of infection along drive lines to the peri-prosthetic mediastinalspace giving rise to a mediastinitis which of course limited the potentialsuccess of transplantation and the need for immunosuppressive agents.Other groups were concerned by the level of serious post-implant

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Transplantation

bleeding which in one report occurred in over 50% of patients9.Gradually, there was agreement that the major complications of deviceimplantation centred on haemorrhage, thromboembolism, infection andmulti-system organ failure. Finally, in the USA, the Food and DrugAdministration banned the use of the Jarvik-7 in January 1990 and, afterrearrangement of the production and patent rights, the TAH wasgranted an Investigational Device Exemption, so that further carefullycontrolled studies of the device (now renamed the CardioWest C-70®)may be undertaken with respect to bridging.

There have been many other approaches over the years towardscirculatory support using mechanical means. They have encompassedpulsatile and continuous blood pumps. The first clinical applications ofcardiopulmonary bypass were made by Gibbon in 195310, when he usedthe heart-lung machine to enable direct closure of an atrioseptal defect.In 1961, there was the first clinical application of a left ventricular assistdevice using a roller pump, cannulation of the left atrium via a trans-septal puncture, and femoral arterial return11. Diastolic pressureaugmentation was investigated using intra-aortic counterpulsation,resulting in the clinical trial of an intra-aortic balloon pump (IABP) byKantrowitz and colleagues in 196712. In the first series of patients inwhom this device was used, there was such a dramatic reversal ofoutcome in patients with cardiogenic shock that a multi-centre studyfollowed and with confirmed results led to the IABP becoming firmlyestablished in clinical practice such that nearly 80000 devices are nowused annually13.

By the middle of the 1970s, intracorporeal and paracorporeal pulsatileventricular assist devices were being investigated for use both as a'bridge to transplant' and as a 'bridge to recovery' following difficulty inweaning from cardiopulmonary bypass. However, it was not until 1984that the first successful bridges to transplant with patient discharge andlong-term follow-up were achieved14-15.

More recently, attempts have been made to provide total supportwithout the need for thoracotomy using an axial flow device mounted ona catheter16. Although initially successful, the device encountered somedifficulty with fracture of the drive line, but design modification andalteration in the insertion technique have allowed the Hemopump® to bere-launched for clinical use as a left ventricular assist device.

Since 1987, over 200 ventricular assist devices have been usedannually, mostly in association with postcardiotomy cardiogenicshock17. Of the patients supported in this particular setting, aroundhalf are subsequently weaned from the devices, but only about half ofthese patients are subsequently discharged from hospital. Support maybe univentricular or biventricular, and has been for variable periods buton average around 3 days. Additionally systems have been used in the

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Artificial heart transplants

Table 1 Mechanical devices used for circulatory support

Device

ECMO

Bio median

Sams

AbiomodBVS

Homo pump

Pierce-Donachy

Novacor

ThermoCardiosyitems

Duration of support

Short

Short/intermediate

Short/intermediate

Short/intermediate

Short

1 ntefrneajcrto/l oog

Intermediate/long

Intermediate/long

Position

Extra corporeal

Extra co rpo red

Extraco rpo real

Extra corporeal

Internal

Paracorporecd

Internal

Internal

Support

Btventncular

Rt, Lt, Bi

Rt, Lt, Bi

Rt, Lt, Bi

Lt

Rt, Lt, Bi

Lt

Lt

Anhcoagulahon

FuD

Moderate

Moderate

Moderate

Moderate

Low

Low

Low

Application

R

P,B

P,B

P,B

P

P,B

B

B

Application R = resuscitation, P — postcardiotomy, B — bridge to transplant

bridge to transplant experience and this is summarised in Table 1. Inparticular, a number of implantable LVAD systems have been used and,in association with prolonged implant time occasioned by the shortageof donor organs, there have been greater efforts to rehabilitate systemusers into their own homes and families. Limitations in the form ofcontrol consoles and battery power supply size have been addressedresulting in wearable control and monitoring packages with portable,wearable battery packs, enabling independent existence and greatermobility.

Early experience in the field of mechanical assist devices was achievedwithout the perceived need for medical device regulation. However, in1976 in the USA, the Medical Device Amendments to the Food, Drugand Cosmetics Act (Public Law 94-295) were enacted, and were laterfollowed by the Safe Medical Devices Act of 199018-19. These actionswere well intentioned, designed to create a regulatory process whichwould oversee the investigation and marketing of devices in such a waythat they would be safe and effective for patients. However, they haveproved to be cumbersome regulations and costly to the investigators.

Scope of circulatory support

Circulatory support is practised in cardiac centres daily. Every time apatient is placed on cardiopulmonary bypass, a circulatory andrespiratory support device is used. However, the intention is only toprovide support for a relatively short and well denned period whilesurgery is performed. Such support is provided by relatively simplemechanical pumps, either based on a roller-head, in which a tube issqueezed by the passage of a roller along a defined portion of the tube, orby centrifugal pumps in which a rotor is spun inside a conical chamberto provide impulsion to the blood in that chamber. The blood is also

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passed through an oxygenator, now usually of membrane type, to ensureadequate oxygenation for the duration of cardiopulmonary bypass.

Such an arrangement allows total support of a patient's circulationduring a period of cardiac standstill, but is limited in that the pumps arerelatively cumbersome, require central access for cannulation of thegreat vessels, require full heparin anticoagulation, and may be used foronly relatively short periods before the oxygenator requires changing orbefore blood trauma leads to irreversible coagulopathies and multi-system organ failure.

Although, for most patients undergoing cardiac surgery, the heartreadily takes over support of the patient at the end of the procedure,there remains a significant group of patients for whom further temporarymechanical assistance is required while the myocardium recovers fromthe insult caused by surgery. Such patients led to the development of theintra-aortic balloon pump (IABP) which was introduced to clinicalpractice in 196812.

The principle of the IABP is to provide offloading of the left ventricleduring cardiac systole and to augment coronary artery blood flow in thediastolic phase. Although the use of the IABP is essentially limited to theleft side of the heart, it is important to recognise that one of the maincauses of right ventricular failure is left ventricular failure. Therefore,aortic counterpulsation may improve right ventricular function byreducing left atrial filling pressure and the gradient across the pulmonaryvascular bed, thus improving right ventricular subendocardial bloodflow. While the IABP may achieve all these ends, and thus facilitatemyocardial recovery, it will not augment a non-existent cardiac output.

The IABP is now well established in clinical use and models areproduced by a number of different companies. Technological advanceshave enabled the pump to follow rapid and irregular heart rhythms, withreal time response to changes. The balloons have also been improvedallowing more simple insertion techniques using a percutaneous,Seldinger wire femoral artery approach. In turn, this has simplifiedremoval of the balloon when no longer required, and permitted areduction in the number of peripheral vascular injuries and complica-tions.

Although the IABP has proved successful in the management ofcardiogenic shock, it remains limited by its own inability to drive thecirculation. The use of left ventricular assist devices (LVAD) to treat thiscondition has theoretical advantage, since the LVAD has greaterpotential to produce haemodynamic improvement and ventricularunloading than does the IABP. In the quest for an ideal device to treatcardiogenic shock, a novel development led to a pump which couldcombine the pump power of a typical LVAD with the safety andsimplicity of the IABP. The result was the Hemopump® — a catheter

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mounted mechanical circulatory assist device, with a miniaturised axialflow pump, based loosely on the principle of the Archimedes screw.

The pump head consists of an inflow catheter which can be placed intothe left ventricle across the aortic valve, with an outflow tract which lieswithin the lumen of the aortic root. The sheathed screw head rotates atspeeds of up to 28 000 rpm, and delivers flow levels of up to 4.51/min.Since the inflow cannula lies within the left ventricular cavity, assistancewith the device results in significant decompression of the LV, and aconsequent reduction in ventricular stroke work.

The pump is marketed by Medtronic, and is currently available only ina 24F size, which is introduced to the aortic lumen through a vasculargraft sutured to the side of the aorta at median sternotomy. There is analternative pump head which will soon be available for introductionfrom the femoral vessels (size 14F) which will provide flows of up to2.51/min with rotational speeds of up to 45 000 rpm.

Evaluation of the Hemopump® continues as an alternative to extra-corporeal VAD support for patients suffering with potentially reversiblecardiogenic shock. More recently, as less invasive approaches to cardiacsurgery have been sought, it has been suggested that the Hemopump®could be used to provide circulatory support, short of full cardio-pulmonary bypass to permit coronary artery bypass surgery to beperformed20.

The remaining circulatory support systems are more cumbersome thanthose already described and their classification is not standardised. Theymay be described in terms of their position (extracorporeal, para-corporeal, heterotopic, or orthotopic), or their use or nature (resusci-tative, external centrifugal and roller pumps, external pulsatile assistdevices, implantable left ventricular assist systems and total artificialhearts).

Resuscitative systems may be deployed rapidly in an acute setting suchas the catheter lab or the intensive care unit in patients who developcardiogenic shock which is not responsive to pharmacological interven-tion or IABP implantation. Femoro-femoral cannulation is required,either by percutaneous or cutdown techniques. This type of supportusually uses an oxygenater with heat exchanger and a centrifugal pump,and is suitable for periods of up to 24 h21>22. The main disadvantage ofthese systems is that they require continuous heparin anticoagulationand that their biventricular support is incomplete.

External centrifugal and roller pumps have also been used extensivelyto provide ventricular support, with both being used in the setting ofpostcardiotomy support, and centrifugal pumps being used as a bridge totransplantation. Cannulae may be placed to any cardiac chamber toprovide the specific support required. However, a patient's mobility isstrictly limited for the duration of support, often remaining-bed-bound,

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if not ventilated. Results are best if the support is for periods of less than1 week, but there are reports of support for as long as 30 days. Again,anticoagulation remains a severe limitation, as does the duration ofsupport.

As an alternative pump source, external pulsatile devices are nowbeing used. Examples of these include the Pierce-Donachy VAX)developed at Pennsylvania State University, the Symbion VAD, andthe Abiomed (BVS 5000) biventncular support system. Impressiveresults have been obtained with many patients supported for longer than30 days.

Implantable LVADs are important devices since they provide valuableinformation as prototypes for non-tethered, permanent mechanicalsystems which may provide an alternative to cardiac transplantation.The two best known systems are the Novacor LV assist device andThermoCardiosystems LV assist device23-24, which are currently underinvestigation as bridges to transplantation. They are not well suited totemporary implantation for support in the postcardiotomy syndromesince they require cannulation of the left ventricle with excision of asignificant muscle mass. They also will only support the left ventricle.The Novacor is powered by batteries worn on a belt around the patient'swaist, while the ThermoCardiosystems device is available either as apneumatically or electrically powered device. Both devices permitconsiderable mobility after implantation, and successful transplantationhas been reported after as long as 370 days of support with the Novacor.The ThermoCardiosystems device has a textured internal surface for theinterface with blood, so that a pseudo-intimal layer develops whichreduces the risk of thromboembolism, and minimises the need foranticoagulant therapy24.

All of these devices are costly, and there are few hospitals which haveall the options available. Proper device selection is essential, and dependson the circumstances leading to cardiogenic shock.

Current state of development of the total artificial heart

Much of the research effort is centred in the USA and, in 1988, theNHLBI awarded 5-year contracts to four different teams to developlong-term, implantable, total heart substitutes. The contracts demandedthat the recipient of such a system should be capable of reasonablynormal, high quality lifestyle, with system response to the varyingcirculatory demands of daily activity. In addition, there should be nodrive lines crossing the body wall.

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The four different systems25"28 were all based on electrically driven,orthotopically placed, blood pumps which eject blood with pulsatileflow into both pulmonary and systemic circulations. Since the pumps areelectric, they may be powered for short periods by in-dwelling nickel-cadmium batteries, but their main power source is derived in each casefrom a transcutaneous energy transfer system (TETS)29>3°. This consistsof an external coil which is worn next to the skin over a secondaryimplanted coil. The passage of high frequency current through theexternal coil induces a current in the implanted coil which may then beused to power the pump.

Electrical energy is converted to pump action by a variety of differentmechamsms, which are essentially variants of pusher-plates. The pumpdeveloped by Pennsylvania State University and Sarns (PSU/Sarns)consists of a reversing motor connected by screw threads to a plate ateach end. Thus the plates can be made to alternately compress two bloodsacs with reciprocal action. Thus the left sac is compressed as the rightsided sac fills and vice versa. As a result of this, left and right sidedphases alternate. The pump developed by the University of Utah consistsof a reversing axial flow pump which directs hydraulic fluid agamstflexible diaphragms in the two sides of the pump. The Abiomed/TexasHeart Institute pump also uses hydraulic fluid, this time moved against adiaphragm by a unidirectional centrifugal pump. Displacement of thepump diaphragms causes blood to be drawn into or expelled from thepump housing. The Cleveland Clinic Foundation/Nimbus pump alsouses a hydraulic circuit connected to pump pusher plates. There is directvolumetric coupling of the hydraulic circuit and the pump chamber forthe Utah and Abiomed/THI pumps, while this is not the case for the lastmentioned. So, although the intricate details of each pump varies fromthe others, the underlying principles are very similar.

Each pump has two chambers corresponding to left and right sides ofthe heart, and each chamber has an inlet and outlet valve in order todirect blood flow in a forward direction. Commercially available valveshave been used in the PSU/Sarns pump and the Utah pump. However,the Abiomed/THI pump uses trileaflet polyurethane valves in both inletand outlet positions, while the CCF/Nimbus uses bioprosthetic valves inan attempt to reduce thrombogenicity of the device.

Complications associated with mechanical circulatorysupport

All approved pulsatile devices contain prosthetic valves. Mechanicaldevices place greater loads on the prosthetic valve than does the natural

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heart, as was demonstrated so dramatically in the strut failure of themitral valve in the first TAH implant31. The design constraints of paracorporeal devices demand long stiff cannulae with relatively smallexternal diameter. This results in high blood velocities during pumpfilling and ejection, with rapid deceleration of blood at the end of pumpsystole and diastole, reversal of flow through the valve, and initiation ofpressure oscillation in the cannulae analogous to the water hammereffect. The result is increased pressure gradients across the valve therebyreducing its fatigue life32.

An additional damaging effect is produced by cavitation which occursas a result of transient reduction in local pressure entailing the rapidproduction and collapse of vapour-filled bubbles, which create highenergy microjets33.

The most important determinant of cavitation and water hammer isthe rate of rise of transvalvar pressure prior to closure of the prosthesis(dP/dT). Methods to overcome this problem include the use of short,wide-bore cannulae, strategies to reduce dP/dT by decreasing drivepressure or by using a driver which increases force gradually.Additionally, elastic mounts for the valves lower the instantaneouschange in velocity and, therefore, reduce the water hammer pressurepulse. A further strategy which may be used to lower the water hammereffect is to use a valve with high static leak. However, this will result inincreased turbulence and its attendant difficulties34.

Thus the major determinants of valve failure in all mechanical devicesare water hammer effect and cavitation, and multiple strategies are usedto reduce valve prosthesis wear.

A significant complication encountered in patients who requiremechanical circulatory support is bleeding. The incidence varies fromreport to report, but for patients requiring ventricular assist devices ortotal artificial hearts as a bridge to transplantation ranges from 29-53%.Although there has been a gradual reduction in frequency and severity ofthis complication, with attention to implant technique and the reversalof systemic heparinisation after cardiopulmonary bypass, there appearsto be a non-specific coagulopathy in which a specific pattern ofhaemostatic abnormalities has not been identified.

Activation of platelets and white blood cells, the fibrinolytic systemand the complement cascade all result from the contact between bloodand artificial surfaces35. As a result, local thrombus formation,thromboembolic complications, consumptive coagulopathy and exces-sive local or general bleeding may ensue.

Platelet activation occurring at the interface with prosthetic materialrenders platelets dysfunctional and refractory. Under the conditions ofnormal cardiopulmonary bypass, this dysfunction is relatively shortlived and haemostatic function returns to normal within 0.5-1 h.

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However, the effects of continuous contact with prosthetic surfaces aremore complex, von Willebrand factor is absorbed onto the artificialsurface resulting in molecular conformation changes which then interactwith the main platelet receptor. This results, in turn, in changes ofplatelet shape with adherence and release of the chemotactic adenosinediphosphate causing platelet-to-platelet aggregation. As time passes, theprosthetic surfaces become coated with protein deposit, termedpassivation, with a progressive reduction of platelet activation.However, activation does persist for several days, and is often associatedwith platelet counts that are low enough to require multiple trans-fusions.

Since there is interference with the normal coagulation mechanisms,and since there is an extensive interface between blood and prostheticmaterials, it is not surprising that thromboembolic complications are ofconcern in the setting of prolonged circulatory support. Thrombosis maybe related to a number of different factors including the device flowpattern, which may cause turbulence shear stress or stasis, the surfacelinings and the thrombogenicity of blood36-37. Procoagulant conditionsmay also prevail in the presence of infection leading to increasedlikelihood of thrombosis. Furthermore, since the pulsatile LVADs havean asynchronous pattern of ejection with the LV, there is increasedlikelihood of stasis and thrombosis either within the pump, or within thenative ventricular cavity, particularly in association with underlyingatrial or ventricular dysrhythmias38.

Device related infection has been a source of concern with reportedrates of 20-75%. All current systems require transcutaneous cannulae,drive lines or power cables and, therefore, there exists potential for thedevelopment of device related infections. However, other factors are alsoimportant, such as the duration of support, the number of trans-cutaneous portals, and anatomic position of the device, and someinfections are related more to the patients pre-implant state and riskfactors than the device itself.

Long hospitalisation prior to implant, multiple sternotomies, chronicdebilitation from heart failure may all contribute to a high rate ofinfection36'39 and contamination may also occur at the time of implantfrom indwelling devices such as vascular cannulae, endotracheal tubes,intra-aortic balloon pumps and urinary catheters. Thrombi formed inthe devices and their connections may also become colonised withbacteria and present a further source of sepsis40.

In contrast to the TAH experience where mediastmitis was a majorconcern, few patients who have received LVADs have developedmediastinitis and, although the incidence of infectious complicationsmay be increased over standard cardiac surgery, the mortality from thesecomplications is relatively low24. However, infection linked with the

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complications of thromboembolism or renal failure has a far moreserious effect with poor prognosis.

Renal failure has been associated with high mortality for patientsundergoing conventional cardiac surgery, and it has also proved asignificant predictor of mortality in patients with pneumatic VADs41.However, it has been shown that renal function may improve with longterm LVAD support24. Similarly, hepatic and pulmonary dysfunctionhave been shown to improve following long term mechanical support.

Gastrointestinal dysfunction has also been reported with a constella-tion of problems. Upper GI bleeding may occur, perhaps as a result ofthe normal physiological stress response, but aggravated, in part, byanticoagulant regimens. Problems have also been encountered withwound dehiscence, herniation of small bowel through the drive sites,adhesions to drive lines, and reduced appetite due either to compressionof abdominal contents by the device, or more generalised factors. As aresult nutritional support may be necessary early after implantation.

The future

This chapter has examined the current state of artificial heart transplantsin its widest sense, ranging from the circulatory support provided on atemporary basis by the cardiopulmonary bypass circuit, through thesimple supportive measures of the intra-aortic balloon pump, andthrough more complex devices in climcal use for ventricular support, tothe more experimental development of the total artificial heart. The fieldis complex with complementary and competing technology. Complica-tions common to all devices pose major challenges and, as totallyimplantable devices move closer to reality, the challenges of powersources and interface between prosthetic materials and blood must bemet.

The goal for all workers in the field of artificial circulatory support isto develop devices and techniques which will restore cardiovascularfunction of an individual so that he may be rehabilitated into society as afully functional individual. A full range of devices may become availablewith separate indications and applications. Mechanical support mayalready be instituted for short periods allowing recovery of the nativeheart from an acute insult such as myocardial infarction or post surgicaldepression and, in due course, permanent implantation may be possiblefor the chronically failing heart which cannot be treated in any otherway. At present, allotransplantation is the ultimate therapy for thefailing heart, but in the future, when conventional pharmacological andsurgical intervention fails, patients may be considered for mechanical

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cardiac replacement. At that stage, a biological donor (allo- or xeno-transplant) may be available, but if not, a completely implantablemechanical replacement may be considered. This may take the form ofTAH or of VAD. Alternatively, the patient may refuse and contemplatedeath, for it is certain that with this technology the greatest question thatmust be answered is this: just because we can do it, does it necessarilyfollow that we should?

The questions posed by such a philosophy are challenging and thepotential cost implications for health care enormous. The devices mustbe proven to be cost-effective and of at least comparable quality to othertherapies currently available if they are to be accepted into standardclinical practice. Whilst cardiac replacement may become establishedpractice, there will also remain a place for portable, paracorporeal assistdevices which may be rotary or pneumatic. These may provide supportfor a group of patients still to be defined until ventricular recovery isachieved41.

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