Total artificial heart: Destination therapy

AbioCor 

The AbioCor (http://www.abiomed.com) implantable replacement heart (Fig. 1) was created to be a totally implantable electric replacement system. As such, it differed greatly in design from the Jarvik-7 (aka Symbion, CardioWest) Heart. The goal was to create an artificial heart that would have completely internal components and provide total circulatory control, thereby allowing the patient to resume a level of activity that approached normal. The idea took over 3 decades to develop, with the final iteration of the device, the AbioCor, currently in clinical trial. Early collaboration with O.H. (Bud) Frazier at the Texas Heart Institute in Houston was important for establishing successful in vivo calf experiments in the early and mid-1990s. Later collaboration with Gray and Dowling from the University of Louisville, Kentucky, was instrumental in achieving reproducible favorable results in the late 1990s to 2000. These studies were submitted to the government for clinical trial approval [1], [2], [3], [4], [5], [6], [7], [8], [9]. On January 20, 2001, the FDA announced its approval for an initial feasibility trial of 15 patients to be conducted at five sites in the United States.

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Fig. 1. The AbioCor totally implantable artificial heart. (Courtesy of AbioMed, Inc., Danvers, MA.)

The AbioCor system consists of internal and external components. The internal components include the thoracic unit, the internal battery, the internal controller, and the internal transcutaneous energy transfer (TET) coil. The external components include the console, radiofrequency (RF) communication box, and the external TET coil. Finally, there are the patient-carried electronics, which consist mainly of the external batteries.

Internal components 

The thoracic unit is a 2.4-lb pump made of plastic and titanium (Fig. 1). It contains two blood-pumping sacs, which fill and empty alternately, thereby supplying pulsatile flow to the aorta and pulmonary artery, respectively. The blood-pumping sacs fill and empty by means of a miniaturized centrifugal pump, which rotates at 5000 to 9000 rpm, alternately shuttling hydraulic fluid back and forth from the left and right sides. The blood moves in a unidirectional pattern by way of four plastic (ie, Angioflex) artificial heart valves. The internal controller is a microprocessor unit that controls the pumping function of the heart. The software installed in the unit provides physiologic parameters by which the device operates, including beat rate, motor speeds, and balance between the right and left sides. Signals are interpreted by the internal controller, which adjust flows on a beat-by-beat basis. The internal battery contains rechargeable lithium ion cells that are capable of providing energy to the pump for 15 to 30 minutes, depending upon the age of the battery, its use over time, and the demands of the pump at the settings established. The size and appearance of the internal controller and internal battery are similar to the original implantable cardiac defibrillators (ICD) and are similarly positioned in the subrectus location. The internal TET coil is a disc-shaped object that is positioned in the infraclavicular region. It receives magnetic waves from the external TET coil and transfers energy to power the implanted system and recharge the internal battery. It enables the entire system to be completely internal with no need for skin-piercing cables.

External components 

The hemodynamic waveforms and power readings of the AbioCor are displayed on the laptop-sized console. The console has a display screen that shows the right and left hydraulic pressure tracings on a continuous basis. It also gives information about the pump's output, stroke volume, and motor speed. On the bottom of the console is a keypad and turn-knobs, which allow the device operator to make manual adjustments in beat rate, motor speed, and balance between the left and right sides of the pump. The radiofrequency communication box is a cassette-sized unit that allows for communication between the AbioCor thoracic unit and the console. It transmits information between the two units. The external TET coil is the critical piece of equipment that tranfers energy across the skin to the internal TET coil. It uses a process known as inductive coupling, which serves to eliminate the need for a skin-piercing transcutaneous cable. The magnetic waves travel invisibly across the skin to the underlying internal TET coil. The energy is carried to the internal battery by cables, keeping the system fully charged at all times as long as the internal and external TET coils are coupled. This technology has enabled patients to be powered without the need for skin-piercing cables, thereby reducing the risk of infection as well as promoting a better sense of body image.

Patient-carried electronics 

For patients who are ambulatory, equipment has been designed to be carried by the patient, using a shoulder harness and utility belt. The main component of the patient-carried electronics (PCE) is the external battery. The external battery is an oblong-shaped unit that weighs about 2 lb and provides approximately 1hour of energy.

Indications 

The purpose of the AbioCor initial feasibility trial is to determine the safety and efficacy of the AbioCor total heart replacement device in a subset of end-stage heart failure patients. The inclusion criteria are as follows:

•Irreversible bi-ventricular heart failure with a predicted life expectancy of <30 days

•Failure to respond to maximal medical therapy

•Age>18 years

•Not eligible for heart transplantation

•“Acceptable” surgical risk

•Positive presurgical evaluation tools (AbioScore and AbioFit)

•Intact psychosocial disposition

•Adequate support network

Patients considered for candidacy included those with chronic heart failure conditions that had time to develop dilated cardiomyopathy. Patients suffering from massive acute myocardial infarction may be considered, as well. In prescreening studies conducted by the company and the investigational centers, approximately one third of the adult patients qualified by virtual fit studies using computed tomographic (CT) or magnetic resonance imaging (MRI) to reconstruct the implant in a three-dimensional manner. In general, these were large male subjects whose body surface area was near or above 2.0 m2. The purpose of the AbioFit is to determine the orientation and alignment of the AbioCor in the recipient—every angle is considered to assure that the unit fits within the bony structures of the thoracic cage (Fig. 2). In addition, it is critical that the device does not compress the soft tissue structures in the area, such as the cavae, branch pulmonary arteries, esophagus, and lobar bronchi.

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Fig. 2. AbioFit-a computer generated three-dimensional reconstruction of the thoracic anatomy based on a computer tomographic (CT) or magnetic resonance image (MRI) with virtual implant of the Abiocor artificial heart. (Courtesy of AbioMed, Inc., Danvers, MA.)

Typical patient profiles considered for the trial include: elderly patients who might otherwise qualify for transplantation except for their age; patients with systemic disorders, such as sarcoidosis, amyloidosis, and connective tissue diseases; patients with multiple and advanced stage comorbidities, for instance, diabetes with end-organ dysfunction; patients with elevated and fixed (irreversible) pulmonary hypertension; patients with prior transplantation and rejection (acute or chronic) who are no longer candidates; and patients on biventricular assist devices who are not candidates. The list of possible patients continues to expand as the pool of heart failure continues to grow. Although the size of the current device limits it overall application, a newer and smaller unit is currently development.

Studies 

Scientific studies of the AbioCor have focused mainly on the mechanical features of the pump and the in vivo experiences at the Texas Heart Institute and the Jewish Hospital in Louisville, Kentucky. The clinical trial, which began with the first implant on July 2, 2001, is nearly mid-way in its course. The status of the seven implants to date are summarized in Table 1. The results of the clinical trial were presented in May 2002 by Dowling at the American Association of Thoracic Surgeons in Washington, DC. Thus far, seven patients have been implanted with four of the seven achieving a successful outcome as defined by the parameters of the study (ie, 60-day survival with improved quality of life at that time). Four patients were able to take excursions outside the hospital, two of the four were successfully discharged from the hospital, and one was able to be discharged to home. Thus far, there have been no device failures and no device-related infections. The causes of death have been directly related to stroke, multiple organ failure, bleeding, and thrombosis. The one remaining patient alive is currently at home, having celebrated his 1-year anniversary on September 13, 2001. Design modifications have included the removal of the cage from the atrial cuffs.

Summary 

The AbioCor is a revolutionary device. It is the only device of its kind in the clinical world. The role of the AbioCor is to provide permanent replacement of the failing heart for patients who are not transplant candidates. The clinical trial has been successful in the majority (four of seven) of patients implanted thus far. Eight more patients are required to complete the initial feasibility phase of the trial, after which it remains to be seen if the trial will expand to additional patient populations. The powerful features of the pump have been demonstrated in the extremely sick group of candidates that enrolled in the study. The TET system has worked extremely well. Lastly, the milestone of discharging a patient to home on a completely internal artificial heart has been realized.

The profile of CardioWest is very different from AbioCor. Although both were initially designed as permanent replacement therapies, the CardioWest has been extremely useful as a bridging device, and it may have a broader application in the future. For the CardioWest, the most favorable groups are patients with cardiomyopathy who decompensate while awaiting transplantation. On the other hand, the AbioCor is indicated in nontransplant candidates with biventricular failure who have a less than 30-day predicted survival. Proper patient selection is critical to achieving success with either device.