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Ann Thorac Surg 1996;61:342-346
© 1996 The Society of Thoracic Surgeons
Department of Surgery, The Pennsylvania State University, College of Medicine, The Milton S. Hershey Medical Center, Hershey, Pennsylvania
Abstract
Background. Pneumatic artificial hearts have played an important role in supporting the circulation in patients before cardiac transplantation. Pneumatic hearts have also been used for permanent cardiac replacement, but most agree they have serious limitations.
Methods. Several groups are now developing electric artificial hearts in which electrical energy crosses the skin using a wireless technique. The electrical energy powers a small direct-current motor, which actuates the blood pump.
Results. Important progress in these devices has resulted in animal survival with electric hearts of more than 1 year.
Conclusions. Extensive bench testing and animal testing will be performed before the initial clinical use of these devices will be initiated. One of the early scientific achievements of the 21st century will be the initial use of the electric artificial heart in humans.
A decade ago, information regarding clinical application of the pneumatic heart was a major news item. The Jarvik and other pneumatic hearts were being used as a bridge to transplantation and as a permanent heart replacement (Fig 1
). The two groups of patients were mutually exclusive. Before bridge use, the patient must fit the rather rigid criteria for a heart transplant recipient. In contrast, the patient considered for a permanent artificial heart should not be considered to be a transplant candidate.
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. Many patients with these devices had complications and were not able to undergo transplantation, and the survival rates after transplantation was considerably less than that after conventional transplantation. However, when one considers that this represents an initial experience, the results are certainly acceptable [1].
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) represent a major improvement over those of the early Jarvik experience. Nevertheless, there remains a divided opinion regarding the optimal bridge technique; most investigators believe that the use of ventricular assist devices, rather than the artificial heart, provides a significant advantage when circulatory support is required for a transplant candidate. The pneumatic artificial heart was first used as a permanent device in 1983. The patient, Barney Clark, MD, lived 112 days and the eyes of the nation were upon him. He was able to get out of bed and walk with minimal assistance. He ultimately died of intestinal complications secondary to antibiotic use. The details of this patient's course and the courses of 3 other patients who had permanent artificial hearts implanted have been described in detail in an article in The Journal of the American Medical Association [2].
Many physicians and most lay people were not aware of the fact that the artificial heart had been developed to the point where animals could be kept alive for a fairly long time. The public was amazed that a device of this type could be used in a patient for permanent circulatory support. The New York Times responded to this activity by writing an editorial that called the artificial heart ``Dracula'' of medical technology. In actual fact, there were a number of very useful pieces of information that came from this experience. These clinical experiences showed that the pumps could fit and function satisfactorily in the chest. In fact, there were several patients in whom the pumps functioned for more than a year, and in 1 patient for more than 2 years. The thromboembolic complication rate was significant. The device provided public awareness of the potential of the artificial heart because patients were shown on the newscasts to be doing quite well and to be able to get out of bed and walk.
In our opinion this experience showed the impracticality of a permanent pneumatic heart. There were two sizable tubes that had to cross the chest wall. A rather bulky power unit provided the primary source of energy, as well as alarms and backups and batteries. Accordingly the patient had to be tethered to a large and heavy power console.
Perhaps most important, this experience stimulated the funding for the electric artificial heart. While we were all watching the news and hearing scientific reports about the implants, our legislators saw what was going on. In a country that is known for its advanced technology, including putting a man on the moon, can we have something better than a pneumatic artificial heart? As a result, focused funding was made available. Several groups in the United States are now developing electric hearts specifically designed for clinical use with a minimum 2-year functional life.
A clinically useful permanent artificial heart must allow its user to have free mobility, to lead a comfortable and relatively normal life, and to return to the workplace. The role model devices are the heart valve, the pacemaker, implantable defibrillators, and the cardiac transplant. Pneumatic artificial hearts require bulky external power units and percutaneous tubes. Although the external power units can be reduced in weight (to a possible 10 kg), this reduction in weight is associated with a reduction in backup systems and alarms. The percutaneous tubes are an integral part of the design and pose a constant threat of infection.
A second design option employs an implantable compact electric motor to drive pusher plates or to pressurize a hydraulic fluid. The electrical energy can be passed through the chest with a fine wire that is tunneled some distance under the skin. Alternatively, the energy required for an artificial heart can be transmitted by a wireless technique, inductive coupling, across the chest wall. This technique was used clinically in the 1950s by Glenn and associates [3]. Modern electronics has considerably improved this technique. Accordingly, the 15 to 20 W required can be transferred from a primary (external) coil to a secondary (internal) coil with a 70% efficiency and with no break required in the skin. Using such a system, the patient would have to carry a rechargeable battery pack. About 1 kg of battery would provide energy to the heart for about 2 hours. The availability of higher energy density batteries would reduce the weight of the battery pack.
The ideal design for an implantable heart would allow all components of the electric heart, including the power supply, to be implanted within the chest or abdomen as with a pacemaker. The only technique to accomplish this using an implantable power supply would be the use of a radioactive plutonium heat source, and a thermal or sterling cycle engine to energize the blood pumps. Needless to say, the Three Mile Island nuclear accident and the Chernobyl disaster have relegated this concept to the back burner for the foreseeable future.
Beginning in 1993 funding has been made available until the year 2000 through the National Institutes of Health for the development of an electrically powered artificial heart. Funding has been made available in a 3-year and a 4-year period, the first being a prototype design phase whereas the second provides funding for mock circulatory loop and animal studies designed to meet stringent Food and Drug Administration requirements. The groups who have obtained funding include ABIOMED in conjunction with the Texas Heart Institute, Nimbus Inc with The Cleveland Clinic Foundation, and our group with the Sarns 3M Healthcare Group.
The general plan of each of these devices [4] is as follows. The artificial heart will be placed in the chest. An implanted brushless direct-current electric motor provides either hydraulic fluid pressure or pusher-plate action to generate the blood pumping. There is an implanted battery to provide temporary power to the device in the event that external power is discontinued. An electronic unit provides control of the device, ensuring that the atrial pressures will be maintained within a physiologic range and providing balance of left and right pumping. The implanted battery provides energy to the device for a period of 30 to 45 minutes in the event that the external coil and battery are unavailable. Accordingly, a patient with one of the devices will be able to take a shower without having any encumbrances at all. Furthermore, the external coil and battery pack can be readily changed.
The Nimbus/Cleveland Clinic Foundation system and the Penn State/3M system both require a compliance chamber. The prototype chamber was developed at The Cleveland Clinic Foundation and consists of a flat elastometric sac in the shape of a flat hot water bottle that is placed between the lung and the chest wall [5]. This type of system requires that there be access to the gas within the device to compensate for gas lost due to diffusion. That is accomplished with an infusion port similar to that used for patients on chemotherapy. The gas space is filled with SF6, an inert gas having a large molecular volume, to impede diffusion. A clever design concept of the ABIOMED/Texas Heart Institute device obviates the need for a separate compliance chamber in this artificial heart.
The ABIOMED/Texas Heart Institute artificial heart uses a high-speed brushless direct-current motor that powers a unidirectional centrifugal pump [6] (Fig 2). The pump runs at a high speed and pressurizes a working or hydraulic fluid. There is an energized rotary valve that moves in such a fashion that the hydraulic fluid first actuates the right ventricle and then the left ventricle. The blood pumps are fabricated of Angioflex, a proprietary type of polyurethane, and use integral Angioflex trileaflet valves. These valves are the result of many years of development and have thin leaflets that function in a very similar fashion to that of a normal aortic or pulmonary valve. This artificial heart uses a unique hydraulic compensation chamber that is positioned within the left atrial chamber, thus obviating the need for a separate compliance chamber. Animal implantation studies are underway. This group now has 1 animal that has lived more than 3 months.
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). This device has several hydraulically actuated spool valves. These valves are passive devices, and they move in accordance with the pressure differential within the hydraulic fluid. By this technique, hydraulic fluid is diverted to actuate the pusher plate of the left or right ventricle. Instead of employing a smooth polyurethane lining, this device has a biolized Hexsyn rubber diaphragm and housing and follows work that has been done at The Cleveland Clinic Foundation for many years. Devices having these linings have been used in animal implantation studies with minimal problems of thromboembolism. Pericardial trileaflet valves are employed. There is a separate gas-filled compliance chamber that is placed between the lung and chest wall. Animal implantations are underway; calves have lived as long as 4 months with this device.
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). The motor rotates four revolutions in one direction and then reverses to counterrotate four revolutions. Pusher plates attached to opposite ends of the threaded shaft directly actuate the left and right ventricles. The blood pumps consist of highly smooth segmented polyurethane sacs with Björk-Shiley Delrin disk inlet and outlet valves of the monostrut type. These valves have performed flawlessly in years of both assist and artificial heart development work; we continue to be very pleased with them. We continue to use the gas-filled compliance chamber similar to that developed by The Cleveland Clinic Foundation group. One of our experimental animals lived for more than 13 months with complete heart replacement with the electric heart of this design.
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What will the research groups be doing between now and the year 2000? Each group must finalize its design, which will take another 12 to 18 months. At that time extensive testing will be performed as required by the Food and Drug Administration. Between 8 and 12 systems will be placed on heavily instrumented mock circulatory loops. The goal is to demonstrate an 80% reliability of the device on the mock circulatory loop. Upon completion of this testing, a series of animal implantation studies will be performed with the finalized design. Artificial hearts will be implanted in approximately 8 animals and should pump for a period of at least 4 months for each animal. Biologic problems such as infection or a non-device-related complication can be excluded. The investigative groups are convinced that this testing is realistic and are eager to proceed. Problems of growth in these calves is significant, and there has also been a problem of calcification within the blood sacks and certain heart valves. Accordingly the realistic period of animal use is probably limited to an average of about 5 to 6 months.
Who will be eligible for these devices when they become available? At the present time, a cardiac transplant is the obvious choice in a patient with end-stage heart disease. The availability of donor hearts is simply not adequate for the need. Approximately 2,000 donor hearts are available where the estimated need is in the range of 10,000 patients per year. Ultimately, we will have permanent assist devices and artificial hearts available on the shelf. The younger patients will get the transplants and the older patients will have the permanent assist devices and the artificial hearts. Patients who cannot be immunosuppressed, for instance diabetics requiring insulin, may be better candidates for mechanical devices. The bridging concept has worked satisfactorily thus far, but clearly it would be better for a patient with end-stage heart disease to be admitted to the hospital and have a permanent device or transplant implanted and go home. When off-the-shelf devices are available, the acutely ill patient will receive one of these rather than a temporary bridge device followed by transplantation, as is currently being performed.
I am frequently asked, ``What is the relative role of the artificial heart versus the long-term ventricular assist device?'' At present, it appears that two thirds of the patients who require mechanical devices will be eligible to have implantable ventricular assist devices. The heart will remain behind as a fail-safe system. In the patients with end-stage heart disease who have thrombus in their left ventricle, thromboembolism will be a major problem. Patients with serious arrhythmias may have problems with right ventricular failure. In these patients it would be much better to remove the diseased ventricles and replace the heart with the mechanical device. Other candidates for heart replacement would include patients with aortic regurgitation, fresh myocardial infarction, left ventricular thrombus, calcified portions of the left ventricle, or a rupture of the ventricular septum that cannot readily be surgically repaired. On the other hand, the ventricular assist device implantation is a simpler procedure and the heart remains as a backup. It is simpler and a more desirable type of support device if the natural heart can be left behind.
Our goal is a patient who is neither confined to a wheelchair nor subjected to transporting a bulky drive unit. However, at the present time we will have to be satisfied with the patient either carrying or wearing a battery pack.
In summary, the results of heart transplantation have been spectacular. There is now and will continue to be an extreme shortage of donor organs. Effective artificial hearts and permanent assist pumps are being developed that will have an important role in the treatment of end-stage heart disease.
Footnotes
Presented at The Third International Conference on Circulatory Support Devices for Severe Cardiac Failure, Pittsburgh, PA, Oct 28-30, 1994.
Address reprint requests to Dr Pierce, Department of Surgery, The Pennsylvania State University, College of Medicine, The Milton S. Hershey Medical Center, 500 University Dr, PO Box 850, Hershey, PA 17033-0850.
References
This article has been cited by other articles:
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  E. M. Hoenicke, R. G. Strange Jr, W. J. Weiss, A. J. Snyder, M. A. Rawhouser, G. Rosenberg, G. A. Prophet, W. E. Pae Jr, B. C. Sun, and W. S. Pierce Modifications in surgical implantation of the Penn State Electric Total Artificial Heart Ann. Thorac. Surg., March 1, 2001; 71(2007): S150 - S155. [Abstract] [Full Text] [PDF]
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R. T.V. Kung and M. Rosenberg Heart Booster: a pericardial support device Ann. Thorac. Surg., August 1, 1999; 68(2): 764 - 767. [Abstract] [Full Text] [PDF]
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H. Mizuhara, T. Koshiji, K. Nishimura, S.-i. Nomoto, K. Matsuda, and T. Ban Evaluation of a compressive-type skeletal muscle pump for cardiac assistance Ann. Thorac. Surg., January 1, 1999; 67(1): 105 - 111. [Abstract] [Full Text] [PDF]
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