Implantable Pneumatic Artificial Hearts

Kevin D. Murray and Don B. Olsen cardiac support and replacement

The first successful artificial heart was designed, fabricated, and eventually implanted into an animal by Kolff at the Cleveland Clinic in 1957 (Akutsu and Kolff, 1957). This initial success sparked a worldwide interest in the possibility of producing a successful replacement for the failing heart. The goal of this early laboratory effort was the development of a permanent replacement for the native heart. However, parallel with these experimental endeavors, routine clinical cardiac surgery was becoming a reality. The introduction of cardiopulmonary bypass (CPB) allowed successful repair of

Biomaterials Science Copyright © 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.

certain congenital cardiac defects and, much later, coronary artery bypass grafting, valve repair and replacement, orthotopic heart transplantation, and, most recently, artificial heart implants.

The advent of successful heart transplantation has provided cardiac replacement for nearly 2400 patients annually in the United States. However, this success falls far short of the minimum annual estímate of 30,000 Americans who need permanent replacement of their irreparable failed hearts. This mismatch of donor hearts and potential recipients provides an undisputed need for another type of long-term myocardial replacement. Congestive heart failure is the only class of heart disease that has continued to increase in incidence (O'Connell and Bristow, 1994). Total artificial hearts (TAHs) and ventricular assist devices (VADs) offer the greatest potential to fill this clinical need by providing permanent cardiac assist or replacement. Interestingly, successful, routine cardiac operative procedures have, on the one hand, limited the population requiring permanent replacement of the native heart, while simultaneously adding a new group of patients who need temporary cardiovascular support, either following an unsuccessful cardiac surgical procedure or to extend the patient's survival while awaiting a donor heart. An extensive review of the clinically used TAHs and VADs was published in 1993 by Rowles et al.

The first clinical use of a TAH was by Cooley in two patients who exhibited myocardial failure and were unweanable after cardiac surgical procedures (Cooley et al., 1981). The goal of these initial TAH implantations was temporary use with eventual orthotopic heart transplantation. Both experiences demonstrated the feasibility of this clinical application of the TAH, despite poor results regarding the patient's ultimate survival.

This pneumatic artificial heart technology was developed in Dr. Kolff's laboratory at the University of Utah. There were numerous designs of pneumatic-powered TAHs. Most notable was the Kwan-Gett heart and the inherent increase in cardiac output in response to an increase in venous return (preload) (Kwan-Gett et al., 1969). Great strides in available materials, design, methods of fabrication, implantation techniques, and noninvasive monitoring of the TAH occurred during the 1970s, culminating in FDA approval in 1981 for human implantation of the Jarvik-7 (J-7; 100-ml stroke volume) artificial heart.

The next clinical use of a TAH was a continuation of the FDA-approved trial of TAH implantation as a permanent replacement in patients with both end-stage heart failure and a contraindication to heart transplantation. The Utah team implanted a permanent TAH in patients beginning with Dr. Barney Clark in 1982 at the University of Utah (DeVries et al., 1984). Similar to Cooley's experience, the procedure was successful in demonstrating that the TAH could replace the native heart with restoration of normal cardiovascular function. The University of Utah's TAH technology was subsequently licensed to Symbion, Inc., for worldwide marketing. The initial IDE was for the continuation of the IDE awarded to the University of Utah for nontransplant candidates only. A significant number of complications developed in all the recipients, which limited survival (ranging from 10 days to 619 days) and resulted in their deaths (Table 1) (Olsen, 1996). This initial clinical experience exposed problems with a pneumati cally powered TAH as a permanent implantable device, although recognized improvements in TAH design, materials, and postoperative patient care offered hope for eliminating these complications.

More recently, the TAH has been used, with increasing success, for temporary cardiac support in the role of a bridge to orthotopic heart transplantation. Despite this clinical success, the one limitation of the TAH, when used as a stopgap measure, is the need for removal of the native heart, necessitating either permanent support with the mechanical heart or the subsequent performance of transplantation. This shortcoming prompted the development, beginning in the 1970s, of VADs. The goal of VAD development was (as the name implies) the augmentation of ventricular performance rather than replacement of the native heart. A significant difference of the VAD system in comparison to the TAH was the potential to remove the VAD system if native myocardial function recovered adequately enough to sustain normal hemodynamics. The advantage of retaining the recipient's heart also contributed to the VAD's major limitation—fit. Since it was not replacing the native heart, there was no intrathoracic anatomic site available where it could be implanted and not interfere with adjacent organs and their function. Several approaches to this problem have included extracorporeal placement, intravascular placement, insertion between muscle layers of the abdominal wall, and overall downsizing of the VADs in an effort to minimize interference with adjacent organs when placed intracorpo-really.

Implantable blood pumps, whether TAHs or VADs, have evolved significantly over the past 2 decades. Improvements in design, materials, fabrication, control, monitoring, and patient selection have all contributed to their clinical success. However, further refinements of currently available devices and the development of improved blood pump concepts are necessary in order to produce the perfect device. There are four questions that beg answers when discussing the future of implantable TAHs and VADs: (1) Are there patients who would benefit from implantable blood pumps? (2) Can implantable, reliable blood pumps restore normal cardiovascular hemodynamics? (3) Can highly reliable implantable blood pumps be designed which cause minimal (or no) complications for the recipient? And finally, (4) can society afford the cost (i.e., development and clinical use) of these devices? The first three questions can be answered "yes," but the answer to the fourth, and potentially the most challenging, is undecided at this time. One report strongly suggests that society can afford the technology when all the factors are considered (Poirier, 1991). It would be regrettable, but the future of the implantable permanent TAH and VAD may very well be determined by financial considerations rather than patient need or laboratory development.

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