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Mechanical and biological cardiac assist devices
SPECIAL LECTURE
Mechanical
and Biological
Cardiac Assist Devices
E. Wolner, MD, and A. Rokitansky, MD
D
UE TO CONSIDERABLE cardiac surgical, medical, and diagnostic progress, operability of patients suffering from severe or end-stage heart diseases has increased during recent years. One important reason for this progress is the clinical availability of mechanical or biological cardiac assist devices, eventually combined with human heart transplantation, offering quick hemodynamic restoration and the chance of a successful life-saving surgical treatment. In principle, hemodynamic restoration can be achieved mechanically by pulsatile (membrane or sack pumps) or nonpulsatile (centrifugal or screw pumps) pumps. In 1965, Spencer described the first clinical use of a left ventricular assist device (LVAD).’ The first clinical total artificial heart (TAH) implantation for bridging to transplantation was performed by Cooley et al in 1969.’ In 1985, Copeland et al bridged a patient who has since fully recovered and returned to work.’ Alternatively, biological cardiac support is possible by the use of a skeletal muscle reinforcing, augmenting, or replacing diseased myocardium. In 1933, Leriche and Fontaine first experimentally demonstrated the feasibility of using the pectoralis muscle to reinforce a myocardial scar after ligation of a coronary artery.4 In 1958, Kantrowitz and McKinnon used a pedicled graft of diaphragm around the mobilized aorta for counterpulsation.’ In 1985, Carpentier and Chachques performed the first successful myocardial reconstruction after resection of a large myocardial fibroma.6 Although the systems are now only under experimental investigation, a biomechanical approach is to use the conditioned autologous muscle to power an artificial ventricular assist device (VAD), which is then completely implantable. In 1987, Chiu et al’ and Acker et als experimentally demonstrated sufficient aortic counterpulsation with such muscle-powered, single- or double-chamber balloon assists. VENTRICULAR
ASSIST SYSTEMS
Mechanical Systems
Artificial blood pump systems consist of three main components: the blood pump for pulsatile or continuous blood flow, the driving unit, and the power supply unit. All available systems for clinical use are still in the nonimplanted or only partially implanted configuration. This means that at least the power supply and control unit are outside the body and are connected to the intracorporeal pump by a transcutaneous line.Y Considering the pumping mode, sufficient cardiac support was clinically achieved with centrifugal or screw
pumps, producing a continuous blood flow, as well as with membrane or sack pumps, producing a pulsatile blood flow up to 10 L/min. All TAH systems used were pulsatile and gas-driven, orthotopically implanted polyurethane membrane pumps with an exterior driving engine. Fully implantable systems for permanent use are under development, and could be proper alternatives for heart transplantation. Biological Systems
Skeletal muscle, such as the latissimus dorsi, the diaphragm, or an intercostal muscle can be used for reinforcing, augmenting, or replacing the diseased myocardium. One clinically used method of muscular cardiomyoplasty is performed by wrapping the left latissimus dorsi skeletal muscle around the left part of the heart by way of a partial resection of the second rib. This surgical technique is called dynamic cardiomyoplasty.6 Via intramuscular electrodes, the skeletal muscle becomes conditioned with a change of fatigue-sensitive glycolytic to fatigue-resistant oxidative fibers, which takes at least 6 weeks. Dynamic cardiomyoplasty may lead to a more forceful systolic contraction, giving the chamber the proper size by replacement of scarred myocardium or by reinforcing a dilated thin left ventricle. Biomechanical Systems
Alternatively, the conditioned skeletal muscle can be used experimentally for driving a mechanical assist device. Such a biomechanical heart support was achieved experimentally by three different system concepts”.‘“: (1) the muscle is wrapped around a blood-filled polyurethane ventricle forming a pouch; (2) the muscle is wrapped around a valve-containing plastic tube; and (3) the muscle powers a fluid-filled chamber, which is connected to the blood-filled pumping chamber. Experimentally, eg, by counterpulsation, each system seems capable of increasing the aortic blood flow.
From the Second Surgical Clinic, University of Vienna, Kenna, Austria. Presented at the Fifth Annual Meeting of the European Association of Cardiothoracic Anaesthesiologists, Vienna, Austria, May 13-16, 1990. Address reprint requests to E. Wolner, MD, Second Surgical Clinic, University of Vienna, Spitalgasse 23, A-1090 Vienna, Austria. Copyright 0 1991 by W B. Saunders Company 1053- 077Ol91 lO504-0019$03.00l0
Journalof Cardiothoracic and Vascular Anesthesia, Vol 5, No 4 (August), 1991:
pp405-408
405
406
WOLNER AND ROKITANSKY
Indication for Cardiac Assist Devices
Use of a mechanical cardiac assist device is principally indicated in a biologically young ( < 50 years) and infectionfret patient developing acute cardiac deterioration (cardiac index of about 1.8 to 2 L/min/m’, a mean blood pressure below 60 mm Hg, pulmonary artery wedge pressures greater than 25 mm Hg, and a urine output less than 20 mL/h), provided an intraarterial balloon pump and cardiac drug treatment fail to improve the patient’s condition.“,” In many cases, even biventricular failure can be treated with a single left ventricular assist and elevated pulmonary artery pressures may be reversible.‘3.‘4 Sufficient function as well as easier implantation favor the use of VADs as the first step. Considering that the original working heart is left in place, another advantage of VADs is the possibility of resuscitation in case of a device failure. TAH implantation is indicated in transplant candidates, when the original heart should be removed for clinical reasons, eg, thrombotic material causing embolism, rupture of the myocardial wall, severe valve dysfunction, or intracardiac shunts. Due to conditioning of the skeletal muscle performing sufficient cardiac support, which takes at least 6 weeks, acute cardiac deterioration may be treated only by mechanical devices. Biological or, eventually, biomechanical assist methods may reach additional importance for long-term cardiac support as an alternative to heart transplantation.
two-stage transplantation is better in patients with acute cardiac deterioration than in patients with chronic detcrioration.‘& Clinical experience with patients who had acutely rejected a previously transplanted heart and were subscquently bridged is particularly disappointing.“’ Permanent Support
Although survival for 622 days is possible with the TAH, it must be kept in mind that due to complications, permanent mechanical heart replacement is still problematic. During an implantation time between 227 and 622 days, all patients developed thromboembolism leading to major or minor strokes. One patient who was on TAH for 112 days developed infection. Thus, thromboembolism and infection are significant limiting factors in the use of artificial hearts as long-term cardiac replacement.‘” On the contrary, the cardiomyoplasty technique using a skeletal muscle reinforcing or replacing the myocardial muscle may lead to long-term biological cardiac support, as was shown experimentally and clinically in 8 cases by Chachques et al” and in 4 cases by Magovern et al.” In a follow-up period of up to 42 months, postoperative heart scans and echocardiographic and hemodynamic examinations showed improvement of ventricular function. A 10% to 15% increase of the ejection fraction was achieved.2’ DISCUSSION
RESULTS
Temporary Support Leading to Cardiac Function Restoration
Quick hemodynamic support by cardiac assist systems provides for multiple organ failure prevention and cardiac decompression with increased coronary artery circulation, which may consequently lead to cardiac functional improvement. Between 1985 and July 1989, 521 patients with cardiogenie shock were supported with a VAD; in 229 cases cardiac function improved and they could be weaned. One hundred twenty-six patients (26%) were discharged.” Mainly by considering the atria1 pressure and the ejection fraction, weaning criteria are determined with the pump on/off procedure.lh Temporary Support Leading to Heart Transplantation
Better results were achieved in the same period by using a mechanical pump system for bridging to transplantation. Due to the sharply increased number of human heart transplantations, a now well-known shortage of donor hearts exists, and the situation seems to have worsened over the last year. As has been demonstrated by clinically performed VAD and TAH implantations, the systems used are sufficient to provide circulatory support in the course of a two-stage heart transplantation. To the present time, approximately 97% of all TAHs and 30% of all VADs have been used for bridging to transplantation. Thirty-one percent of 187 TAH-bridged patients and 47% of the 169 VAD-bridged patients are still alive.“.” Although pulsatile VAD bridging was used in 73% of the cases reported, clinical experience has shown that centrifugal VAD also leads to a patient discharge rate of 44%. Outcome of a
Mechanical Systems
Complications following mechanical assist device application have mainly consisted of bleeding problems, infection, hemolysis, thrombus formation, thromboembolism, immunologic stimulation, and device dysfunction. Bleeding. In the early postoperative period, bleeding with an incidence of 30% to 60% is a significant problem. Because of hemorrhage, 40% of 97 Jarvik 7170 TAH implants required reoperation.” In 1988, Farrar et al reported 29 patients who received VADs as a bridge to transplantation. Twenty-one underwent a two-stage transplantation. About 40% of these VAD patients had severe bleeding problems.*’ The bleeding hazard in patients on artificial circulation is increased by the special anastomoses and the needed anticoagulation as well as by the reduced number of platelets during the early postoperative period. The surgical anastomoses between organic tissue and artificial materials are susceptible to insufficiency. This is caused by the sewn link between tissue and plastic being under permanent increased mechanical stress due to the movements of the pump. Additionally, in order to avoid thrombus formation, physiological sealing of these anastomoses is suppressed by drug-induced anticoagulation. Surgical hemostasis by close-running sutures should achieve optimal tightness. Infection. Due to transcutaneous driving lines, catheters, intracorporeal artificial surfaces, surrounding damaged tissue, and organ failure, the risk of infection during TAH pumping is high (30% to 40%). These transplantationpreventing infections are highly resistant to antibiotics and persist until the foreign body is removed.24
407
CARDIAC ASSIST DEVICES
Thirty-six percent of 106 Jarvik 7/70 TAH patients developed infections.” Most of the patients on TAH, who could not undergo transplantation, suffered from incurable infections.” Considering the transplantation-preventing effect and the often seen incurability of infections, aseptic and antiseptic measures during bridging are most important. Reduction of transcutaneous lines offered by completely implantable systems seems to be the future goal of development. Hemolysis. Due to a reduced driving gas pressure, hemolysis rate has decreased with the use of modem driving units for pulsatile pumping. More frequently, hemolysis is seen in centrifugal and screw pumps, eg, in the 20 Hemopump (Nimbus Medical Inc, California) bridged cases free plasma hemoglobin was between 10 and 30 mgldL. Thrombus formation, thromboembolism, and anticoagulation. Introduction of artificial surfaces into the blood-
stream induces platelet, complement, and coagulation cascade activation. These hemostatic mechanisms are triggered by the artificial pumps when blood flow changes and shear stress is inevitably present. In relation to the pumping duration, nonpulsatile centrifugal pumps are much more susceptible to thrombus formation than pulsatile pumps. Use of polyurethanes with high biocompatibility and hemodynamically improved designs diminished thrombus formation, but medical anticoagulation as well as antiaggregation continue to be necessary and the thromboembolic problem still exists. Risks of thrombus formation leading to transient ischemic attacks during TAH bridging is around ll%.” Sites of thrombus formation were mainly the atria1 artificial cuffs, the valve rings, or the driving shaft of the centrifugal pump. Thrombus formation in the ventricle chambers is an infrequent event. Blood clot formation in the ventricles seems to be caused by microfractures or pitlike lesions of the artificial plastic surface that occur after intensive mechanical stress. The future goal is to develop thromboresistant or stress-resistant plastic materials. Aside from blood contact with artificial surfaces, slow blood flow is an additional factor for thrombus formation. For thrombus prevention in centrifugal pumps, a flow of less than 2 Wmin should be avoided.16 In 1986, Copeland et al hypothesized that increasing the heart rate would reduce thrombus formation. A 57% reduction of thrombus formation was observed comparing the small Jarvik 7/70 with the Jarvik 7/100 device. Additionally, an interrelation between thrombotic events and infection seems to be evident. In the pathophysiologic view, bacteremia may trigger the hemostatic cascade mechanisms; in contrast, small thrombotic material represents optimal conditions for bacterial growth. Immunologic alterations. Stimulation of the immuno-
logic system during artificial circulation may be caused by different mechanisms such as damaged blood cells, contact between blood and the artificial surface, administration of blood and blood products, and clinical or subclinical infections. Two patients who had rejected their transplant and then received an artificial heart for more than 240 days developed very active immunorejection profiles.” Therefore, it may be concluded that long-term bridging in intensive care unit conditions may lead to an additional stimulation of the immunologic system. Device dysfunction. Reliability of the available devices for clinical use is very high. Only the valves, fixed in the stiff ventricle plastic housing, as well as the percutaneous driving shaft of the intracardial Hemopump, showed increased susceptibility for material breakage. PaShould bridging to transplantation be recommended? tients having undergone a two-stage transplantation now have a long-term survival rate of between 30% and 40% and resemble high-risk transplant candidates, whereas onestage transplanted patients have a survival rate between 70% and 80%. Considering the reduced availability of donor hearts, two-stage transplantation may lead to a waste of donor organs.26 In contrast, Joyce et al believe that as many as 10% of currently harvested hearts are wasted because of the unavailability of a suitable recipient.*’ Considering the bridging indications for clinical TAH or VAD application and the outcome of the reported cases, use of mechanical circulatory support should be restricted to biologically young patients with acute cardiac deterioration, having the best chances of survival. Biological Systems
Because of conditioning of the skeletal muscle, the patient does not fully benefit in the first postoperative weeks.” Therefore, acute cardiac deterioration is not an indication for dynamic cardiomyoplasty, but this method may gain importance as an alternative procedure to heart transplantation in the future. Future
Offering an additional alternative to heart transplantation as well as for infection prevention and improvement of patients’ quality of living, implantation of TAH or VAD components is the future goal of development. In this way, today’s trend is focused on the construction of small completely implantable electric, electrohydraulic, or musclepowered biomechanical pumps. Transcutaneous energy supply, using electric induction, has been successfully demonstrated experimentally without skin lesions due to permanent electric streams.
REFERENCES
1. Spencer FC, Eisentian B, Trinkle JK, et al: Assisted circulation for cardiac failure following intracardiac surgery with cardiopulmonary bypass. J Thorac Cardiovasc Surg 49:56-73,1965 2. Cooley DA, Liotta D, Hallmann GL, et al: First clinical implantation of a total artificial heart. Trans Am Sot Artif Intern Organ 15:68-72, 1969
3. Copeland JG, Levinson MM, Smith R, et al: The total artificial heart as a bridge to transplantation. JAMA 256:29912995, 1986 4. Leriche R, Fontaine R: Essai experimental de traitement de certains infarctus du myocarde et de I’anevirsme du coeur par une greffe de muscle strie. Bull Sot Nat1 Chir 59:229-234, 1933
408
5. Kantrowitz A, McKinnon WWP: The experimental use of the diaphragm as an auxilliary myocardium. Surg Forum 9:266-268, 1958 6. Carpentier A, Chachques JC: Myocardial substitution with a stimulated skeletal muscle: First successful clinical case. Lancet 1:1267, 1985 7. Chiu RJC, Walsh GL, Dewar ML, et al: Implantable extraaortic balloon assist powered by transformed fatigue-resistant skeletal muscle. J Thorac Cardiovasc Surg 94:694-701, 1987 8. Acker M, Anderson WA, Hammond RL, et al: Skeletal muscle ventricles in circulation. J Thorac Cardiovasc Surg 94:163174,1987 9. Portner PM, Oyer PE, Pennington DG, et al: Implantable electric left ventricular assist system: Bridge to transplantation and the future. Ann Thorac Surg 47:142-150,1989 10. Kochamba G, Desrosiers C, Dewar M, et al: The musclepowered dual-chamber counterpulsator rheologically superior implantable cardiac assist device. Ann Thorac Surg 45:620-625, 1988 Il. Norman JC, Cooley DA, Igo SR, et al: Prognostic indices for survival during postcardiotomy intraaortic balloon pumping. J Thorac Cardiovasc Surg 74:709-720,1977 12. Pae WE, Pierce WS, Myers LJ, et al: Staged cardiac transplantation: Total artificial heart or ventricular assist pump? Circulation 78:66-72, 1988 13. Trubel W, Losert U, Schima H, et al: Total artificial heart bridging: A temporary support for deteriorating HTX candidates. Thorac Cardiovasc Surgeon 35:277-282,1987 14. Pae WE, Rosenberg G, Donachy JH, et al: Mechanical circulatory assistance for postoperative cardiogenic shock: A three year experience. Trans Am Sot Artif Intern Organ 26:256-261, 1980 15. Paw WE: VAD Registry. Hershey, PA, 1990
WOLNER AND ROKITANSKY
16. Termuhlen DR, Swartz MT, Pennington DC;, et al: f’redictors for weaning patients from ventricular assist devices. Tram Am Sot Artif Intern Organ 34:131-139, 1988 17. Joyce L: TAH Registry. Minneapolis, MN, 1990 18. Cabrol C, Leger PH, Gandjbakhch I, et al: Clinical application and patient selection in the use of a total artificial heart as a bridge for transplantation. Pacing Clin Electrophysiol I1:933-941. 1988 19. Olsen DB, Riebman JB, DePaulis R, et al: Registry and tabulations of orthotopic total artificial hearts in humans. Trans Am Sot Artif Intern Organ 32:182-189, 1987 20. DeVries W: The permanent artificial heart. JAMA 259:849854, 1988 21. Chachques JC, Grandjean P, Schwarzt K, et al: Effect of latissimus dorsi dynamic cardiomyoplasty on ventricular function. Circulation 78:203-216, 1988 (suppl III) 22. Magovern GJ, Heckler FR, Park SB, et al: Paced skeletal muscle for dynamic cardiomyoplasty. Ann Thorac Surg 45:614-622, 1988 23. Farrar DJ, Litwak P, Lawson JH, et al: In vivo evaluations of a new thromboresistant polyurethane for artificial heart blood pumps. J Thorac Cardiovasc Surg 95:191-200,1988 24. Peters G: Plastikinfektionen der Staphylokokken. Dt Arzteblatt 85:286-290, 1988 25. Joyce LD, Jhonson KE, Pierce WS, et al: Summary of the world experience with clinical use of total artificial hearts as heart support devices. J Heart Transplant 5:229-235,1986 26. Annas GJ: No cheers for temporary artificial hearts. Hastings Center Rep 15:27-28, 1985 27. Joyce LD, Jhonson KE, Pierce WS, et al: Summary of the world experience with clinical use of total artificial hearts as cardiac support devices. J Heart Transplant 5:229-235,1986
Mechanical
and Biological
Cardiac Assist Devices
E. Wolner, MD, and A. Rokitansky, MD
D
UE TO CONSIDERABLE cardiac surgical, medical, and diagnostic progress, operability of patients suffering from severe or end-stage heart diseases has increased during recent years. One important reason for this progress is the clinical availability of mechanical or biological cardiac assist devices, eventually combined with human heart transplantation, offering quick hemodynamic restoration and the chance of a successful life-saving surgical treatment. In principle, hemodynamic restoration can be achieved mechanically by pulsatile (membrane or sack pumps) or nonpulsatile (centrifugal or screw pumps) pumps. In 1965, Spencer described the first clinical use of a left ventricular assist device (LVAD).’ The first clinical total artificial heart (TAH) implantation for bridging to transplantation was performed by Cooley et al in 1969.’ In 1985, Copeland et al bridged a patient who has since fully recovered and returned to work.’ Alternatively, biological cardiac support is possible by the use of a skeletal muscle reinforcing, augmenting, or replacing diseased myocardium. In 1933, Leriche and Fontaine first experimentally demonstrated the feasibility of using the pectoralis muscle to reinforce a myocardial scar after ligation of a coronary artery.4 In 1958, Kantrowitz and McKinnon used a pedicled graft of diaphragm around the mobilized aorta for counterpulsation.’ In 1985, Carpentier and Chachques performed the first successful myocardial reconstruction after resection of a large myocardial fibroma.6 Although the systems are now only under experimental investigation, a biomechanical approach is to use the conditioned autologous muscle to power an artificial ventricular assist device (VAD), which is then completely implantable. In 1987, Chiu et al’ and Acker et als experimentally demonstrated sufficient aortic counterpulsation with such muscle-powered, single- or double-chamber balloon assists. VENTRICULAR
ASSIST SYSTEMS
Mechanical Systems
Artificial blood pump systems consist of three main components: the blood pump for pulsatile or continuous blood flow, the driving unit, and the power supply unit. All available systems for clinical use are still in the nonimplanted or only partially implanted configuration. This means that at least the power supply and control unit are outside the body and are connected to the intracorporeal pump by a transcutaneous line.Y Considering the pumping mode, sufficient cardiac support was clinically achieved with centrifugal or screw
pumps, producing a continuous blood flow, as well as with membrane or sack pumps, producing a pulsatile blood flow up to 10 L/min. All TAH systems used were pulsatile and gas-driven, orthotopically implanted polyurethane membrane pumps with an exterior driving engine. Fully implantable systems for permanent use are under development, and could be proper alternatives for heart transplantation. Biological Systems
Skeletal muscle, such as the latissimus dorsi, the diaphragm, or an intercostal muscle can be used for reinforcing, augmenting, or replacing the diseased myocardium. One clinically used method of muscular cardiomyoplasty is performed by wrapping the left latissimus dorsi skeletal muscle around the left part of the heart by way of a partial resection of the second rib. This surgical technique is called dynamic cardiomyoplasty.6 Via intramuscular electrodes, the skeletal muscle becomes conditioned with a change of fatigue-sensitive glycolytic to fatigue-resistant oxidative fibers, which takes at least 6 weeks. Dynamic cardiomyoplasty may lead to a more forceful systolic contraction, giving the chamber the proper size by replacement of scarred myocardium or by reinforcing a dilated thin left ventricle. Biomechanical Systems
Alternatively, the conditioned skeletal muscle can be used experimentally for driving a mechanical assist device. Such a biomechanical heart support was achieved experimentally by three different system concepts”.‘“: (1) the muscle is wrapped around a blood-filled polyurethane ventricle forming a pouch; (2) the muscle is wrapped around a valve-containing plastic tube; and (3) the muscle powers a fluid-filled chamber, which is connected to the blood-filled pumping chamber. Experimentally, eg, by counterpulsation, each system seems capable of increasing the aortic blood flow.
From the Second Surgical Clinic, University of Vienna, Kenna, Austria. Presented at the Fifth Annual Meeting of the European Association of Cardiothoracic Anaesthesiologists, Vienna, Austria, May 13-16, 1990. Address reprint requests to E. Wolner, MD, Second Surgical Clinic, University of Vienna, Spitalgasse 23, A-1090 Vienna, Austria. Copyright 0 1991 by W B. Saunders Company 1053- 077Ol91 lO504-0019$03.00l0
Journalof Cardiothoracic and Vascular Anesthesia, Vol 5, No 4 (August), 1991:
pp405-408
405
406
WOLNER AND ROKITANSKY
Indication for Cardiac Assist Devices
Use of a mechanical cardiac assist device is principally indicated in a biologically young ( < 50 years) and infectionfret patient developing acute cardiac deterioration (cardiac index of about 1.8 to 2 L/min/m’, a mean blood pressure below 60 mm Hg, pulmonary artery wedge pressures greater than 25 mm Hg, and a urine output less than 20 mL/h), provided an intraarterial balloon pump and cardiac drug treatment fail to improve the patient’s condition.“,” In many cases, even biventricular failure can be treated with a single left ventricular assist and elevated pulmonary artery pressures may be reversible.‘3.‘4 Sufficient function as well as easier implantation favor the use of VADs as the first step. Considering that the original working heart is left in place, another advantage of VADs is the possibility of resuscitation in case of a device failure. TAH implantation is indicated in transplant candidates, when the original heart should be removed for clinical reasons, eg, thrombotic material causing embolism, rupture of the myocardial wall, severe valve dysfunction, or intracardiac shunts. Due to conditioning of the skeletal muscle performing sufficient cardiac support, which takes at least 6 weeks, acute cardiac deterioration may be treated only by mechanical devices. Biological or, eventually, biomechanical assist methods may reach additional importance for long-term cardiac support as an alternative to heart transplantation.
two-stage transplantation is better in patients with acute cardiac deterioration than in patients with chronic detcrioration.‘& Clinical experience with patients who had acutely rejected a previously transplanted heart and were subscquently bridged is particularly disappointing.“’ Permanent Support
Although survival for 622 days is possible with the TAH, it must be kept in mind that due to complications, permanent mechanical heart replacement is still problematic. During an implantation time between 227 and 622 days, all patients developed thromboembolism leading to major or minor strokes. One patient who was on TAH for 112 days developed infection. Thus, thromboembolism and infection are significant limiting factors in the use of artificial hearts as long-term cardiac replacement.‘” On the contrary, the cardiomyoplasty technique using a skeletal muscle reinforcing or replacing the myocardial muscle may lead to long-term biological cardiac support, as was shown experimentally and clinically in 8 cases by Chachques et al” and in 4 cases by Magovern et al.” In a follow-up period of up to 42 months, postoperative heart scans and echocardiographic and hemodynamic examinations showed improvement of ventricular function. A 10% to 15% increase of the ejection fraction was achieved.2’ DISCUSSION
RESULTS
Temporary Support Leading to Cardiac Function Restoration
Quick hemodynamic support by cardiac assist systems provides for multiple organ failure prevention and cardiac decompression with increased coronary artery circulation, which may consequently lead to cardiac functional improvement. Between 1985 and July 1989, 521 patients with cardiogenie shock were supported with a VAD; in 229 cases cardiac function improved and they could be weaned. One hundred twenty-six patients (26%) were discharged.” Mainly by considering the atria1 pressure and the ejection fraction, weaning criteria are determined with the pump on/off procedure.lh Temporary Support Leading to Heart Transplantation
Better results were achieved in the same period by using a mechanical pump system for bridging to transplantation. Due to the sharply increased number of human heart transplantations, a now well-known shortage of donor hearts exists, and the situation seems to have worsened over the last year. As has been demonstrated by clinically performed VAD and TAH implantations, the systems used are sufficient to provide circulatory support in the course of a two-stage heart transplantation. To the present time, approximately 97% of all TAHs and 30% of all VADs have been used for bridging to transplantation. Thirty-one percent of 187 TAH-bridged patients and 47% of the 169 VAD-bridged patients are still alive.“.” Although pulsatile VAD bridging was used in 73% of the cases reported, clinical experience has shown that centrifugal VAD also leads to a patient discharge rate of 44%. Outcome of a
Mechanical Systems
Complications following mechanical assist device application have mainly consisted of bleeding problems, infection, hemolysis, thrombus formation, thromboembolism, immunologic stimulation, and device dysfunction. Bleeding. In the early postoperative period, bleeding with an incidence of 30% to 60% is a significant problem. Because of hemorrhage, 40% of 97 Jarvik 7170 TAH implants required reoperation.” In 1988, Farrar et al reported 29 patients who received VADs as a bridge to transplantation. Twenty-one underwent a two-stage transplantation. About 40% of these VAD patients had severe bleeding problems.*’ The bleeding hazard in patients on artificial circulation is increased by the special anastomoses and the needed anticoagulation as well as by the reduced number of platelets during the early postoperative period. The surgical anastomoses between organic tissue and artificial materials are susceptible to insufficiency. This is caused by the sewn link between tissue and plastic being under permanent increased mechanical stress due to the movements of the pump. Additionally, in order to avoid thrombus formation, physiological sealing of these anastomoses is suppressed by drug-induced anticoagulation. Surgical hemostasis by close-running sutures should achieve optimal tightness. Infection. Due to transcutaneous driving lines, catheters, intracorporeal artificial surfaces, surrounding damaged tissue, and organ failure, the risk of infection during TAH pumping is high (30% to 40%). These transplantationpreventing infections are highly resistant to antibiotics and persist until the foreign body is removed.24
407
CARDIAC ASSIST DEVICES
Thirty-six percent of 106 Jarvik 7/70 TAH patients developed infections.” Most of the patients on TAH, who could not undergo transplantation, suffered from incurable infections.” Considering the transplantation-preventing effect and the often seen incurability of infections, aseptic and antiseptic measures during bridging are most important. Reduction of transcutaneous lines offered by completely implantable systems seems to be the future goal of development. Hemolysis. Due to a reduced driving gas pressure, hemolysis rate has decreased with the use of modem driving units for pulsatile pumping. More frequently, hemolysis is seen in centrifugal and screw pumps, eg, in the 20 Hemopump (Nimbus Medical Inc, California) bridged cases free plasma hemoglobin was between 10 and 30 mgldL. Thrombus formation, thromboembolism, and anticoagulation. Introduction of artificial surfaces into the blood-
stream induces platelet, complement, and coagulation cascade activation. These hemostatic mechanisms are triggered by the artificial pumps when blood flow changes and shear stress is inevitably present. In relation to the pumping duration, nonpulsatile centrifugal pumps are much more susceptible to thrombus formation than pulsatile pumps. Use of polyurethanes with high biocompatibility and hemodynamically improved designs diminished thrombus formation, but medical anticoagulation as well as antiaggregation continue to be necessary and the thromboembolic problem still exists. Risks of thrombus formation leading to transient ischemic attacks during TAH bridging is around ll%.” Sites of thrombus formation were mainly the atria1 artificial cuffs, the valve rings, or the driving shaft of the centrifugal pump. Thrombus formation in the ventricle chambers is an infrequent event. Blood clot formation in the ventricles seems to be caused by microfractures or pitlike lesions of the artificial plastic surface that occur after intensive mechanical stress. The future goal is to develop thromboresistant or stress-resistant plastic materials. Aside from blood contact with artificial surfaces, slow blood flow is an additional factor for thrombus formation. For thrombus prevention in centrifugal pumps, a flow of less than 2 Wmin should be avoided.16 In 1986, Copeland et al hypothesized that increasing the heart rate would reduce thrombus formation. A 57% reduction of thrombus formation was observed comparing the small Jarvik 7/70 with the Jarvik 7/100 device. Additionally, an interrelation between thrombotic events and infection seems to be evident. In the pathophysiologic view, bacteremia may trigger the hemostatic cascade mechanisms; in contrast, small thrombotic material represents optimal conditions for bacterial growth. Immunologic alterations. Stimulation of the immuno-
logic system during artificial circulation may be caused by different mechanisms such as damaged blood cells, contact between blood and the artificial surface, administration of blood and blood products, and clinical or subclinical infections. Two patients who had rejected their transplant and then received an artificial heart for more than 240 days developed very active immunorejection profiles.” Therefore, it may be concluded that long-term bridging in intensive care unit conditions may lead to an additional stimulation of the immunologic system. Device dysfunction. Reliability of the available devices for clinical use is very high. Only the valves, fixed in the stiff ventricle plastic housing, as well as the percutaneous driving shaft of the intracardial Hemopump, showed increased susceptibility for material breakage. PaShould bridging to transplantation be recommended? tients having undergone a two-stage transplantation now have a long-term survival rate of between 30% and 40% and resemble high-risk transplant candidates, whereas onestage transplanted patients have a survival rate between 70% and 80%. Considering the reduced availability of donor hearts, two-stage transplantation may lead to a waste of donor organs.26 In contrast, Joyce et al believe that as many as 10% of currently harvested hearts are wasted because of the unavailability of a suitable recipient.*’ Considering the bridging indications for clinical TAH or VAD application and the outcome of the reported cases, use of mechanical circulatory support should be restricted to biologically young patients with acute cardiac deterioration, having the best chances of survival. Biological Systems
Because of conditioning of the skeletal muscle, the patient does not fully benefit in the first postoperative weeks.” Therefore, acute cardiac deterioration is not an indication for dynamic cardiomyoplasty, but this method may gain importance as an alternative procedure to heart transplantation in the future. Future
Offering an additional alternative to heart transplantation as well as for infection prevention and improvement of patients’ quality of living, implantation of TAH or VAD components is the future goal of development. In this way, today’s trend is focused on the construction of small completely implantable electric, electrohydraulic, or musclepowered biomechanical pumps. Transcutaneous energy supply, using electric induction, has been successfully demonstrated experimentally without skin lesions due to permanent electric streams.
REFERENCES
1. Spencer FC, Eisentian B, Trinkle JK, et al: Assisted circulation for cardiac failure following intracardiac surgery with cardiopulmonary bypass. J Thorac Cardiovasc Surg 49:56-73,1965 2. Cooley DA, Liotta D, Hallmann GL, et al: First clinical implantation of a total artificial heart. Trans Am Sot Artif Intern Organ 15:68-72, 1969
3. Copeland JG, Levinson MM, Smith R, et al: The total artificial heart as a bridge to transplantation. JAMA 256:29912995, 1986 4. Leriche R, Fontaine R: Essai experimental de traitement de certains infarctus du myocarde et de I’anevirsme du coeur par une greffe de muscle strie. Bull Sot Nat1 Chir 59:229-234, 1933
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