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The Selection of a Cardiac Valve Substitute
EDITORIAL
The Selection of a Cardiac Valve Substitute Russell M. Nelson, M.D., Ph.D. tory. The unknown quality of ultimate durability of the xenograft cannot be assessed at this time, but Davila and co-workers found that of the 13 valves that failed in 10 patients (approximately 1failure per 1,000 valve months), 7 had been destroyed by bacterial endocarditis. Gross and microscopic inspection strongly suggested infection in 5 more valves, but cultures proved negative. In their paper "Hemodynamic Evaluation of Lillehei-Kaster and Starr-Edwards Prostheses" (p 336), Pyle and associates compared the hemodynamic performance of the LilleheiKaster tilting-disc valve in 71 patients with the early model Starr-Edwards valves in 63 patients. The Lillehei-Kaster valves functioned well with the exception of the smaller sizes; valves less than 25 mm in the aortic position or less than 27.5 mm in the mitral position had relatively high gradients and small calculated valve areas. The Lillehei-Kaster valves showed no hemodynamic advantage over the StarrEdwards valves, with the possible exception of the larger mitral valve replacements. The Lillehei-Kaster mitral valves with an outside diameter of 28.5 and 32.0 mm demonstrated a statistically significantly larger calculated valve area in vivo than similar-sized Starr-Edwards valves. To show this difference, however, Pyle and colleagues had to delete the hemodynamic performance values obtained with 3 valves that developed limited disc motion secondary to thrombus or impingement by ventricular muscle. These 3 valves represent 16% of the Lillehei-Kaster valves in this size range in the series, an apparently high failure rate. The clinical follow-up presented on these 71 patients is not included in this hemodynamic study, but is referenced for publication elsewhere. In "Durability of Prosthetic Heart Valves" (p 323), Clark and associates attempted to answer the critical question of valve durability by invitro evaluation of most available cardiac reFrom the Division of Thoracic Surgery, University of Utah College of Medicine, LDS Hospital, 325 Eighth Ave, Salt placement valves, utilizing accelerated fatigue Lake City, UT 84143, testing. Cycling the mechanical valves 33 to 35
The surgeon who replaces a diseased cardiac valve today does so knowing that the ideal replacement does not yet exist. The surgeon may choose either a mechanical valve, an example of human engineering and ingenuity, or a tissue valve, which represents man's attempt to couple his own ingenuity with that of nature. Although these man-made valves have functioned satisfactorily as replacements, both types have limitations. The important papers by Davila, Clark, Pyle, and their associates in this issue explore the challenge of cardiac valve replacement from different perspectives and allow us the privilege of adding these viewpoints to the work of others and to our personal experience in dealing with these choices. Without actually answering the question they posed as their title, "Is the Hancock Porcine Valve the Best Cardiac Valve Substitute Today?," Davila and associates (p 303) report their experience with 221 patients followed for 36 to 75 months after xenograft valve replacement. Despite only limited or no anticoagulation in all patients with aortic valve replacement and in half the patients with mitral valve replacement, the overall incidence of thromboembolism remained low (2.24 emboli per 100 patient years) and valve thrombosis never occurred. It is not reported whether any of the 19 patients who sustained the 21 emboli were among those who received anticoagulation or whether the xenograft valves used early in the study had been washed with antibiotics, a procedure now known to increase the risk of thromboembolism. Bacterial endocarditis developed in 10 patients 6 to 62 months after valve replacement, but 6 made apparent full recovery with appropriate medical therapy. This high degree of success in eradicating infection from the Hancock valve compares favorably with autogenous valves and vastly exceeds that achieved by others with mechanical valves. Hemodynamic function generally was satisfac-
291 0003-4975/78/0026-0401$01.00 1978 by Russell M. Nelson
292 The Annals of Thoracic Surgery Vol 26 No 4 October 1978
times per second in a 37°C water bath with physiological flows across the valve and a pressure differential of 100 mm Hg, they could stress almost all of the valves to the point of failure. They found that the valve components that failed in the testing system were the same components that had been reported as failing clinically. Two valves performed exceptionally well; the Lillehei-Kaster tilting-disc valve showed virtually no wear after cycling the equivalent of nearly 20 years, and the Bjork-Shiley Pyrolite disc valve failed only after cycling the equivalent of more than 24 years. Despite the efficacy of the in vitro testing of mechanical valves, the methods of Clark and associates did not define the durability of the tissue valves. Using a testing medium of glutaraldehyde at the concentration used for valve storage by the respective manufacturers, they subjected the porcine xenografts of Hancock and Carpentier-Edwards and the Ionescu-Shiley porcine pericardial protheses to their system of accelerated fatigue testing. Valve failure manifested by multiple leaflet fenestrations or leaflet tears or both occurred by cycling equivalent to less than 2 years in the Hancock and Ionescu valves and less than 1 year with the Carpentier-Edwards valve. This finding seems to be at variance with the clinical experience with these prostheses. The authors appreciated this discrepancy and analyzed the reasons for it. They pointed out that the absolute consistency of valve leaflet motion within the rigid constraints of the testing chamber tended to apply repeated stress to the identical portion of the valve leaflet with each cycle. In vivo, however, the point of leaflet bending varies due to changing flow rates across the valve, shifts in the annular shape, and pressure gradient changes, all of which produce variations in human application but were not duplicated by the testing apparatus. Other differences may include possible wear acceleration because of the rapid cycling rates and the influence of the medium, glutaraldehyde, rather than blood or plasma protein. Also, it is possible that the effects of vibration at these unphysiologically rapid rates may stress tissue valves beyond that which is expected at more normal rates. In addition, we
know that the velocity of motion of leaflets is in direct proportion to the forces concentrated at any given point for a given fluid flow rate. Thus, the "power" acting at any point on the leaflet at a given flow rate will be greater if the leaflet movement is accelerated. (A string breaks more easily when it is "snapped.") Therefore, the studies of Clark and colleagues could not be expected to quantitate the lifespan of the tissue valves. Yet, they leave us with our expectation that, in time, tissue valves will wear out. A cardiac surgeon uses a substitute valve only after judging that the patient may live longer with these unknown qualities of the replacement compared with the known quality of the deformed or worthless valve to be removed. An alternative still to be considered is to repair the defective valve if possible. In selected patients, valve repair may obviate the need for any type of replacement valve. Mitral stenosis without extensive leaflet calcification or fusing of the chordae tendineae can frequently be repaired, giving the patient several years of excellent valve function. However, methods of valvuloplasty used in the past for mixed mitral stenosis and regurgitation do not appear to equal the effectiveness of presently available valve replacements. For pure mitral regurgitation, newer techniques of valvuloplasty may be added to the older techniques of annulorrhaphy , particularly those employing the Carpentier ring. Except for an occasional patient with congenital stenosis, aortic valve disease does not lend itself well to repair. When a valve must be replaced, the selection of an appropriate substitute is a matter of individual judgment and consideration. For example, infection appears to be more manageable with tissue valves than with mechanical valves, suggesting a preference for tissue valves in patients requiring valve replacement because of autogenous or prosthetic valve endocarditis. Because mechanical prostheses require anticoagulant therapy, tissue valves are now preferred in those patients for whom anticoagulant therapy is inadvisable or contraindicated. Such patients include those with peptic ulceration, metrorrhagia, liver disease, and intracranial problems, as well as the young woman who
293 Editorial: Nelson: Cardiac Valve Substitute
wishes to bear a child, the individual with an occupation or activity subject to trauma, or the patient who because of emotional or situational factors cannot or will not be able to handle a regimen of anticoagulation safely. When a tissue valve is selected, the advantages of low thrombogenicity, lack of noise, and freedom from anticoagulation are offset to some extent by the prediction of less durability and ultimate failure with the possibility of another operation in the future. Other circumstances favor mechanical valves. A relatively small annulus is an example. Inasmuch as all substitute valves are relatively stenotic, this factor assumes critical importance in the smaller sizes. Presently available tissue valves smaller than 23 mm generally should not be used in the aortic position. Although improvements are being made, the porcine valve xenograft results in a smaller effective valve orifice for a given annulus size compared with certain mechanical valves. For example, mechanical valves with an outside diameter of 25 mm, such as the Bjork-Shiley,
may have almost a 20% greater in vivo valve area than the Hancock or Ionescu-Shiley heterografts. Other mechanical valves, such as the Starr-Edwards and Lillehei-Kaster, have calculated valve areas similar to or even smaller than the tissue valves and, therefore, offer no apparent hemodynamic advantage to the patient with a small annulus. The advantage of durability is offset to some extent by thrombogenicity, noise, and the need for anticoagulation when a mechanical prosthesis is selected. It seems advisable to discuss the selection process with the patient, thereby including his views in the matrix of information that must be assimilated by the surgeon before making a final selection. The ultimate responsibility for choosing a valve replacement rests with the surgeon. The lack of a definite superiority between tissue valves and mechanical valves can be frustrating. But the availability of several excellent valves offers a diversity of options almost beyond our hopes just over a decade ago.
The Selection of a Cardiac Valve Substitute Russell M. Nelson, M.D., Ph.D. tory. The unknown quality of ultimate durability of the xenograft cannot be assessed at this time, but Davila and co-workers found that of the 13 valves that failed in 10 patients (approximately 1failure per 1,000 valve months), 7 had been destroyed by bacterial endocarditis. Gross and microscopic inspection strongly suggested infection in 5 more valves, but cultures proved negative. In their paper "Hemodynamic Evaluation of Lillehei-Kaster and Starr-Edwards Prostheses" (p 336), Pyle and associates compared the hemodynamic performance of the LilleheiKaster tilting-disc valve in 71 patients with the early model Starr-Edwards valves in 63 patients. The Lillehei-Kaster valves functioned well with the exception of the smaller sizes; valves less than 25 mm in the aortic position or less than 27.5 mm in the mitral position had relatively high gradients and small calculated valve areas. The Lillehei-Kaster valves showed no hemodynamic advantage over the StarrEdwards valves, with the possible exception of the larger mitral valve replacements. The Lillehei-Kaster mitral valves with an outside diameter of 28.5 and 32.0 mm demonstrated a statistically significantly larger calculated valve area in vivo than similar-sized Starr-Edwards valves. To show this difference, however, Pyle and colleagues had to delete the hemodynamic performance values obtained with 3 valves that developed limited disc motion secondary to thrombus or impingement by ventricular muscle. These 3 valves represent 16% of the Lillehei-Kaster valves in this size range in the series, an apparently high failure rate. The clinical follow-up presented on these 71 patients is not included in this hemodynamic study, but is referenced for publication elsewhere. In "Durability of Prosthetic Heart Valves" (p 323), Clark and associates attempted to answer the critical question of valve durability by invitro evaluation of most available cardiac reFrom the Division of Thoracic Surgery, University of Utah College of Medicine, LDS Hospital, 325 Eighth Ave, Salt placement valves, utilizing accelerated fatigue Lake City, UT 84143, testing. Cycling the mechanical valves 33 to 35
The surgeon who replaces a diseased cardiac valve today does so knowing that the ideal replacement does not yet exist. The surgeon may choose either a mechanical valve, an example of human engineering and ingenuity, or a tissue valve, which represents man's attempt to couple his own ingenuity with that of nature. Although these man-made valves have functioned satisfactorily as replacements, both types have limitations. The important papers by Davila, Clark, Pyle, and their associates in this issue explore the challenge of cardiac valve replacement from different perspectives and allow us the privilege of adding these viewpoints to the work of others and to our personal experience in dealing with these choices. Without actually answering the question they posed as their title, "Is the Hancock Porcine Valve the Best Cardiac Valve Substitute Today?," Davila and associates (p 303) report their experience with 221 patients followed for 36 to 75 months after xenograft valve replacement. Despite only limited or no anticoagulation in all patients with aortic valve replacement and in half the patients with mitral valve replacement, the overall incidence of thromboembolism remained low (2.24 emboli per 100 patient years) and valve thrombosis never occurred. It is not reported whether any of the 19 patients who sustained the 21 emboli were among those who received anticoagulation or whether the xenograft valves used early in the study had been washed with antibiotics, a procedure now known to increase the risk of thromboembolism. Bacterial endocarditis developed in 10 patients 6 to 62 months after valve replacement, but 6 made apparent full recovery with appropriate medical therapy. This high degree of success in eradicating infection from the Hancock valve compares favorably with autogenous valves and vastly exceeds that achieved by others with mechanical valves. Hemodynamic function generally was satisfac-
291 0003-4975/78/0026-0401$01.00 1978 by Russell M. Nelson
292 The Annals of Thoracic Surgery Vol 26 No 4 October 1978
times per second in a 37°C water bath with physiological flows across the valve and a pressure differential of 100 mm Hg, they could stress almost all of the valves to the point of failure. They found that the valve components that failed in the testing system were the same components that had been reported as failing clinically. Two valves performed exceptionally well; the Lillehei-Kaster tilting-disc valve showed virtually no wear after cycling the equivalent of nearly 20 years, and the Bjork-Shiley Pyrolite disc valve failed only after cycling the equivalent of more than 24 years. Despite the efficacy of the in vitro testing of mechanical valves, the methods of Clark and associates did not define the durability of the tissue valves. Using a testing medium of glutaraldehyde at the concentration used for valve storage by the respective manufacturers, they subjected the porcine xenografts of Hancock and Carpentier-Edwards and the Ionescu-Shiley porcine pericardial protheses to their system of accelerated fatigue testing. Valve failure manifested by multiple leaflet fenestrations or leaflet tears or both occurred by cycling equivalent to less than 2 years in the Hancock and Ionescu valves and less than 1 year with the Carpentier-Edwards valve. This finding seems to be at variance with the clinical experience with these prostheses. The authors appreciated this discrepancy and analyzed the reasons for it. They pointed out that the absolute consistency of valve leaflet motion within the rigid constraints of the testing chamber tended to apply repeated stress to the identical portion of the valve leaflet with each cycle. In vivo, however, the point of leaflet bending varies due to changing flow rates across the valve, shifts in the annular shape, and pressure gradient changes, all of which produce variations in human application but were not duplicated by the testing apparatus. Other differences may include possible wear acceleration because of the rapid cycling rates and the influence of the medium, glutaraldehyde, rather than blood or plasma protein. Also, it is possible that the effects of vibration at these unphysiologically rapid rates may stress tissue valves beyond that which is expected at more normal rates. In addition, we
know that the velocity of motion of leaflets is in direct proportion to the forces concentrated at any given point for a given fluid flow rate. Thus, the "power" acting at any point on the leaflet at a given flow rate will be greater if the leaflet movement is accelerated. (A string breaks more easily when it is "snapped.") Therefore, the studies of Clark and colleagues could not be expected to quantitate the lifespan of the tissue valves. Yet, they leave us with our expectation that, in time, tissue valves will wear out. A cardiac surgeon uses a substitute valve only after judging that the patient may live longer with these unknown qualities of the replacement compared with the known quality of the deformed or worthless valve to be removed. An alternative still to be considered is to repair the defective valve if possible. In selected patients, valve repair may obviate the need for any type of replacement valve. Mitral stenosis without extensive leaflet calcification or fusing of the chordae tendineae can frequently be repaired, giving the patient several years of excellent valve function. However, methods of valvuloplasty used in the past for mixed mitral stenosis and regurgitation do not appear to equal the effectiveness of presently available valve replacements. For pure mitral regurgitation, newer techniques of valvuloplasty may be added to the older techniques of annulorrhaphy , particularly those employing the Carpentier ring. Except for an occasional patient with congenital stenosis, aortic valve disease does not lend itself well to repair. When a valve must be replaced, the selection of an appropriate substitute is a matter of individual judgment and consideration. For example, infection appears to be more manageable with tissue valves than with mechanical valves, suggesting a preference for tissue valves in patients requiring valve replacement because of autogenous or prosthetic valve endocarditis. Because mechanical prostheses require anticoagulant therapy, tissue valves are now preferred in those patients for whom anticoagulant therapy is inadvisable or contraindicated. Such patients include those with peptic ulceration, metrorrhagia, liver disease, and intracranial problems, as well as the young woman who
293 Editorial: Nelson: Cardiac Valve Substitute
wishes to bear a child, the individual with an occupation or activity subject to trauma, or the patient who because of emotional or situational factors cannot or will not be able to handle a regimen of anticoagulation safely. When a tissue valve is selected, the advantages of low thrombogenicity, lack of noise, and freedom from anticoagulation are offset to some extent by the prediction of less durability and ultimate failure with the possibility of another operation in the future. Other circumstances favor mechanical valves. A relatively small annulus is an example. Inasmuch as all substitute valves are relatively stenotic, this factor assumes critical importance in the smaller sizes. Presently available tissue valves smaller than 23 mm generally should not be used in the aortic position. Although improvements are being made, the porcine valve xenograft results in a smaller effective valve orifice for a given annulus size compared with certain mechanical valves. For example, mechanical valves with an outside diameter of 25 mm, such as the Bjork-Shiley,
may have almost a 20% greater in vivo valve area than the Hancock or Ionescu-Shiley heterografts. Other mechanical valves, such as the Starr-Edwards and Lillehei-Kaster, have calculated valve areas similar to or even smaller than the tissue valves and, therefore, offer no apparent hemodynamic advantage to the patient with a small annulus. The advantage of durability is offset to some extent by thrombogenicity, noise, and the need for anticoagulation when a mechanical prosthesis is selected. It seems advisable to discuss the selection process with the patient, thereby including his views in the matrix of information that must be assimilated by the surgeon before making a final selection. The ultimate responsibility for choosing a valve replacement rests with the surgeon. The lack of a definite superiority between tissue valves and mechanical valves can be frustrating. But the availability of several excellent valves offers a diversity of options almost beyond our hopes just over a decade ago.