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The advantages and disadvantages of prosthetic valves for aortic valve replacement
The Advantages for
and Disadvantages of Prosthetic Aortic Valve Replacement
Valves
By GEORGE E. DUVOISIN AND DWIGHT C. MCGOON
C
ONCERN and uncertainty are the prevalent attitudes regarding the selection of the proper substitute valve for insertion after excision of diseased cardiac valves. The current focus for this indecision is on the aortic valve, because valvular grafts have been more widely employed and acclaimed in the aortic than the mitral location and have there challenged the prosthetic valve most pointedly. The chief choice now lies between the ball-valve prosthesis, because it has achieved significant successand has been generally the most popular, and the homograft aortic valve. In spite of this uncertainty, the surgeon is forced to reach a decision by the time the diseased valve of his specific patient has been excised and the aortic root prepared for the substitute valve. A choice at that time is inescapable. Fortunately, the record of either substitute valve-the ball valve or the homograft-has been generally good, and each offers the symptomatic patient with aortic valve disease a considerably improved outlook over that which would be his plight if operation was unavailable. But precisely because this choice cannot be avoided, it is appropriate to review and weigh the advantages and disadvantages of a prosthetic substitute valve for the aortic area. PROSTHETICVALVES Sauvage and co-workers L have presented a simple classification of cardiac valves: those with central flow and those with lateral flow. With the former, there is an unobstructed central pathway, whereas with the latter, because a central obstruction remains during systole, lateral flow must occur. Central-flow valves. The natural valve is of this type. Historically and instinctively this design was the first and most promising choice. One would even now speculate that this design, contrived by nature, will be the one that will ultimately prove best for a substitute valve. The reason for the early dissatisfaction with such prosthetic valves was that the materials used, synthetic cloth or Silastic rubber, were not adequately durable to withstand the stresses of repeated flexion. Sufficiently durable materials have not yet been developed. An ideal material, if such can ever be developed, would have the properties of durability, flexibility, elasticity, nonporosity, and nonthrombogenicity, thus simulating the natural valvular cusps. In addition, it should be able to be mounted on a rigid stent or support to assure conformity of the distorted aortic root to the prosthesis and thus give assurance of the competence of the prosthesis in all instances. Lateral @co u&es. Due to the unsatisfactory performance of the prosthetic valves of natural- or central-flow design, surgeons abandoned them and began to use instead valves that allow lateral flow, the initial and persistently From
the
Mayo
Clinic
and
Mayo
Foundation,
Section
of
Surgery,
Rochester,
Minnesota.
294 PROGRESS IN CARDIOVASCULAR
DISEASES,
VOL. XI, No. 4 (JAN.), 1969
295
PROSTHETIC AORTIC VALVES
most popular of which, in the aortic area, at least, has been the ball-valve prosthesis. This transition was made easier by the realization that for certain other mechanical functions found in nature, man had made practical improvements in design, such as the jet engine over the flapping wing of the bird, and the screw over the fish’s tail. The ball valve enjoyed immediate, though imperfect, success. It was apparent from the first that the lateral-flow principle resulted in a transaortic valvular peak systolic gradient of from 5 to 40 mm. Hg, but this appeared to be well within the margin of reserve that the left ventricle could tolerate under conditions of normal activity. The lateral-flow characteristic also may be disadvantageous because the resultant accelerated flow of blood near the walls of the aortic sinuses has been implicated in the development of intimal hypertrophy of these regions, which may progress to encroach on the coronary arterial orifices. This complication has been noted only rarely. In the absence of leaks about the valve, the mildly accelerated destruction of blood elements and especially red blood cells has not caused clinically detectable difficulty. Other designs of prosthetic valves have appeared-sutureless sewing rim, the low-profile disk and lens valves, a centrally hinged “butterfly” valve, and others. But the arguments for their preferability over the ball valve were not strong enough to counter conclusively the theoretic attractiveness of the ball principle as far as durability was concerned, and also its tendency to revolve unconfined and thus to present constantly changing surfaces for contact with the cage during its excursions. It is appropriate to note that the homogrnft and heterograft valves as used clinically in most practices are also actually prostheses of the natural- or central-flow design, for they are nonvital. Even when efforts to preserve viability of homograft valves have been made, it has been difficult to prove viability. Considered as such, the structural material of these “prosthetic” grafted valves approaches the desirable qualities of the ideal material mentioned previously. CLINICAL
DATA
Kesults concerning the patient provide the most useful index of successwith a particular valve substitute. A review of the 1,530 operations performed at the Mayo Clinic up to January 1967 included 550 single aortic-valve replacements with the Starr-Edwards prosthesis, associated with a hospital mortality of 6 per cent. Aortic homografts were used in 73 instances, with hospital deaths in 7 per cent. Despite the similarity of these percentages, it should be mentioned that the homograft group represents a somewhat selected series. Homograft replacement was avoided in patients with severe coronaryartery disease because they might not tolerate the more prolonged coronary perfusion required for insertion of homograft valves. Likewise, neither patients with aneurysms of the ascending aorta nor those with severe dilatation of the aortic root were candidates for this procedure. The unfavorable influence of such factors on the Starr-Edwards group is evident when concomitant additional procedures are excluded. For instance, in 1966 there were no hos-
Table Procedure
l-Causes
or valve
Primary cause Embolism (fatal or disabling) Subacute bacterial endocarditis Aortic insufficiency Fatal anticoagulant complication Serum hepatitis Cardiac Sudden death Coronary artery disease Myocardial disease Associated valvular disease Dissecting aneurysm Pacemaker complication Noncardiac Psychiatric stntc Carcinoma Trauma Uncertain Total:
of Failure
in Aortic
Valve Replacement
Starr-Edwards (518 patients)”
Homograft ( 68 patients ) *
“3 19 14 3 3
1 0 7 0 0
18 (6)t
0 2 0
11 4
:j 1 1
0 0 0
1 2 2 2 107
0 u 0 0 10
*Number of patients surviving insertion (traced as long as 52 months in the case of Starr-Edwards valve and 27 monthes in the case of homografts). +Nrlmher of autopsies in parentheses.
pita1 deaths among 91 patients with uncomplicated aortic valve disease (that is, only calcific aortic stenosis or aortic insufficiency) treated by insertion of a Starr-Edwards prosthesis. Reports from other centers indicate that operative mortality after the use of homografts range from 6 per cent to 8 per cent,“,‘: while the range after prosthetic valve insertions is 6 per cent to 22 per cent.“-’ Once patients have survived aortic valve replacement, adverse factors may compromise the successof the long-term result (Table I ). One of the prominent problems encountered in the early experience with Starr-Edwards prostheses was subacute bacterial endocarditis, often with a fatal outcome. This problem has been nearly eliminated from present-day operations. The disappearance of the complication of infection may be associated with the general reduction in wound infections in the general hospital population and attributed to improved techniques of sterility and cleanliness and the routine use of newer more potent antibiotics. Sudden death occurred in I8 patients of the Starr-Edwards group who were doing well clinically at the time of their demise, Autopsy obtained in a third of these casesfailed to reveal the cause of the sudden fatal catastrophic event. Coronary artery disease accounted for a significant number of deaths (11 deaths), as might be expected in this age group3 There are several important considerations relative to the choice between the ball valve and the grafted valve. The ball valve seems to hold an advantage in assurance of immediate adequate function, in technical ease of insertion, in availability, and in late development of incompetence. The single advantage of the grafted valve is its lesser threat of thromboembolism. In
PROSTHETIC
AORTIC
VALVES
397
durability, a sufficient length of follow-up is not available to establish a decisive advantage for either valve. Mechanical malfunctions pertinent to the question of durability of the Starr-Edwards valve were limited at this institution to 14 instances of a leak around the prosthesis. The most serious mechanical defect in a prosthetic valvc~ concerns the recent reports of ball variance by Starr. No evidence of this malfunction in Starr-Edwards prostheses has been noted in the series at this institution, a result similar to the experience at the Cleveland Clinic.’ Hall vnriance is a poorly understood phenomenon confined to the aortic rather than the mitral Starr-Edwards prostheses. Lipid absorption by the Silastic ball causes swelling, followed by grooving, sticking, or even fracture and embolization of the poppet.:’ Early diagnosis has been stressed by Starr to allow replacement of defective valves prior to fatal prosthetic dysfunction;“’ the c>ssential basis for early detection of ball variance rests on the diminished amplitude of the first ljrosthetic sound both on clinical examination and phonecardiographic analysis. Starr reported that 20 per cent of patients with the now discontinued Model 1000 nortic prosthesis had a ball variance an nverage of 40 months after operation. Ball variance led to the change in the design of the Starr-Edwards aortic prosthesis to Slodel 2300, in which the silicone rubber ball is replaced by a hollow metal poppet composed of Stellite. This valve is also cushioned by Dacrou-covered struts which permit tissue ingrowth into the fabric. This latter fcbature resulted in aortic valve gradients averaging 30 I~IIL I-Ig in 6 patients studied 6 months or more after 3peratiou, presumably on the basis of overgrowth of neointima on the fabric.& The cloth covering has now been changed to Teflon. Thus, the metal ball eliminates the problem of ball variance, but the long-term effect of tissue ingrowth in the new Teflon-covered prosthesis is unknown. The fact that ball variance is more common in patients who have fistulae about their valves, thus causing increased trauma to the ball. indicates that details of technique of insertion of the prosthesies also may be prominent factors contributing to lutcxr ball varinuce. If this is true, the incidence of ball variance may bc higher \vhen injury to the ball has been permitted during operation, as by needle points or trauma on the cage, or where cnlcific masses or suture knots may ha\.c~ been left to project against the ball and thus injure it. Tl~~~oml~oen~bolism rcmnins the prominent cause of failure iu the StarrEd\fv:lrds group ( Table 1) . In fact, this complication was the primar). reason that some surgeons turucd to homogriift aortir-valvr replacement. The threat of rAAism has been met 1,~ using anticoagulant therapy during the postoperative period. Although the risk of embolism has been previousl!, sho\vn to 1~ the same whether or not patients received anticoagulants,11 analysis of the anticoagulant programs clearly demonstrates that the risk of embolism is rcalattd to the degree of optimd anticoagulant control as judged by prothrombin-time values. Discontinuing therapy led to the highest incidence of c~mbolism. Suboptimal prothrombin values provided some protection. but tllr most striking fact was that less than 2 ptJr cent of patients with optimal control of anticoagulant therapy suffer embolism ( Fig. I), Vnfortunately, tlris (legrec of control is difficult to achirve, iI\ only 20 per cent of :unticoagu-
DUVOISIX
AND
LfCGOON
Suboptimal
Optimal 2
3 Years
4
5
.6
postoperot1ve
Fig. l-Incidence of thromboembolism, depending on type of anticoa ulant COP Dotted lines on all graphs indicate less than 20 casesobserved i uring that period. trol.
hated patients were in this category, The risk of anticoagulant therapy is also proposed as an objection to prosthetic valves, but in our experience only three deaths in the 483 anticoagulated survivors in the Starr-Edwards group could be attributed to anticoagulant therapy (Table 1). Furthermore, difficulty in anticoagulant management su5cient to require cessation of therapy occurs in only 4 per cent of anticoagulated patients.*’ The necessity of anticoagulant therapy after insertion of the cloth-covered Model 2300 aortic prosthesis is currently under evaluation by Starr using a double-blind study:’ It has been postulated that the neointima will encapsulate the fabric of the cage and base of the valve, thus eliminating any tendency for thrombus to form and propagate. Aortic insu5ciency is the major detriment evident after insertion of homografts (TabIe 1). Factors reIevant to this condition include 1) difficult operative technique, 2) method of preservation, and 3) unfavorable aortic root changes. There is no question that aortic homografts require longer perfusion times and are technically more di5cult to insert than prosthetic valves. Furthermore, immediate competence is not assured as with prosthetic substitutes, ahhough surgeons indicate that the incidence of diastolic murmurs diminishes as experience increases. The method of preservation definitely influences the strength of the homograft. Maim and associates13have presented evidence in a well-conceived study demonstrating the superior tensile strength of homografts sterilized by exposure to a high-energy electron beam as opposed to betapropiolactone. I3 Unfortunately, the electron beam is not widely available to surgeons. The sterilization and storage methods may contribute
PROSTHETIC
AORTIC
VALVES
499
to the late degenerative changes, although this factor has yet to be fully evaluated. Problems of storage are averted with the use of fresh aortic homografts, which have shown no tendency for rejection in the hands of .-lngell and Shumway.l+ However, the problems in procuring an adequate supply of fresh valves is even more challenging than for preserved valves. In fact, Angel1 and Shumway recently indicated that they plan to use porcine heterografts because of the difhculty in obtaining fresh human aortic valves.” Late degenerative changes to which homografts are subject cause considerable concern for the eventual durability of this valve. Cusp rupture occurs in only about 2 per cent of cases2 But, more significantly, ROSS” encountered fairly heavy deposits of calcium in the aortic wall in 7 of 10 homografts inserted up to 4 years previously. In an unspecified number of valves, calcification extended to involve the aortic cusps themselves. These findings have influenced Ross to insert a pulmonary autograft to replace the aortic valve. and to use an aortic homograft to replace the pulmonary valve. M’hereas prosthetic valves are suitable for any situation involving aortic valve replacement, homografts are less satisfactory in the presence of an aneurysm of the ascending aorta, or where the aortic root is significantly dilated or deformed. One approach designed to overcome the problem of postoperative insufficiency of tissue graft valves has been to mount homografts or heterografts on rigid stcnts.lG This modification introduces an additional foreign body but provides a rigid ring that facilitates insertion and prevents insufficiency secondary to dilatation of the nortic ring. A definition of what constitutes clinically significant postoperative aortic insufficiency has yet to be standardized. Therefore it is difficult to evaluate the mass of conflicting data to obtain an accurate appraisal of its true incidence. Leaks after both prosthetic”J” and homograft insertions’,l+“,l~ have been reported by many investigators, and one can conclude only that the problem exists. ROSS’~ reported that diastolic murmurs develop in 30 per cent of patients after homograft insertion, although only 9 per cent have aortic ins&iciency. Likewise Barrutt-Boyes’ reported murmurs in 42 per cent of his operative survivors, but only 1s per cent of patients with murmurs had other evidence of aortic insufhciency. In the hlayo Clinic series, the incidence of diastolic murmurs after homograft replacement of the aortic valve ~3s considerably higher than after the use of a Starr-Edwards prosthesis ( Fig. 2). Indeed, in this respect the homograft curve parallels that of the Teflon \~lvcs that were abandoned in I(963 for reasons of mechanical failure in the proswere clearly associntcd with clinically significant thesis.’ T Not all murmurs valve malfunction. When hernodynalnically significant aortic insufficiency is defined as that situation iii which au aortic diastolic murmur is associated with a systolic blood pressure more than twice the diastolic level, then the incidence of insufficiency is suggestively higher ( p = 0.05) after homograft than after Starr-Edwards aortic valve replacement ( Fig. 3). Although this tabulation of data provides some interesting points for discussion, no decision can be made concerning the superiority of the prosthesis versus the homograft. The ultimate valve substitute will excel solely on its comparative superiority of overall clinical results. Let US then turn to clinical
Years
Fig. 2-Aortic murmurs.
valve replacement
(623 operations),
Yeats
Fig. 3-Aortic valve replacement cant aortic insufficiency.
postoperative
Actuarial
incidence
of diastolic
posfoperalive
(623 operations).
Actuarial
incidence
of sibqlifi-
data and examine what can be expected after insertion of ball \.alr.es \.ersus honlogafts. The information we seek is relatively simple: given a group of patients who have aortic valve replacement with a Starr-Edwards prosthrsis. ,\‘
PROSTHETIC
AOHTIC
v.iI,\‘ES
Starr-Edwards
04
2
3 years
Fig. 4-Aortic valve replacement
I-___4
2
5
6
portoperatlve
(623 operations).
Survivor&p
with a good result.
years later. The crucial question is: How many patients with homograft replacement have a similar good result at a comparable time interval after operation? Obviously, the longer a patient is traced, the greater the chance that death or faihrre will occur. Despite the simplicity of this approach, the actual statistical method is more complexzu Nevertheless, accurate comparison of data (such as by actuarial curves) is the essential ingredient for arriving at a choice between methods of aortic valve replacement. Comparison of the actuarial curves for the Starr-Edwards and the homograft group from the Mayo Clinic series shows that there is no significant difference between the incidence of good results achieved in each group (Fig. 4). Naturally, these results will be more meaningful when the homograft group has been followed up for a longer time and when it contains a larger number of patients. As pointed out previously, the homograft series contains a somewhat selected group of more favorable patients. The fate of homografts beyond this relatively short follow-up period has not been examined by the actuarial method, whereas hard data on the Starr-Edwards vah extends to more than 4 years of follow-up. Furthermore, it is important to recognize that the Starr-Edwards curve portraying survival with a good result tends to flatten after the first 2 years, indicating that the risk of death or a poor result is significantly reduced after that time. Therefore, we know not only that a satisfactory result is possible as long as 4 years after operation but also that the shape of the curve indicates the probable maintenance of this good result. Whether the homograft valve will parallel this good result is a matter of conjecture and concern until accurate statistical data are available. SUMMARY
The ideal substitute for the diseased aortic valve has yet to be discovered At the moment the most significant choice involves the Starr-Edwards prosthesis and the homograft aortic valve, each with its own deficiencies. Aortic
insufficiency has followed a number of homograft insertions; the threat of cusp degeneration and calcification remains as a further deterrent to widespread adoption of homografts. Thromboembolism and, more recently, ball variance in Starr-Edwards valves detract from the benefits of such technical assets as ease of insertion and dependable valvular competence. The clothcovered aortic prosthesis has been designed, but has not yet been proved, to overcome these deficiencies. An accurate statistical appraisal of clinical results has shown that either valve inserted in the patient with aortic valve disease will provide a comparable chance for a good result for the first 2 postoperative years. No further data are available for the homograft (from this institution) at this time, but actuarial curves for the Starr-Edwards prosthesis demonstrate a persistent good result for an additional 2 years. A similar lack of deterioration for homograft aortic valves can be ascertained only by the results of future analysis. ACKNOWLEDGMENT The operations reviewed Wallace, and F. H. Ellis, homografts were performed
in this report were performed by Drs. J. W. Kirklin, R. B. Jr., as well as the authors. Most of the procedures employing by Drs. J. \V. Kirklin and R. B. Wallace.
REFERENCES 1. Sauvage, L. R., Viggers, R. F., Robel, S. B., Wood, S. J., Berger, K., and Wesolowski, S. A.: Prosthetic heart valve replacement. Ann. N.Y. Acad. Sci. 146: 289, 1968. 2. Barratt-Boyes, B. G.: Homograft replacement for aortic valve disease. hlod. Cont. Cardiov. Dis. 36:1, 1967. 3. Ross, D.: Homograft replacement of the aortic valve. Brit. J. Surg. 54:842, 1967. 4. Starr, A., and Herr, R. E.: Accumulated experience with the Starr-Edwards prostheses 1960-1968. Read at the Second National Conference on Prosthetic Heart Valves, Los Angeles, California, May 30 to June 1, 1968. 5. Cooley, D. A., Bloodwell, R. D., Beall, A. L., Bricker, D. L., Hallman, G. L., and DeBakey, M. E.: Long term results after cardiac valve replacement: Clinical experience with 2000 patients. Read at the Second National Conference on Prosthetic Heart Valves, Los Angles, California, May 30 to June 1, 1968. 6. Magovern, G. J., and Kent, E. M.: Mechanical hxation (suturelen) of the aortic and mitral valves: A review of five years’ experience. Read at the Second National Conference on Prosthetic Heart Valves, LOS Angeles, California, May 30 to June 1, 1968. 7. Ross, J., Jr., and Braunwald, E.: Aortic stenosis. Circulation 38 ( Suppl 5) : 61, 1968. 8. Effler, D. B., Grover, L. K., and Cumulative experience Favaloro, R. G.:
with the ball valve prosthesis. Read at the Second National Conference on Prosthetic Heart Valves, Los Angeles, California, May 30 to June 1, 1968. 9. Ablaza, S. C. G., Blanco, G., Maranhao. V., and Goldberg, EL: Fatal extrusion of the ball from a Starr-Edwards aortic valve prosthesis: Report of a case. J. Thorac. Cardiov. Surg. 50:401, 1965. 10. Hylen, J. C., Kloster, F. E., Herr, R. E., Hull, P. Q., Ames, A. W., Starr, A., and Griswold, H. E.: Phonocardiographic diagnosis of aortic ball variance. Circulation 38:90, 1968. 11. Duvoisin, G. E., Brandenburg, R. O., and McGoon, D. C.: Factors affecting thromboembolism associated with prosthetic heart valves. Circulation 35 ( Suppl 1) :70, 1967. 12. Duvoisin, G. E., and McGoon, D. C.: Unpublished data. 13. Maim, R. J., Bowman, F. O., Jr., Harris, P. D., and Kowalik, A. T. W.: An evaluation of aortic valve homografts sterilized by electron beam energy. J. Thorac. Cardiov. Surg. 54:471, 1967. 14. Angel], W. W., and Shumway, N. E.: The homograft prosthesis. Read at the Second National Conference on Prosthetic Heart Valves, Los Angeles, California, May 30 to June 1, 1968. 15. Ross, D. N.: The replacement of the aortic valve with biological tissue. Ann.
PROSTHETIC
AORTIC
VALVES
Thorac. Surg. 5:383, 1968. 16. Geha, A. S., Titus, J. L., and McGoon, D. C.: Fixation of aortic valve homografts with metal rings. J. Thorac. Cardiov. Surg. 54:605, 1967. 17. Duvoisin, G. E., Wallace, R. B., Ellis, F. H., Jr.. Anderson, M. W., and McGoon, D. C.: Late results of cardiac-valve replacement. Circulation 37 (Suppl. 2):75, 1968. 18. Favaloro, R. G., Effler, D. B., Groves, I,. K.. Suarez. E., and Shirey, E. K.: Surgi-
303 cal repair of leaking prosthetic heart valvrs. Ann. Thorac. Surg. 3:503, 1967. 19. Bigelow, W. G., Trimble, A. S.. Aldridge, H. E., Bedard, P., Spratt, E. H., and Lansdown, E. L.: The problem of insufficiency following homograft replacement of the aortic valve. J. Thorac. Cardiov. Surg. 54:478, 1967. 20. Berkson, J., and Gage, R. P.: Calculation of survival rates for cancer. Proc. Staff Meet. Mayo. Clin. 25:270, 1950.
and Disadvantages of Prosthetic Aortic Valve Replacement
Valves
By GEORGE E. DUVOISIN AND DWIGHT C. MCGOON
C
ONCERN and uncertainty are the prevalent attitudes regarding the selection of the proper substitute valve for insertion after excision of diseased cardiac valves. The current focus for this indecision is on the aortic valve, because valvular grafts have been more widely employed and acclaimed in the aortic than the mitral location and have there challenged the prosthetic valve most pointedly. The chief choice now lies between the ball-valve prosthesis, because it has achieved significant successand has been generally the most popular, and the homograft aortic valve. In spite of this uncertainty, the surgeon is forced to reach a decision by the time the diseased valve of his specific patient has been excised and the aortic root prepared for the substitute valve. A choice at that time is inescapable. Fortunately, the record of either substitute valve-the ball valve or the homograft-has been generally good, and each offers the symptomatic patient with aortic valve disease a considerably improved outlook over that which would be his plight if operation was unavailable. But precisely because this choice cannot be avoided, it is appropriate to review and weigh the advantages and disadvantages of a prosthetic substitute valve for the aortic area. PROSTHETICVALVES Sauvage and co-workers L have presented a simple classification of cardiac valves: those with central flow and those with lateral flow. With the former, there is an unobstructed central pathway, whereas with the latter, because a central obstruction remains during systole, lateral flow must occur. Central-flow valves. The natural valve is of this type. Historically and instinctively this design was the first and most promising choice. One would even now speculate that this design, contrived by nature, will be the one that will ultimately prove best for a substitute valve. The reason for the early dissatisfaction with such prosthetic valves was that the materials used, synthetic cloth or Silastic rubber, were not adequately durable to withstand the stresses of repeated flexion. Sufficiently durable materials have not yet been developed. An ideal material, if such can ever be developed, would have the properties of durability, flexibility, elasticity, nonporosity, and nonthrombogenicity, thus simulating the natural valvular cusps. In addition, it should be able to be mounted on a rigid stent or support to assure conformity of the distorted aortic root to the prosthesis and thus give assurance of the competence of the prosthesis in all instances. Lateral @co u&es. Due to the unsatisfactory performance of the prosthetic valves of natural- or central-flow design, surgeons abandoned them and began to use instead valves that allow lateral flow, the initial and persistently From
the
Mayo
Clinic
and
Mayo
Foundation,
Section
of
Surgery,
Rochester,
Minnesota.
294 PROGRESS IN CARDIOVASCULAR
DISEASES,
VOL. XI, No. 4 (JAN.), 1969
295
PROSTHETIC AORTIC VALVES
most popular of which, in the aortic area, at least, has been the ball-valve prosthesis. This transition was made easier by the realization that for certain other mechanical functions found in nature, man had made practical improvements in design, such as the jet engine over the flapping wing of the bird, and the screw over the fish’s tail. The ball valve enjoyed immediate, though imperfect, success. It was apparent from the first that the lateral-flow principle resulted in a transaortic valvular peak systolic gradient of from 5 to 40 mm. Hg, but this appeared to be well within the margin of reserve that the left ventricle could tolerate under conditions of normal activity. The lateral-flow characteristic also may be disadvantageous because the resultant accelerated flow of blood near the walls of the aortic sinuses has been implicated in the development of intimal hypertrophy of these regions, which may progress to encroach on the coronary arterial orifices. This complication has been noted only rarely. In the absence of leaks about the valve, the mildly accelerated destruction of blood elements and especially red blood cells has not caused clinically detectable difficulty. Other designs of prosthetic valves have appeared-sutureless sewing rim, the low-profile disk and lens valves, a centrally hinged “butterfly” valve, and others. But the arguments for their preferability over the ball valve were not strong enough to counter conclusively the theoretic attractiveness of the ball principle as far as durability was concerned, and also its tendency to revolve unconfined and thus to present constantly changing surfaces for contact with the cage during its excursions. It is appropriate to note that the homogrnft and heterograft valves as used clinically in most practices are also actually prostheses of the natural- or central-flow design, for they are nonvital. Even when efforts to preserve viability of homograft valves have been made, it has been difficult to prove viability. Considered as such, the structural material of these “prosthetic” grafted valves approaches the desirable qualities of the ideal material mentioned previously. CLINICAL
DATA
Kesults concerning the patient provide the most useful index of successwith a particular valve substitute. A review of the 1,530 operations performed at the Mayo Clinic up to January 1967 included 550 single aortic-valve replacements with the Starr-Edwards prosthesis, associated with a hospital mortality of 6 per cent. Aortic homografts were used in 73 instances, with hospital deaths in 7 per cent. Despite the similarity of these percentages, it should be mentioned that the homograft group represents a somewhat selected series. Homograft replacement was avoided in patients with severe coronaryartery disease because they might not tolerate the more prolonged coronary perfusion required for insertion of homograft valves. Likewise, neither patients with aneurysms of the ascending aorta nor those with severe dilatation of the aortic root were candidates for this procedure. The unfavorable influence of such factors on the Starr-Edwards group is evident when concomitant additional procedures are excluded. For instance, in 1966 there were no hos-
Table Procedure
l-Causes
or valve
Primary cause Embolism (fatal or disabling) Subacute bacterial endocarditis Aortic insufficiency Fatal anticoagulant complication Serum hepatitis Cardiac Sudden death Coronary artery disease Myocardial disease Associated valvular disease Dissecting aneurysm Pacemaker complication Noncardiac Psychiatric stntc Carcinoma Trauma Uncertain Total:
of Failure
in Aortic
Valve Replacement
Starr-Edwards (518 patients)”
Homograft ( 68 patients ) *
“3 19 14 3 3
1 0 7 0 0
18 (6)t
0 2 0
11 4
:j 1 1
0 0 0
1 2 2 2 107
0 u 0 0 10
*Number of patients surviving insertion (traced as long as 52 months in the case of Starr-Edwards valve and 27 monthes in the case of homografts). +Nrlmher of autopsies in parentheses.
pita1 deaths among 91 patients with uncomplicated aortic valve disease (that is, only calcific aortic stenosis or aortic insufficiency) treated by insertion of a Starr-Edwards prosthesis. Reports from other centers indicate that operative mortality after the use of homografts range from 6 per cent to 8 per cent,“,‘: while the range after prosthetic valve insertions is 6 per cent to 22 per cent.“-’ Once patients have survived aortic valve replacement, adverse factors may compromise the successof the long-term result (Table I ). One of the prominent problems encountered in the early experience with Starr-Edwards prostheses was subacute bacterial endocarditis, often with a fatal outcome. This problem has been nearly eliminated from present-day operations. The disappearance of the complication of infection may be associated with the general reduction in wound infections in the general hospital population and attributed to improved techniques of sterility and cleanliness and the routine use of newer more potent antibiotics. Sudden death occurred in I8 patients of the Starr-Edwards group who were doing well clinically at the time of their demise, Autopsy obtained in a third of these casesfailed to reveal the cause of the sudden fatal catastrophic event. Coronary artery disease accounted for a significant number of deaths (11 deaths), as might be expected in this age group3 There are several important considerations relative to the choice between the ball valve and the grafted valve. The ball valve seems to hold an advantage in assurance of immediate adequate function, in technical ease of insertion, in availability, and in late development of incompetence. The single advantage of the grafted valve is its lesser threat of thromboembolism. In
PROSTHETIC
AORTIC
VALVES
397
durability, a sufficient length of follow-up is not available to establish a decisive advantage for either valve. Mechanical malfunctions pertinent to the question of durability of the Starr-Edwards valve were limited at this institution to 14 instances of a leak around the prosthesis. The most serious mechanical defect in a prosthetic valvc~ concerns the recent reports of ball variance by Starr. No evidence of this malfunction in Starr-Edwards prostheses has been noted in the series at this institution, a result similar to the experience at the Cleveland Clinic.’ Hall vnriance is a poorly understood phenomenon confined to the aortic rather than the mitral Starr-Edwards prostheses. Lipid absorption by the Silastic ball causes swelling, followed by grooving, sticking, or even fracture and embolization of the poppet.:’ Early diagnosis has been stressed by Starr to allow replacement of defective valves prior to fatal prosthetic dysfunction;“’ the c>ssential basis for early detection of ball variance rests on the diminished amplitude of the first ljrosthetic sound both on clinical examination and phonecardiographic analysis. Starr reported that 20 per cent of patients with the now discontinued Model 1000 nortic prosthesis had a ball variance an nverage of 40 months after operation. Ball variance led to the change in the design of the Starr-Edwards aortic prosthesis to Slodel 2300, in which the silicone rubber ball is replaced by a hollow metal poppet composed of Stellite. This valve is also cushioned by Dacrou-covered struts which permit tissue ingrowth into the fabric. This latter fcbature resulted in aortic valve gradients averaging 30 I~IIL I-Ig in 6 patients studied 6 months or more after 3peratiou, presumably on the basis of overgrowth of neointima on the fabric.& The cloth covering has now been changed to Teflon. Thus, the metal ball eliminates the problem of ball variance, but the long-term effect of tissue ingrowth in the new Teflon-covered prosthesis is unknown. The fact that ball variance is more common in patients who have fistulae about their valves, thus causing increased trauma to the ball. indicates that details of technique of insertion of the prosthesies also may be prominent factors contributing to lutcxr ball varinuce. If this is true, the incidence of ball variance may bc higher \vhen injury to the ball has been permitted during operation, as by needle points or trauma on the cage, or where cnlcific masses or suture knots may ha\.c~ been left to project against the ball and thus injure it. Tl~~~oml~oen~bolism rcmnins the prominent cause of failure iu the StarrEd\fv:lrds group ( Table 1) . In fact, this complication was the primar). reason that some surgeons turucd to homogriift aortir-valvr replacement. The threat of rAAism has been met 1,~ using anticoagulant therapy during the postoperative period. Although the risk of embolism has been previousl!, sho\vn to 1~ the same whether or not patients received anticoagulants,11 analysis of the anticoagulant programs clearly demonstrates that the risk of embolism is rcalattd to the degree of optimd anticoagulant control as judged by prothrombin-time values. Discontinuing therapy led to the highest incidence of c~mbolism. Suboptimal prothrombin values provided some protection. but tllr most striking fact was that less than 2 ptJr cent of patients with optimal control of anticoagulant therapy suffer embolism ( Fig. I), Vnfortunately, tlris (legrec of control is difficult to achirve, iI\ only 20 per cent of :unticoagu-
DUVOISIX
AND
LfCGOON
Suboptimal
Optimal 2
3 Years
4
5
.6
postoperot1ve
Fig. l-Incidence of thromboembolism, depending on type of anticoa ulant COP Dotted lines on all graphs indicate less than 20 casesobserved i uring that period. trol.
hated patients were in this category, The risk of anticoagulant therapy is also proposed as an objection to prosthetic valves, but in our experience only three deaths in the 483 anticoagulated survivors in the Starr-Edwards group could be attributed to anticoagulant therapy (Table 1). Furthermore, difficulty in anticoagulant management su5cient to require cessation of therapy occurs in only 4 per cent of anticoagulated patients.*’ The necessity of anticoagulant therapy after insertion of the cloth-covered Model 2300 aortic prosthesis is currently under evaluation by Starr using a double-blind study:’ It has been postulated that the neointima will encapsulate the fabric of the cage and base of the valve, thus eliminating any tendency for thrombus to form and propagate. Aortic insu5ciency is the major detriment evident after insertion of homografts (TabIe 1). Factors reIevant to this condition include 1) difficult operative technique, 2) method of preservation, and 3) unfavorable aortic root changes. There is no question that aortic homografts require longer perfusion times and are technically more di5cult to insert than prosthetic valves. Furthermore, immediate competence is not assured as with prosthetic substitutes, ahhough surgeons indicate that the incidence of diastolic murmurs diminishes as experience increases. The method of preservation definitely influences the strength of the homograft. Maim and associates13have presented evidence in a well-conceived study demonstrating the superior tensile strength of homografts sterilized by exposure to a high-energy electron beam as opposed to betapropiolactone. I3 Unfortunately, the electron beam is not widely available to surgeons. The sterilization and storage methods may contribute
PROSTHETIC
AORTIC
VALVES
499
to the late degenerative changes, although this factor has yet to be fully evaluated. Problems of storage are averted with the use of fresh aortic homografts, which have shown no tendency for rejection in the hands of .-lngell and Shumway.l+ However, the problems in procuring an adequate supply of fresh valves is even more challenging than for preserved valves. In fact, Angel1 and Shumway recently indicated that they plan to use porcine heterografts because of the difhculty in obtaining fresh human aortic valves.” Late degenerative changes to which homografts are subject cause considerable concern for the eventual durability of this valve. Cusp rupture occurs in only about 2 per cent of cases2 But, more significantly, ROSS” encountered fairly heavy deposits of calcium in the aortic wall in 7 of 10 homografts inserted up to 4 years previously. In an unspecified number of valves, calcification extended to involve the aortic cusps themselves. These findings have influenced Ross to insert a pulmonary autograft to replace the aortic valve. and to use an aortic homograft to replace the pulmonary valve. M’hereas prosthetic valves are suitable for any situation involving aortic valve replacement, homografts are less satisfactory in the presence of an aneurysm of the ascending aorta, or where the aortic root is significantly dilated or deformed. One approach designed to overcome the problem of postoperative insufficiency of tissue graft valves has been to mount homografts or heterografts on rigid stcnts.lG This modification introduces an additional foreign body but provides a rigid ring that facilitates insertion and prevents insufficiency secondary to dilatation of the nortic ring. A definition of what constitutes clinically significant postoperative aortic insufficiency has yet to be standardized. Therefore it is difficult to evaluate the mass of conflicting data to obtain an accurate appraisal of its true incidence. Leaks after both prosthetic”J” and homograft insertions’,l+“,l~ have been reported by many investigators, and one can conclude only that the problem exists. ROSS’~ reported that diastolic murmurs develop in 30 per cent of patients after homograft insertion, although only 9 per cent have aortic ins&iciency. Likewise Barrutt-Boyes’ reported murmurs in 42 per cent of his operative survivors, but only 1s per cent of patients with murmurs had other evidence of aortic insufhciency. In the hlayo Clinic series, the incidence of diastolic murmurs after homograft replacement of the aortic valve ~3s considerably higher than after the use of a Starr-Edwards prosthesis ( Fig. 2). Indeed, in this respect the homograft curve parallels that of the Teflon \~lvcs that were abandoned in I(963 for reasons of mechanical failure in the proswere clearly associntcd with clinically significant thesis.’ T Not all murmurs valve malfunction. When hernodynalnically significant aortic insufficiency is defined as that situation iii which au aortic diastolic murmur is associated with a systolic blood pressure more than twice the diastolic level, then the incidence of insufficiency is suggestively higher ( p = 0.05) after homograft than after Starr-Edwards aortic valve replacement ( Fig. 3). Although this tabulation of data provides some interesting points for discussion, no decision can be made concerning the superiority of the prosthesis versus the homograft. The ultimate valve substitute will excel solely on its comparative superiority of overall clinical results. Let US then turn to clinical
Years
Fig. 2-Aortic murmurs.
valve replacement
(623 operations),
Yeats
Fig. 3-Aortic valve replacement cant aortic insufficiency.
postoperative
Actuarial
incidence
of diastolic
posfoperalive
(623 operations).
Actuarial
incidence
of sibqlifi-
data and examine what can be expected after insertion of ball \.alr.es \.ersus honlogafts. The information we seek is relatively simple: given a group of patients who have aortic valve replacement with a Starr-Edwards prosthrsis. ,\‘
PROSTHETIC
AOHTIC
v.iI,\‘ES
Starr-Edwards
04
2
3 years
Fig. 4-Aortic valve replacement
I-___4
2
5
6
portoperatlve
(623 operations).
Survivor&p
with a good result.
years later. The crucial question is: How many patients with homograft replacement have a similar good result at a comparable time interval after operation? Obviously, the longer a patient is traced, the greater the chance that death or faihrre will occur. Despite the simplicity of this approach, the actual statistical method is more complexzu Nevertheless, accurate comparison of data (such as by actuarial curves) is the essential ingredient for arriving at a choice between methods of aortic valve replacement. Comparison of the actuarial curves for the Starr-Edwards and the homograft group from the Mayo Clinic series shows that there is no significant difference between the incidence of good results achieved in each group (Fig. 4). Naturally, these results will be more meaningful when the homograft group has been followed up for a longer time and when it contains a larger number of patients. As pointed out previously, the homograft series contains a somewhat selected group of more favorable patients. The fate of homografts beyond this relatively short follow-up period has not been examined by the actuarial method, whereas hard data on the Starr-Edwards vah extends to more than 4 years of follow-up. Furthermore, it is important to recognize that the Starr-Edwards curve portraying survival with a good result tends to flatten after the first 2 years, indicating that the risk of death or a poor result is significantly reduced after that time. Therefore, we know not only that a satisfactory result is possible as long as 4 years after operation but also that the shape of the curve indicates the probable maintenance of this good result. Whether the homograft valve will parallel this good result is a matter of conjecture and concern until accurate statistical data are available. SUMMARY
The ideal substitute for the diseased aortic valve has yet to be discovered At the moment the most significant choice involves the Starr-Edwards prosthesis and the homograft aortic valve, each with its own deficiencies. Aortic
insufficiency has followed a number of homograft insertions; the threat of cusp degeneration and calcification remains as a further deterrent to widespread adoption of homografts. Thromboembolism and, more recently, ball variance in Starr-Edwards valves detract from the benefits of such technical assets as ease of insertion and dependable valvular competence. The clothcovered aortic prosthesis has been designed, but has not yet been proved, to overcome these deficiencies. An accurate statistical appraisal of clinical results has shown that either valve inserted in the patient with aortic valve disease will provide a comparable chance for a good result for the first 2 postoperative years. No further data are available for the homograft (from this institution) at this time, but actuarial curves for the Starr-Edwards prosthesis demonstrate a persistent good result for an additional 2 years. A similar lack of deterioration for homograft aortic valves can be ascertained only by the results of future analysis. ACKNOWLEDGMENT The operations reviewed Wallace, and F. H. Ellis, homografts were performed
in this report were performed by Drs. J. W. Kirklin, R. B. Jr., as well as the authors. Most of the procedures employing by Drs. J. \V. Kirklin and R. B. Wallace.
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with the ball valve prosthesis. Read at the Second National Conference on Prosthetic Heart Valves, Los Angeles, California, May 30 to June 1, 1968. 9. Ablaza, S. C. G., Blanco, G., Maranhao. V., and Goldberg, EL: Fatal extrusion of the ball from a Starr-Edwards aortic valve prosthesis: Report of a case. J. Thorac. Cardiov. Surg. 50:401, 1965. 10. Hylen, J. C., Kloster, F. E., Herr, R. E., Hull, P. Q., Ames, A. W., Starr, A., and Griswold, H. E.: Phonocardiographic diagnosis of aortic ball variance. Circulation 38:90, 1968. 11. Duvoisin, G. E., Brandenburg, R. O., and McGoon, D. C.: Factors affecting thromboembolism associated with prosthetic heart valves. Circulation 35 ( Suppl 1) :70, 1967. 12. Duvoisin, G. E., and McGoon, D. C.: Unpublished data. 13. Maim, R. J., Bowman, F. O., Jr., Harris, P. D., and Kowalik, A. T. W.: An evaluation of aortic valve homografts sterilized by electron beam energy. J. Thorac. Cardiov. Surg. 54:471, 1967. 14. Angel], W. W., and Shumway, N. E.: The homograft prosthesis. Read at the Second National Conference on Prosthetic Heart Valves, Los Angeles, California, May 30 to June 1, 1968. 15. Ross, D. N.: The replacement of the aortic valve with biological tissue. Ann.
PROSTHETIC
AORTIC
VALVES
Thorac. Surg. 5:383, 1968. 16. Geha, A. S., Titus, J. L., and McGoon, D. C.: Fixation of aortic valve homografts with metal rings. J. Thorac. Cardiov. Surg. 54:605, 1967. 17. Duvoisin, G. E., Wallace, R. B., Ellis, F. H., Jr.. Anderson, M. W., and McGoon, D. C.: Late results of cardiac-valve replacement. Circulation 37 (Suppl. 2):75, 1968. 18. Favaloro, R. G., Effler, D. B., Groves, I,. K.. Suarez. E., and Shirey, E. K.: Surgi-
303 cal repair of leaking prosthetic heart valvrs. Ann. Thorac. Surg. 3:503, 1967. 19. Bigelow, W. G., Trimble, A. S.. Aldridge, H. E., Bedard, P., Spratt, E. H., and Lansdown, E. L.: The problem of insufficiency following homograft replacement of the aortic valve. J. Thorac. Cardiov. Surg. 54:478, 1967. 20. Berkson, J., and Gage, R. P.: Calculation of survival rates for cancer. Proc. Staff Meet. Mayo. Clin. 25:270, 1950.