Original Article
Twelve-Year Comparison of a Bjork–Shiley Mechanical Heart Valve with Porcine Bioprostheses
N Engl J Med 1991; 324:573-579February 28, 1991
- Abstract
- Abstract
Background.
Patients undergoing heartvalve replacement may receive a mechanical prosthesis, necessitating lifelong anticoagulant treatment, or a porcine bioprosthesis, which involves no absolute need for anticoagulants.
Methods.
We carried out a randomized, prospective trial to compare the durability of the Bjork—Shiley mechanical prosthesis (spherical tilting-disk model) and the incidence of valve-related complications with those variables in the Hancock and the Carpentier-Edwards porcine prostheses. The mitral valve was replaced in 261 patients, the aortic valve in 211, and both in 61; the survivors have been followed up for a mean of 12 years.
Results.
We found a trend toward improved actuarial survival after 12 years with the Bjork—Shiley prosthesis, but this trend was not statistically significant (group with Bjork—Shiley valve vs. group with porcine valve [mean ±SE], 51.5±3.2 vs. 44.4±3.2 percent; P = 0.08). There was no significant difference in the actuarial incidence of reoperation after 5 years, but after 12 years significantly more patients with a porcine prosthesis had undergone reoperation (8.5±2.0 vs. 37.1 ±4.1 percent, P<0.001). An analysis combining death and reoperation as end points for an actuarial assessment of survival with the original prosthesis intact confirmed that the patients with Bjork—Shiley prostheses had improved survival (48.6±3.2 vs. 30.0±3.0 percent after 12 years, P<0.001). Bleeding requiring hospitalizaron or blood transfusion was significantly more frequent in the patients with Bjork—Shiley prostheses (18.6±3.2 vs. 7.1 ±2.3 percent after 12 years, P<0.01). There was no significant difference after 12 years in the actuarial occurrence of embolism (Bjork—Shiley vs. porcine, 21.1 ±3.1 vs. 26.4±3.5 percent) or endocarditis (3.7±1.4 vs. 4.6±1.6 percent).
Conclusions.
Survival with an intact valve is better among patients with the Bjork—Shiley spherical tilting-disk prosthesis than among patients with porcine bioprostheses, but use of the Bjork—Shiley valve carries an attendant increased risk of bleeding associated with the need for anticoagulant treatment. (N Engl J Med 1991; 324:573–9.)
Media in This Article
Figure 1
Actuarial Survival among All Patients after 12 Years.Figure 2
Actuarial Survival among All Patients with Original Prosthesis Intact (I.e., Survival without Reoperation) after 12 Years.Article Activity
- Article
WE have previously reported the initial results of a randomized trial begun in 1975 that compared mechanical with porcine prostheses for heartvalve replacement. After a median follow-up of 5.6 years, there were no significant differences in the rates of overall survival, valve failure, or valve-related complications.1
The Department of Veterans Affairs (formerly the Veterans Administration) began a similar study in 1977, and also found no significant difference in survival after a mean follow-up of five years. There was, however, a significantly lower incidence of valve-related complications in patients receiving a porcine prosthesis, mainly because patients receiving a mechanical prosthesis had an increased risk of bleeding associated with anticoagulation.2
Degeneration of bioprosthetic heart valves is more common five or more years after valve replacement.3 4 5 6 7 We report on survival, valve failure, and valve-related complications in the Edinburgh Heart Valve Trial after a mean follow-up period of 12 years.
Methods
The patients and methods have been described in detail previously.1 In brief, 541 patients undergoing valve replacement in the period from 1975 to 1979 were randomly assigned to receive either a mechanical Bjork—Shiley 60-degree spherical tilting-disk valve or a porcine bioprosthetic valve. The Bjork—Shiley spherical disk valve preceded the convexo-concave model, which was introduced in 1979 and recently withdrawn from the market because of an unacceptably high rate of strut fracture. Two hundred sixty-one patients underwent mitral-valve replacement, 211 underwent aortic-valve replacement, and 61 received both valves. Eight patients required tricuspid-valve replacement; because of the small number of these patients, they were excluded from further analysis in this report and our initial report.1 (In our previous report we had not identified some errors in our computerized data base. The current report therefore includes one patient with aortic-valve replacement who was omitted from the previous report and one patient undergoing mitral-valve and aortic-valve replacement previously identified as undergoing only mitral-valve replacement.)
At the time of surgery, if there were no technical reasons that precluded random assignment of a patient to a type of prosthesis —e.g., the presence of a small aortic root — an envelope was opened that indicated prosthesis assignment. Randomization was performed in groups of 10 patients to ensure an even distribution of patients among the three participating surgeons and among treatment groups if the trial had to be terminated prematurely. Of the 533 patients studied, 267 received the Bjork—Shiley prosthesis. Initially, the patients assigned to a porcine bioprosthesis received a Hancock prosthesis (107 patients), but after January 1977 such patients received a Carpentier—Edwards prosthesis because of a substantial cost advantage (159 patients). The clinical characteristics of the patients are summarized in Table 1Table 1
Base-Line Characteristics of Patients Undergoing Replacement of the Aortic Valve, Mitral Valve, or Both, with a Bjork—Shiley or Porcine Prosthesis..Postoperative Anticoagulation
All patients received warfarin for two months after surgery, and patients with Bjork—Shiley prostheses continued to receive warfarin throughout the study. Patients with a porcine bioprosthesis subsequently received warfarin at the discretion of their cardiologist. The therapeutic prothrombin-time ratio used in the United Kingdom ranges from 2.0 to 4.5 and is based on human brain thromboplastin.8
Collection of Follow-up Data
At each hospital follow-up visit, the patient's functional class (New York Heart Association classification), drug therapy, physical findings, chest radiographs, and electrocardiograms were evaluated. Follow-up assessment was conducted by the referring cardiologist or the local, hospital-based physician for the majority of patients and by the family doctor for a small minority. Since our initial report on follow-up for a median of 5.6 years,1 all patients have also completed an annual questionnaire, with subsequent clarification by telephone from the patient or family doctor if necessary. The questionnaires were analyzed by a single observer blinded to the type of prosthesis implanted.
Valve-Related Complications
A minor embolism was indicated by an episode of transient neurologic deficit, amaurosis fugax, limb ischemia of sudden onset, or any other clinical event that could reasonably be attributed to embolism. A major embolism was defined as an embolism resulting in residual neurologic deficit, limb ischemia requiring surgery, or death. Major bleeding was defined as bleeding necessitating hospitalization or blood transfusion. Minor bleeding was not recorded during the initial five years of follow-up but was identified according to the answers to the annual questionnaire sent later to all surviving patients; it was indicated by epistaxis, hematuria, severe bruising, or gastrointestinal bleeding so severe that the patient notified his or her doctor.9 Infective endocarditis was recorded if it was diagnosed clinically and required parenteral antibiotics or if it was found at reoperation or at autopsy. A reoperation was recorded if prosthetic-valve replacement was necessary because of symptomatic valve dysfunction, a leak near the prosthesis, or infective endocarditis. A reoperation for replacement of a native valve or for coronary-artery bypass surgery was not recorded as a complication related to a prosthetic valve. Deaths occurring after hospital discharge after initial valve replacement were classified as "cardiac," "noncardiac," or "possibly cardiac" on the basis of stated clinical data and information obtained at autopsy. Information from the death certificate filed with the Registry of Deaths for Scotland was available for patients (the majority) whose death occurred in Scotland.
Statistical Analysis
Statistical analysis was largely performed as previously described, according to standard methods for assessing survival.1 The end points considered were death, reoperation, and the development of certain valve-related complications. Values for the actuarial occurrence of events at any point in the study are expressed as means ±SE. Any patient undergoing reoperation for prosthetic-valve replacement was considered to be lost to follow-up at that time with respect to the valve-related complications, but continued to be included for the assessment of survival. Only the first occurrence of any specified complication was considered in analysis. To obtain an estimate of "valve survival" (survival with the prosthesis intact), the occurrence of either death or reoperation for valve failure also served as an end point. The log-rank test was used to test for differences between the types of valves in all of the above end points. In addition, proportional-hazard models10 were used to investigate the following variables as risk factors in valve survival: age, sex, functional class, the site of the valve being replaced, atrial fibrillation, previous valve surgery, previous embolism, and pathogenesis of valve disease.
All surviving patients were followed at least until 1989 (except five patients who were lost to follow-up), and the mean duration of follow-up has been 12 years. An initial analysis comparing the two types of porcine valves showed their results to be very similar; there were no statistically significant differences between the groups with these valves in any of the end points considered in this study. For example, actuarial survival among patients receiving the Hancock prosthesis was 44.5 ±4.8 percent after 12 years, as compared with 43.8±4.3 percent among those receiving the Carpentier—Edwards prosthesis. Furthermore, there was no statistically significant difference in any end point between the patients receiving the Bjork—Shiley prosthesis in the corresponding early and late periods of the trial. We therefore present the results in patients receiving the Bjork—Shiley prosthesis, as compared with those receiving a porcine prosthesis.
Late Postoperative Deaths
Of the 533 patients included in our analysis, 44 died before leaving the hospital, as described in our previous report (Table 2Table 2
Deaths after Operation in Patients Receiving a Bjork—Shiley or Porcine Prosthesis for Aortic-Valve, Mitral-Valve, or Combined Aortic-Valve—Mitral-Valve Replacement.). There have been 235 deaths after hospital discharge (late deaths), including those occurring after reoperation. The causes of these late deaths (cardiac, possibly cardiac, noncardiac, or unknown) are summarized in Table 2. Many possibly cardiac deaths were sudden, information from an autopsy was not usually available, and the mode of death could not be precisely determined from the death certificate. Death was due to a noncardiac cause in 24 patients with the Bjork—Shiley prosthesis and in 25 patients with a porcine prosthesis. Of the 49 noncardiac deaths, 28 were due to carcinoma, 11 to pneumonia or obstructive airways disease, 4 to disorders manifested by dementia or diffuse cerebrovascular disease, 3 to trauma or suicide, 2 to peripheral vascular disease, and 1 to pyelonephritis.Analysis of the actuarial survival of all patients in the study showed a trend toward better survival among those with a Bjork—Shiley prosthesis (Fig. 1Figure 1
Actuarial Survival among All Patients after 12 Years.), but this trend was not statistically significant (patients alive after 12 years [group with Bjork—Shiley valve vs. group with porcine valve], 51.5±3.2 vs. 44.4±3.2 percent; P = 0.08) (Table 3Table 3
Actuarial Survival and Occurrence of Valve-Related Events after 5 Years and 12 Years in Patients Receiving a Bjork—Shiley or Porcine Prosthesis.).Similarly, analysis of actuarial survival after 12 years (Table 3) showed no significant difference between the types of prostheses in the subgroups of patients undergoing mitral-valve replacement (Bjork—Shiley vs. porcine, 44.2±4.9 vs. 40.4±4.4 percent) or Aortic Valve replacement (56.1 ±4.9 vs. 52.3±5.4 percent).
Late Postoperative Events
Reoperation
Eighty-five patients have undergone reoperation for prosthetic-valve replacement — 17 patients with the Bjork—Shiley valve and 68 with porcine prostheses (27 with the Hancock valve and 41 with the Carpentier—Edwards valve). After 5 years there was no significant difference in the actuarial incidence of reoperation, but after 12 years the incidence was significantly increased in patients with porcine prostheses (Bjork—Shiley vs. porcine, 8.5±2.0 vs. 37.1±4.1 percent) (Table 3). There was also a significant difference in the actuarial incidence of reoperation after 12 years (Table 3) in the subgroups undergoing single mitral-valve replacement (Bjork—Shiley vs. porcine, 9.9±3.2 vs. 43.1 ±6.0 percent) or single aortic-valve replacement (4.2±2.1 vs. 22.6±5.7 percent).
Mechanical valve failure may lead to death before reoperation can be performed. We therefore conducted an analysis in which death or reoperation served as the end point for an actuarial assessment of valve survival. This analysis also showed no significant difference after 5 years, but did reveal significantly better survival with the original prosthesis intact after 12 years in patients who had undergone replacement with a Bjork—Shiley valve (Bjork—Shiley vs. porcine, 48.6±3.2 vs. 30.0±3.0 percent) (Fig. 2Figure 2
Actuarial Survival among All Patients with Original Prosthesis Intact (I.e., Survival without Reoperation) after 12 Years. and Table 3). An actuarial analysis of death and reoperation among patients undergoing single mitral-valve replacement showed no difference after 5 years but demonstrated significantly improved survival with the original prosthesis intact after 12 years in those receiving a Bjork—Shiley prosthesis (Bjork—Shiley vs. porcine, 41.6±4.7 vs. 23.8±3.8 percent) (Fig. 3Figure 3
Actuarial Survival among Patients with Original Prosthesis Intact after 12 Years, According to Site of Implantation (Aortic vs. Mitral Valve). and Table 3). Analysis of these events in patients undergoing single aortic-valve replacement showed similar results, with no significant difference after 5 years and a trend in favor of those with the Bjork—Shiley prosthesis after 12 years (Bjork—Shiley vs. porcine, 55.2±5.0 vs. 41.1 ±5.4 percent) (Fig. 3 and Table 3), but this trend did not quite reach the conventional level for statistical significance (P = 0.052).Multivariate analysis confirmed that the rate of reoperation with porcine prostheses was significantly higher in all treatment groups, and that the rate of reoperation was significantly higher in patients undergoing single mitral-valve replacement than in those undergoing single aortic-valve replacement. The age of the patient was also an important variable in predicting reoperation; younger patients were more likely to require reoperation, with the relative risk of reoperation increasing by 55 percent for each 10 years, continuously over the whole range of ages studied.
In almost all cases of reoperation for failure of a porcine valve, failure was due to severe regurgitation caused by rupture of one or more of the cusps of the prosthesis and, in a minority of cases, by severe stenosis and calcification of the cusps. Fourteen patients with both aortic-valve and mitral-valve porcine prostheses required reoperation, 12 because of structural deterioration of the valve; an aortic prosthesis had failed in 2 patients, a mitral prosthesis in 7, and both prostheses in 3.
Sixteen patients undergoing reoperation died before discharge, for a hospital mortality rate of 19 percent. Of the 69 patients in whom reoperation was successful, 19 subsequently died (Table 2). Actuarial survival after reoperation was 73.4±4.9 percent after one year and 64.1 ±5.6 percent after three years, as compared with 84.6±1.6 percent after one year and 77.7±1.8 percent after three years for the initial operation.
Bleeding
Eighty-one episodes of bleeding were recorded in 65 patients, including 55 episodes of major bleeding in 41 patients. The actuarial incidence of both major bleeding and all bleeding was significantly higher in the patients with the Bjork—Shiley prosthesis (Table 3). As noted above, minor episodes of bleeding were not effectively recorded in the first five years of observation but were recorded from the answers to the annual questionnaire sent in the latter part of the study. There was a significant difference in favor of porcine prostheses in the actuarial incidence of major bleeding and all bleeding after 5 and 12 years in patients receiving a single aortic-valve replacement, but not in those receiving a single mitral-valve replacement (Table 3). The protocol for anticoagulation in patients with porcine prostheses varied, but overall the 132 patients with porcine mitral prostheses took warfarin for 25 percent of the total patient-years of follow-up and the 102 patients with porcine aortic prostheses took warfarin for 15 percent of this period.
Embolism
A total of 114 episodes of embolism were recorded in 90 patients, including 40 episodes of major embolism in 35 patients. There was no important difference in the actuarial incidence of major embolism or any embolism between the Bjork—Shiley-valve group and the porcine-valve group after 5 years or 12 years (Table 3). There was a significantly lower actuarial incidence of major embolism with the Bjork—Shiley prosthesis among patients undergoing single aortic-valve replacement, but this was balanced by a lower incidence of major embolism with the porcine prostheses among those undergoing single mitral-valve replacement (Table 3).
Bacterial Endocarditis
Bacterial endocarditis occurred on 25 occasions in 19 patients, occurring twice in 6 patients. Five patients were successfully treated medically, and seven died without reoperation. Seven patients underwent reoperation, during which one died. Only 5 of the 12 patients who were successfully treated medically or surgically remain alive. There was no difference in the actuarial incidence of endocarditis between the prosthesis groups (Table 3).
All Major Events
Actuarial analysis of survival free from all major events — i.e., death, reoperation, major bleeding, major embolism, and endocarditis — showed no significant difference after five years. However, it did show a significant difference in favor of the Bjork—Shiley prosthesis after 12 years in all patients with this type of valve and in the subgroup undergoing single mitral-valve replacement, but not in the subgroup undergoing single aortic-valve replacement (Table 3).
Discussion
After a mean follow-up period of 12 years in a large, prospective, randomized trial, we observed a trend toward improved actuarial survival among patients undergoing valve replacement with the Bjork—Shiley spherical tilting-disk prosthesis as compared with patients receiving the Hancock or Carpentier—Edwards porcine prosthesis, but this trend did not reach statistical significance. Although overall mortality in these two groups was not significantly different, the need for reoperation involving valves implanted in both the aortic and mitral positions was significantly higher in the group receiving porcine valves, because of an increased incidence of valve failure five or more years after implantation. The most common reason for failure of the porcine valves was cusp rupture resulting in prosthetic-valve regurgitation. Jamieson et al. found an increased incidence of porcine-valve failure in children and young adults,11 and we also found an increased incidence of reoperation in the younger patients in the present study.
We considered the possibility that confounding factors might operate so that the failure rate of porcine valves might appear to be increased as compared with the failure rate of Bjork—Shiley valves. A Bjork—Shiley valve may fail suddenly, causing death before the patient can be brought to surgery.12 In most instances of sudden death in our study an autopsy was not performed, and some of these deaths may have been due to sudden but unrecognized failure of the prosthetic valve.13 No cases of strut failure were identified in our study; however, autopsy data were not always available for cases of sudden death. Failure of bioprosthetic valves is usually gradual; the patient is likely to present with symptoms and is thus more likely to undergo operation. We therefore performed an actuarial analysis in which we used death or reoperation as an end point for assessment of valve survival. This analysis also showed that valve survival was significantly improved in patients with the Bjork—Shiley prosthesis, both in the whole group and in the subgroup undergoing single mitral-valve replacement, but the difference did not quite reach statistical significance in the subgroup undergoing single aortic-valve replacement. Failure of the porcine prostheses occurred more frequently when they were implanted in the mitral position, and indeed when reoperation for valve failure was required in patients with both aortic and mitral porcine prostheses, the mitral prosthesis was found to have failed twice as often as the aortic prosthesis — a finding also reported in other studies.14 15 16 All patients in our study received the spherical tilting-disk (60-degree) model of the Bjork—Shiley valve, and our results cannot necessarily be extrapolated to other models of this prosthesis, such as the convexo-concave valve recently withdrawn from the market because of problems with strut fracture.
In our previous report we showed the treatment groups to be well balanced,1 but the trial was potentially compromised by the change in the type of porcine prosthesis used after the trial had begun. This introduced the possibility that the time of operation might bias results.17 , 18 The results of replacement with the Bjork—Shiley valve or the two types of porcine valve were not, however, significantly different during the early and late periods of the trial.1
The differences that we have observed between prostheses became apparent only after follow-up beyond five years, which emphasizes the importance of prolonged periods of observation in the evaluation of heart-valve prostheses. Unrandomized studies with extended periods of follow-up have also reported a high incidence of failure of porcine prosthetic valves five or more years after implantation.4 5 6 7
Embolism, bleeding, and bacterial endocarditis are important complications that can occur after heartvalve replacement. Both major and minor episodes of bleeding were more common in our patients who had received a Bjork—Shiley prosthesis, all of whom received anticoagulants, but the incidence of embolism and bacterial endocarditis was equally distributed. Reoperation for replacement of a failing prosthesis was associated with a hospital mortality rate of 19 percent and with increased late mortality in survivors. The increased risk of reoperation would appear to be a high price to pay for the reduced risk of bleeding through avoidance of the use of anticoagulants.
In the Veterans Affairs Cooperative Study2 similar to ours, 575 patients were randomly assigned to receive either a Bjork—Shiley or a Hancock prosthesis for single valve replacement. After a mean follow-up of five years there was no significant difference in survival between the two groups. In that study, bleeding, embolism, endocarditis, and valve malfunction resulting in either death or reoperation were considered major events. Actuarial analysis showed significantly improved "event-free" survival in patients in whom a porcine prosthesis was implanted in the mitral position, because of a much higher incidence of bleeding in patients who received the Bjork—Shiley prosthesis. The other events were equally distributed. We believe that all valve-related complications cannot be considered equally,19 but to facilitate comparison of our study with the Veterans Affairs study, we similarly compared freedom from death, reoperation, major bleeding, major embolism, and endocarditis as end points for actuarial analysis. We found a significantly increased incidence of these events in the patients with porcine prostheses among all patients studied and in the subgroup undergoing single mitral-valve replacement. This was due to an increased incidence of reoperation after five years, which more than offset the increased incidence of major bleeding in patients receiving a Bjork—Shiley prosthesis. There was no significant difference in actuarial occurrence of death and major events in patients undergoing single Aortic Valve replacement. Rahimtoola3 has noted that the higher incidence of bleeding in the Veterans Affairs study than in ours is probably due to a higher level of anticoagulation in that study and to differences in "patient-related and health care delivery factors." In the patients in the Veterans Affairs study who received warfarin, anticoagulation was initially controlled by keeping the prothrombin-time ratio in the range of 2.0 to 2.5; we used a range based on that recommended in the United Kingdom — 2.0 to 4.5, approximately equivalent to a range of 1.3 to 2.0 with reagents commonly used in the United States.20 , 21
What recommendations can be made from the results of our study about the selection of a type of prosthesis for patients requiring valve replacement? We would recommend that patients undergoing mitral-valve replacement should receive a mechanical prosthesis. Patients in whom the risk posed by anticoagulants is unacceptably high may receive a bioprosthesis, but at the increased risk of a further operation at a later date. Apart from implantation in this very small group, insertion of a porcine bioprosthesis in the mitral position appears to be contraindicated. We have not shown such a clear advantage to the use of a mechanical prosthesis in patients undergoing single aortic-valve replacement, but would recommend the use of a mechanical prosthesis in young patients, in whom the risk of porcine-valve failure is higher and for whom the durability of the prosthesis is of paramount importance. Porcine valves may be considered for elderly patients whose life expectancy may not exceed that of the prosthesis used.
Supported in part by the Scottish Home and Health Department.
Source Information
From the Departments of Cardiology (P.B., H.C.M.) and Cardiac Surgery (D.J.W.), Royal Infirmary of Edinburgh, and the Medical Statistics Unit (R.J.P.), University of Edinburgh. Address reprint requests to Dr. Miller at the Department of Cardiology, Royal Infirmary, Edinburgh EH3 9YW, Scotland.
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