After a mean follow up period of 20 years in a large prospective randomised trial we have observed no difference in survival among patients undergoing valve replacement with the Bjork-Shiley tilting disc prosthesis as compared with the Hancock or Carpentier-Edwards porcine prostheses. In our previous report after a mean follow up period of 12 years we had observed a trend towards improved survival with the Bjork-Shiley prosthesis, but this trend did not continue and with further follow up the survival curves have converged. Although there was no difference in overall mortality, there were important differences in morbidity.
The need for reoperation was significantly greater for all patients who had received a porcine prosthesis. As in our previous report we considered that there might be confounding factors to bias the comparison of the failure rates of porcine and Bjork-Shiley valves. Occasionally a valve may fail suddenly, causing death before the patient can be brought to surgery, and as necropsy was rarely done in this study it is possible that some failures of prosthetic valves went unrecognised. We therefore undertook an analysis with death or reoperation as an end point for assessment of valve survival. This analysis showed that valve survival was significantly better in patients with the Bjork-Shiley prosthesis in the whole group and in each of the subgroups undergoing single aortic valve, single mitral valve, and combined mitral valve and aortic valve replacement.
In our previous report, at a mean follow up of 12 years, improved valve survival in patients undergoing single mitral valve replacement had become apparent at 8–10 years2
; in the current report after a mean follow up of 20 years, significantly improved valve survival in those undergoing single aortic valve replacement became apparent after 12–14 years. In the smaller subgroup of patients undergoing combined mitral valve and aortic valve replacement, improved valve survival with the Bjork-Shiley prosthesis became apparent after 8–10 years and became significant with the accumulation of more prolonged follow up data.
Non-randomised observational studies with extended periods of follow up have also reported a high incidence of failure of porcine prosthetic valves, with failure occurring more commonly and earlier in patients undergoing mitral valve replacement than aortic valve replacement.3–5
When reoperation for valve failure was required in patients in our series with both aortic and mitral porcine prostheses, the mitral prosthesis was found to have failed more than twice as often as the aortic prosthesis—a finding also reported in other studies.6,7
Reoperation for valve failure was associated with substantial mortality: 14% at 30 days and 22% at one year after reoperation. Even though the 30 day mortality of 9% in the later years of follow up was only half of that before 1987, reoperation carried a high risk of death. Our results are comparable with the UK heart valve registry which reports a 30 day mortality rate for reoperation of 16.6% over the years 1986 to 1997.8
From a single US centre, Akins and colleagues reported a mortality for reoperation of 6.8% for mitral, 7.8% for aortic, and 14.3% for double prosthetic valve replacement over the period 1985 to 1997.9
Bleeding occurred more often in patients receiving the Bjork-Shiley prosthesis, all of whom received oral anticoagulants. Major bleeding episodes were more common with the Bjork-Shiley prosthesis in the single aortic valve and single mitral valve groups, but there was no significant difference in the group undergoing combined mitral valve and aortic valve replacement, which involved a smaller number of patients. When all episodes of bleeding (both major and minor) were considered, there was a higher incidence of bleeding with the Bjork-Shiley prosthesis in patients undergoing single aortic valve replacement but not in those undergoing single mitral valve replacement. This may reflect the higher proportion of patients undergoing single mitral valve replacement with a porcine prosthesis who were also receiving warfarin. With increasing time from operation a greater proportion of porcine valve replacement patients received warfarin. This was mainly because of patient factors such as the development of atrial fibrillation or chamber enlargement rather than factors directly related to the prosthetic valve.
The increased risk of bleeding in patients receiving a Bjork-Shiley prosthesis partly offsets the increased risk of reoperation in those receiving a porcine prosthesis. We found no difference in the occurrence of embolism or endocarditis between the groups. Analysis of survival free from a major event showed a significantly better outcome with the Bjork-Shiley prosthesis in the group undergoing single mitral valve and combined aortic valve and mitral valve replacement, but the difference did not reach significance in the single aortic valve replacement group. The level of anticoagulation required with the current generation of bileaflet mechanical prostheses is lower than for the Bjork-Shiley prosthesis10–12
and the risk of bleeding is reduced, so the relative benefit of mechanical prostheses may now be even more pronounced.
The Department of Veterans Affairs has carried out a similar study to our own, starting in 1977 and comparing the Bjork-Shiley with the Hancock porcine prosthesis in an all male population of patients undergoing single valve replacement.13,14
After a follow up period of 15 years Hammermeister and colleagues reported significantly improved survival in patients who had undergone aortic valve replacement with the Bjork-Shiley prosthesis.15
After 15 years, 34% of those receiving a Bjork-Shiley prosthesis versus 21% of those receiving a Hancock bioprosthesis were still alive. As in our study, Hammermeister and colleagues found no difference in survival in those patients undergoing mitral valve replacement. The difference in survival in patients who had undergone aortic valve replacement with the Hancock bioprosthesis compared with the mechanical valve was, they found, “probably due to more deaths from primary valve failure (8 v
0)”. They also found that “almost all of the excess deaths with bioprosthesis after aortic valve replacement occurred in the 10 to 15 year time period”. Both an increased need for reoperation in patients who had undergone aortic valve replacement with a bioprosthesis and inferior valve survival after about 10 years compared with the mechanical valve were also identified in our study. The Veterans Affairs study enrolled twice as many patients undergoing single aortic valve replacement as in our study, and all were male. Thus, the greater statistical power afforded by a larger cohort and the influence of other patient variables may have operated to produce the difference in survival seen in the Veterans Affairs study. Hammermeister and colleagues also reported an increased risk of bleeding in the Veterans Affairs study in those patients receiving the Bjork-Shiley prosthesis compared with those receiving a bioprosthesis. The absolute risks of bleeding in the Veterans Affairs study were higher than we observed. This may reflect a higher intensity of anticoagulation used in the USA during the major period of the trial compared with that used in the UK, owing to differences in the thromboplastins used to standardise the prothrombin assay.15,16
There was no difference in the incidence of endocarditis or embolism.15,17
Peterseim and colleagues recently reported a large retrospective study comparing outcomes in patients undergoing aortic valve replacement; they found no difference in survival after 10 years of follow up but a significantly increased incidence of reoperation in patients receiving a porcine prosthesis compared with those receiving a St Jude mechanical prosthesis.18
Our study emphasises the need for prolonged follow up of patients undergoing valve replacement in order to show significant differences between treatment groups. Trends towards improved survival seen after 12 years did not develop further with prolonged follow up, probably as a result of the overwhelming effect of patient variables in this aging group of individuals. However, important differences in valve survival did become apparent in all treatment groups during this extended period of follow up.
On the basis of this study, we recommend that patients undergoing single mitral valve or combined aortic valve and mitral valve replacement should receive mechanical prostheses because of their superior durability compared with bioprostheses. Furthermore many such patients will have, or will develop, coexisting conditions such as atrial fibrillation for which anticoagulation is necessary. Patients receiving a single aortic valve replacement who would be expected to live 10 years or more should receive a mechanical valve unless anticoagulation treatment is contraindicated. Our results also have important implications for health care provision, particularly in the UK where there are long waiting lists for cardiac surgery. Patients requiring prosthetic valve replacement use scarce cardiac surgery resources. Mechanical valves have proved to be more durable over several generations of prostheses19
over prolonged periods of follow up, and their use should reduce the need for further cardiac surgery in those in whom they are implanted.