Studies Comparing Mechanical With Biological Prostheses

There have been two large randomised trials comparing results of mechanical valve with porcine valve replacement. Both trials used the Bjork-Shiley mechanical valve before the introduction of the convexo-concave model which subsequently proved liable to strut fracture. The Department of Veterans Affairs (VA) trial randomised 575 male patients undergoing single valve replacement in 13 centres between 1977 and 1982 to receive either a Bjork-Shiley tilting disc prosthesis or a Hancock porcine prosthesis. Three hundred and ninety four patients underwent aortic valve replacement and 181 patients mitral valve replacement. All patients receiving the Bjork-Shiley prosthesis received anticoagulants, but for those patients receiving a porcine prosthesis only those requiring anticoagulants for another reason (for example, atrial fibrillation) received warfarin. After a mean duration of follow up of 15 years Hammermeister and colleagues9 reported significantly improved survival at 15 years for those who had undergone aortic valve replacement with a Bjork-Shiley prosthesis (79% v 66%), but no significant difference for those who had undergone mitral valve replacement (fig 13.2). There was a significantly increased risk of reoperation with the Hancock prosthesis, both for patients who had undergone aortic valve and mitral valve replacement. There was no significant difference in the occurrence of thromboembolism or endocarditis, but there was a significantly greater occurrence of major bleeding with those receiving a Bjork-Shiley prosthesis as a result of the greater use of anticoagulants.

The Edinburgh heart valve trial randomised male and female patients undergoing valve replacement between 1975 and 1979 to receive a Bjork-Shiley or porcine (Hancock or Carpentier-Edwards) prosthesis.10 After a mean follow up period of 20 years, we reported results in 533 patients; 261 patients who had undergone mitral valve replacement, 211 aortic replacement, and 61 combined aortic and mitral valve replacement.11 We found no difference in patient survival between biological or mechanical valve recipients when all patients were considered together or when the subgroups

Years after valve replacement

Figure 13.2 The Veterans Affairs randomised trial comparing outcome following valve replacement with a mechanical (Bjork-Shiley) versus a porcine bioprosthetic valve. Cumulative mortality curves show significantly higher mortality over 15 years of follow up with bioprostheses for those undergoing aortic valve replacement (AVR). For mitral valve replacement (MVR) cumulative mortality was initially higher with mechanical prostheses but with more prolonged follow up the mortality curves converged. Reproduced from Hammermeister K, etal. JAm Coll Cardiol 2000:36;1152-8, with permission of the publisher.

Years after valve replacement

Figure 13.2 The Veterans Affairs randomised trial comparing outcome following valve replacement with a mechanical (Bjork-Shiley) versus a porcine bioprosthetic valve. Cumulative mortality curves show significantly higher mortality over 15 years of follow up with bioprostheses for those undergoing aortic valve replacement (AVR). For mitral valve replacement (MVR) cumulative mortality was initially higher with mechanical prostheses but with more prolonged follow up the mortality curves converged. Reproduced from Hammermeister K, etal. JAm Coll Cardiol 2000:36;1152-8, with permission of the publisher.

undergoing aortic valve replacement, mitral valve replacement, and combined aortic and mitral valve replacement were considered separately. As in the VA study we found an increased need for reoperation with the porcine prostheses. An actuarial analysis using death or reoperation as combined end points showed a lower event rate and therefore improved valve survival with the Bjork-Shiley prosthesis. The increased need for re-operation for valve failure occurred after 8-10 years in those who had received a porcine mitral prosthesis, and at 10-12 years in those who had received a porcine aortic prosthesis (fig 13.3). Interestingly, when a patient who had received combined aortic and mitral valve replacement with porcine prosthesis required reoperation for valve failure, it was invariably the mitral prosthesis which had failed. There was a significantly increased risk of bleeding in those with the Bjork-Shiley prosthesis. When we performed an analysis examining the combined end points of death, reoperation, endocarditis, major embolism, and major bleeding as end points, we found survival free from major events was significantly better in those who received a Bjork-Shiley prosthesis.

INFLUENCE OF PATIENT'S AGE ON DURABILITY OF PORCINE PROSTHESES

Biological valves have a higher failure rate in younger patients. Burdon and colleagues7 found that after 15 years of follow up only a third of patients who had received a bioprosthesis for

U 0 2 4 6 8 1012141618 20 Years after randomisation

AVR: Bjork-Shiley 109 92 85 78 72 68 58 51 46 37 20 Porcine 102 83 81 77 67 60 44 33 25 17 7

MVR: Bjork-Shiley 129 99 85 76 71 63 53 44 40 32 19 Porcine 132 101 90 81 69 39 27 16 10 6 3

Figure 13.3 The Edinburgh heart valve trial. Cumulative occurrence of death or reoperation for patients undergoing aortic valve replacement or mitral valve replacement. Significantly more patients receiving a porcine prosthesis had one of these events compared with those receiving the mechanical Bjork-Shiley prosthesis. The curves separated at 8-10 years for mitral valve replacement and at 10-12 years for aortic valve replacement.

aortic valve replacement between the ages of 16-39 years remained free of structural valve deterioration, compared with more than 90% of those over 70 at the time of implantation. In the Edinburgh trial we found an increased risk of porcine valve failure in younger patients with a relative risk of approximately 1.5 for every 10 years of age. Bioprostheses for mitral valve replacement have proved less durable than for aortic valve replacement in all age groups.

Peterseim and colleagues12 reported a large non-randomised series from a single centre of predominantly elderly patients undergoing aortic valve replacement with the patients receiving either a mechanical or porcine prosthesis. There was no difference in prosthesis survival between mechanical and porcine prostheses up to 10 years after implantation. Beyond 10 years an increased need for reoperation became apparent in the patients who had received a porcine prosthesis. The risk of bleeding was significantly increased in those who had received a mechanical prosthesis. However, at 10 years patient survival was only 50%, and in this older population with relatively limited life expectancy and low incidence of porcine prosthetic valve failure, aortic valve replacement with a porcine prosthesis appeared to confer an advantage compared with mechanical prostheses.

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