Prosthetic Valve Choice
Abstract & Commentary
Synopsis: A 15-year survival rate was better with a mechanical aortic valve replacement because primary valve failure was virtually absent.
Source: Hammermeister K, et al. J Am Coll Cardiol 2000;36:1152-1158.
Previous studies comparing mechanical to tissue valves have shown no difference in overall survival for up to 12 years. Thus, the 15-year report of the Veterans Affairs randomized trial is of interest. From 1977 to 1982, 575 men were randomized to either a Bjork-Shiley mechanical valve or a porcine bioprosthesis for single-valve aortic or mitral replacements. The primary end points of the long-term follow-up were death and valve-related complications. Follow-up was 97% complete at the 18th year since randomization. Operative mortality was 8% and did not differ between the two valve types. Aortic valve replacement (AVR) with a mechanical valve resulted in a significantly lower 15-year mortality vs. a bioprosthetic valve (66 vs 79%; P = 0.02), but there was no difference with mitral valve replacement (MVR). The cause of death was prosthesis related in 37-57% of cases of AVR and MVR with both valve types. Primary valve failure was more common with bioprosthetic valves in both positions: AVR, 23 vs. 0%, P < 0.001; MVR 53 vs. 31%, P = 0.01. In MVR, perivalvular regurgitation was more common with mechanical valves; 17 vs. 7%, P = 0.05. In AVR, re-operation was more common with a bioprosthetic valve; 29 vs. 10%, P = 0.004. The incidence of other complications such as systemic embolism, endocarditis, and valve thrombosis was not different between the two valve types. Hammermeister and colleagues concluded that 15-year survival was better with a mechanical AVR because primary valve failure was virtually absent. Primary valve failure was more common with a bioprosthesis in either position and was more prevalent in those younger than 65 years old.
Comment by Michael H. Crawford, MD
The previous large, long-term comparative studies were inconclusive regarding survival of patients with mechanical vs. bioprosthetic valves. The five and 11-year reports from the VA trial showed no difference in survival for the two valve types in either position. The Edinburgh trial showed a trend for increased survival with mechanical valves at 12 years (P = 0.08). Thus, this 15-year VA follow-up study is of interest.
Ideally, valve replacement should be a one-time event, consequently, this type of long-term data is important for selecting the right valve for each patient. Unfortunately, the two valves used in this study are no longer the most widely used and concern would be reasonable that these results are not applicable to modern valves. On the other hand, there is no data suggesting that current valves are superior to the older designs in any way that would effect major outcomes. Also, one mechanical valve is probably more similar to another mechanical valve than it is to a bioprosthetic valve and vice versa. Currently, this is the best data available and several major clinical points are made.
Overall mortality was high in this series ranging from 66% to 81% over the 15-year follow-up. The majority of deaths were due to factors other than the valve replacement per se; about 25% were other cardiac causes, 20% noncardiac, and 10% undetermined. This is not surprising given the comorbidities reported in this male, veteran population. However, about 45% of the deaths were valve related and with AVR the lower primary valve failure rate resulted in a better survival with a mechanical valve. This advantage for mechanical AVR was greatest in those younger than age 65 because virtually all the primary valve failures in porcine valves occurred in the younger patients. Thus, in patients younger than age 65, AVR should be with a mechanical valve unless chronic anticoagulation is not feasible or desirable.
The major drawback to mechanical valves is the need for anticoagulation and the subsequent risk of bleeding that was demonstrably higher for mechanical valves in both positions in this study. Clinically, significant bleeds occurred in about half the patients with mechanical valves over 15 years. However, the incidence of bleeding with bioprosthetic valves in both positions was substantial at about 30%. Hammermeister et al speculate that many of the bioprosthetic valve patients were on anticoagulants for other reasons, such as atrial fibrillation. If one analyses VA trial data on atrial fibrillation patients treated with anticoagulants, the bleeding rate over 15 years would calculate to 23%. The high bleeding rates with both types of valves may be related to the fact that INRs were not used during the early part of this study and thus, anticoagulant control may have been suboptimal. Bleeding rates in the INR era are probably lower.
In patients older than age 65, no advantage to a mechanical valve was demonstrated, so such patients may be good candidates for bioprosthetic valves especially if they do not need anticoagulants for other reasons. Also of interest is the fact that thromboembolic events were uncommon (< 20%) and were equal between the two valve types. Thus, the concept that there are fewer embolic events with bioprosthetic valves is disproven by this study. In addition, valve thrombosis was rare (1-2%) and was not more common with mechanical valves.
In summary, with AVR, a mechanical valve makes sense for those younger than age 65 and a porcine valve for those older than age 65, provided that they do not need anticoagulation for some other reason. Patients who need anticoagulation for other reasons should get mechanical valves in either position at any age. With MVR, there is no survival advantage of one valve type over the other, so patient-related factors become more important. On the other hand, for MVR, primary valve failure was significantly lower with mechanical valves (5% vs 44%). MVR freedom from re-operation has to be weighed against freedom from anticoagulation.
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