Aortic Valve Replacement in Older Adults: Mechanical or Bioprosthetic Valve?

Abstract & Commentary

By Andrew J. Boyle, MBBS, PhD, Assistant Professor of Medicine, Interventional Cardiology, University of California, San Francisco. Dr. Boyle reports no financial relationships relevant to this field of study.

Source: Brennan JM, et al. Long-term safety and effectiveness of mechanical versus biologic aortic valve prostheses in older patients: Results from the Society of Thoracic Surgeons adult cardiac surgery national database. Circulation 2013;127:1647-1655.

The decision to choose a mechanical or a tissue valve for patients undergoing surgical aortic valve replacement (AVR) is a complex one that involves synthesizing clinical factors such as risk of bleeding, likelihood of reoperation, and patient preference. Increasingly, older patients with more comorbidities are being referred for AVR surgery. Comparative data between valve types with long-term follow-up in this high-risk cohort are needed to inform our treatment decisions. Accordingly, Brennan and colleagues searched the Society of Thoracic Surgeons (STS) database and identified a cohort of Medicare-linked patients between 65 and 80 years of age undergoing elective or urgent AVR with a mechanical or biological prosthesis from January 1, 1991 until December 28, 1999. They excluded patients undergoing concomitant non-coronary artery bypass graft (CABG) cardiac surgical procedures, those with a prior history of any valve replacement, patients at health maintenance organizations and military hospitals where no patients were linked to Medicare records, those with potential linkage to multiple Medicare files, and those with index procedures that occurred outside a period of fee-for-service Medicare enrollment. The primary endpoint was all-cause mortality. Secondary endpoints were rehospitalization for aortic valve reoperation, stroke, hemorrhagic stroke, hemorrhage, and endocarditis. These were analyzed up to 2007, resulting in at least 8 years of follow-up for all patients.

The study included 39,199 patients who received biological (n = 24,410) or mechanical (n = 14,789) aortic valve prostheses in 605 hospitals. The median age was 73 years and mean follow-up was 12.6 years (range, 8-17 years). Bioprostheses were used with increasing frequency over time among progressively older patients, with a 20% absolute increase from 1991 to 1999. Compared with patients who received mechanical valves, those who received bioprosthetic valves were older (74 vs 71 years; P < 0.001), with a higher prevalence of heart failure (43.7% vs 39.9%; P < 0.0001) and significant coronary artery disease (70.1% vs 65.6%; P < 0.001), and were more likely to have combined AVR + CABG (60% vs 55%; P < 0.0001). The authors performed propensity weighting, after which the baseline and operative characteristics were similar between groups. They quantified hazard ratios (HR) on these risk-adjusted groups.

The 12-year rate of all-cause mortality after AVR was very high for both groups: 70.5% for patients who received bioprosthetic valves and 60.3% for those who received mechanical valves (HR 1.29; 95% confidence interval [CI], 1.26-1.32). After risk adjustment, patients who received bioprosthetic valves experienced a similar long-term mortality rate as those who received mechanical valves (HR 1.04; 95% CI, 1.01-1.07); however, mortality rates were higher beyond 9 years of follow-up in patients treated with bioprosthetic valves. The absolute risk of long-term mortality varied widely across patient subgroups and was particularly high among patients with either preoperative renal failure (12-year mortality, 65.2%) or reduced left ventricular ejection fraction (12-year mortality, 74.1%).

By 12 years, reoperation was observed in 5.2% of patients with bioprosthetic valves and 2.3% of those with mechanical valves. After risk adjustment, bioprosthetic valves were associated with a more than two-fold increase in the long-term rate of reoperation compared with mechanical valves (HR 2.55; 95% CI, 2.1-3.0). This effect was larger among younger patients. Patients who received bioprosthetic valves also experienced a higher risk of endocarditis (HR 1.60; 95% CI, 1.31-1.94), except among the oldest patients (75-80 years; HR 1.17; 95% CI, 0.85-1.60) and those with renal failure (HR 0.69; 95% CI, 0.29-1.66).

However, the adjusted rate of stroke was significantly lower among patients with bioprosthetic valves (HR 0.87; 95% CI, 0.82-0.93). Bioprosthetic valves were associated with a lower adjusted rate of both all-cause bleeding (HR 0.66; 95% CI, 0.62-0.70) and hemorrhagic stroke (HR 0.57; 95% CI, 0.49-0.65). The authors conclude that among patients undergoing AVR, long-term mortality rates were similar for those who received bioprosthetic vs mechanical valves. Bioprostheses were associated with a higher long-term risk of reoperation and endocarditis but a lower risk of stroke and hemorrhage. These risks varied as a function of a patient’s age and comorbidities.

Commentary

Degenerative calcific aortic stenosis is a disease that increases in prevalence with increasing age. This paper from Brennan et al presents data specific to the aging population (65-80 years old), and is thus a welcome adjunct to randomized trial data that tend to enroll younger patients. However, because this cohort has substantial burden of comorbidities, the accuracy of the results relies heavily on statistical matching between groups. This was performed in a rigorous fashion, but statistics cannot account for all comorbidities, and thus the results must be interpreted with caution. Most clinicians err on the side of caution and recommend bioprostheses to patients who are frail or have poor compliance with medications, and neither of these clinical judgments can be captured in observational database studies like this one. This is likely to bias the results in favor of mechanical valves, but to what degree remains unknown.

The unadjusted outcome data are striking in this group: mortality 66%, stroke 14%, and bleeding 17%. This underscores the high-risk profile of aging patients in general. In addition, the uniquely high-risk nature of this group also means that the data in this paper may not be generalizable to younger age groups. How should we choose between mechanical and bioprosthetic valves in patients over 65? The choice should continue to be made on a case-by-case basis, taking into account a patient’s overall risk of bleeding and stroke, other clinical comorbidities, and ability to take warfarin long-term. Both appear to be reasonable alternatives in the appropriate patients.