Letter regarding article by Gammie et al, "influence of hospital procedural volume on care process and mortality for patients undergoing elective surgery for mitral regurgitation".

BACKGROUND Few studies have examined the procedural volume-outcome relationship for heart valve surgery. None have examined process of care factors that may be mediators of this association. METHODS AND RESULTS This was a retrospective review of outcomes for 13,614 patients having elective surgery for mitral regurgitation between 2000 and 2003 in 575 North American centers participating in the Society of Thoracic Surgeons National Cardiac Database. Hospital annual mitral valve volume varied widely from 22 cases per year in the lowest-volume quartile to 394 in the highest. Unadjusted mortality rates decreased from 3.08% in the lowest-volume category to 1.11% in the highest-volume category. The risk-adjusted odds ratio for mortality in the highest-volume category compared with the lowest was 0.48 (95% confidence interval 0.28 to 0.82). The rates of mitral valve repair increased from 47.7% in the lowest-volume quartile to 77.4% in high-volume hospitals (P<0.0001). Similarly, the rates of bioprosthetic valve use for patients aged >65 years rose from 59% in the lowest-volume quartile to 75% in the highest-volume quartile (P=0.0002). The association between volume and mortality was still significant but attenuated when the risk adjustment was modified to adjust for mitral valve repair versus replacement. CONCLUSIONS Hospital procedural volume was associated with higher frequency of valve repair, higher frequency of prosthetic valve usage in elderly patients, and lower adjusted operative mortality. Differences in care process may contribute to improved outcomes in higher-volume centers.