Volume and outcome in coronary artery bypass graft surgery: true association or artefact?

Abstract Objectives : To examine the evidence for a relation between volume of coronary artery bypass graft surgery and hospital death rates, and to assess the degree to which this could be due to confounding because of differences in case mix. Subjects : People receiving coronary artery bypass graft surgery in the United States. Design : A systematic review of empirical studies examining the relation between volume and outcome of coronary artery bypass graft surgery. Studies were scored according to degree of adjustment for case mix. Above 200 procedures a year was regarded as high volume. Results : Fifteen studies were identified, all of which used observational data from the United States for 1972-92. Six were included in the analysis, one was included in a sensitivity analysis, and eight were excluded because of duplicate analyses of data sources and methods of reporting results. The seven studies analysed reported a reduced mortality with increased volume. Studies with better adjustment for case mix, however, indicated less reduction in mortality with increased volume (P=0.04). The apparent advantages of higher volume also decreased over time (P<0.001). Conclusions : The evidence for reduced mortality in hospitals with a high volume of coronary artery bypass graft surgery is based entirely on observational studies. These studies may have overestimated the benefit of increased volume because of poor adjustment for case mix. It signals the need for caution in interpreting the results of observational studies that examine the relation between volume and outcome.