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Volume and outcome in coronary artery bypass graft surgery: true association or artefact?
A J Sowden1, J J Deeks, T A Sheldon
1NHS Centre for Reviews and Dissemination, University of York.
Insights
High-volume hospitals may show lower death rates for coronary artery bypass graft (CABG) surgery. However, this association may be overestimated due to case mix differences in observational studies.
Area of Science:
- Cardiovascular Surgery
- Health Services Research
- Medical Quality Improvement
Background:
- Hospital volume is often associated with patient outcomes.
- Coronary artery bypass graft (CABG) surgery is a major cardiac procedure where volume-outcome relationships are frequently studied.
- Confounding by case mix can influence the observed association between surgical volume and mortality.
Purpose of the Study:
- To evaluate the evidence linking CABG surgery volume to hospital mortality rates.
- To determine if case mix differences confound the observed volume-outcome relationship.
Main Methods:
- Systematic review of empirical studies on CABG surgery volume and outcomes in the United States.
- Studies were assessed based on their adjustment for patient case mix.
- High volume was defined as over 200 procedures annually.
Main Results:
- Seven observational studies (US, 1972-1992) reported reduced mortality with increased CABG surgery volume.
- Studies with better case mix adjustment showed a smaller reduction in mortality (P=0.04).
- The perceived benefit of higher volume decreased over time (P<0.001).
Conclusions:
- Evidence for reduced mortality in high-volume CABG hospitals relies solely on observational data.
- Observational studies may overestimate the benefits of high surgical volume due to inadequate case mix adjustment.
- Caution is advised when interpreting volume-outcome relationships from observational studies.
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 over-estimated 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.