Response to Letter Regarding Article, “Off-Pump Versus On-Pump Coronary Artery Bypass Graft Surgery: Differences in Short-Term Outcomes and in Long-Term Mortality and Need for Subsequent Revascularization”
We would like to thank Bollati et al for their interest in our study.1 Their concerns about the validity of our study are (1) that the logistic regression analyses used to risk-adjust the outcomes are based on stepwise models, and (2) that there is no c statistic reported for the propensity model used to test for selection bias.
We do not have space for a discussion of the pros and cons of stepwise analyses, but when we developed models based on all available independent variables, the results were almost identical to the findings with stepwise analyses. For example, when all variables were used in the model for subsequent revascularization, the hazard ratio changed from 1.50 (P<0.001) to 1.51 (P<0.001).
With respect to point 2, as indicated in one of the references2 cited by Bollati et al, the warning sign from the c statistic in a propensity model is a very high value (close to 1.0), because this means that there are few pairs of patients undergoing the interventions who have similar sets of risk factors. Our c statistic was 0.60, which indicates that the populations of on-pump and off-pump patients are not so dissimilar that they cannot be compared after adjustment. It should also be noted that in addition to the type of propensity analysis mentioned by Bollati et al, we matched patients exactly based on characteristics related to outcomes, and this analysis yielded very similar findings to the risk-adjusted analysis.
Furthermore, Bollati et al suggest that our finding that off-pump surgery has superior short-term outcomes (lower mortality and complication rates) and worse long-term outcomes (higher subsequent revascularization rates) is counterintuitive and indicative of invalid analyses. First, because off-pump surgery is associated with more incomplete revascularization and fewer grafts per diseased vessel, it is not surprising that there were fewer short-term problems (as fewer vessels were attempted) and more longer-term problems associated with both incomplete revascularization and lower graft patency. Second, our findings are very consistent with those of randomized clinical trials referenced in our study.
In conclusion, we are confident that the validity of our study1 is not compromised by the methods used or the statistics resulting from the application of those methods.