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LETTER TO EDITOR |
Department of Pediatric Cardiac Surgery, Alder Hey Hospital, Liverpool L12 2AP, United Kingdom
I read with great interest the article by Perek et al1 in the December issue assessing the early results of isolated coronary artery bypass grafting (CABG) in women and determining the reasons for high morbidity and mortality in the female gender after coronary artery surgery. Although the authors suggest that females do worse after coronary artery surgery yet there are some inherent weaknesses in their study like most other published studies2–7 and therefore extreme caution needs to be exercised while interpreting the findings of their study.
Many studies examining gender differences outcomes after CABG report a higher unadjusted postoperative morbidity and mortality for women compared with men.2,3 The debate has centered on whether gender differences in outcome are attributable to female gender per se or to a higher prevalence of unfavorable risk factors in women. A number of investigations report that gender has no influence on adverse postoperative outcomes after CABG after adjusting for gender dissimilarities in preoperative risk.4,5 An equivalent number of investigations report an increase in postoperative risk for women, despite application of risk-adjustment strategies.1,2 Finally, a few investigations report no difference in operative mortality even with statistically significant gender differences in the preoperative risk profiles.6,7
Conflicting results among published studies with regard to the influence of gender on postoperative morbidity and mortality after CABG are influenced by a number of factors: dissimilar study designs and data collection methods, different study sample sizes, and variable application of statistical methods to analyze the data sets. Most of the investigations are nonrandomized observational studies from large databases, which inherently entail systematic differences in baseline characteristics between women and men. Differing approaches to risk adjustment and the model-selection processes among these investigations can also lead to variability in the final model chosen.
It is to be noted that in the study of Perek et al1 and those previously published2–7 regression adjustment has been used as the statistical analysis tool for drawing conclusions. Although standard diagnostic procedures for regression would not reveal an imbalanced representation of the levels of one covariate within the levels of another, sub classification by propensity makes this imbalance evident. This demonstrates an advantage of propensity modeling over regression adjustment in that the investigator may find that there is effectively no overlap in the distributions of covariates among the groups under investigation. Application of regression adjustment in this circumstance would result in problems in drawing valid conclusions from the data without making assumptions that involve extrapolation. On the other hand, with the application of propensity score methods to retrospective data, baseline characteristics can be summarized as a single score that approximates the background characteristics for the individual patient. As a result covariates are, in general, balanced between the women and men, similar to the balance achieved in a randomized controlled trial. This allows for a "fair" comparison between women and men with regard to morbidity and mortality outcomes after CABG.
In my opinion dissimilarities in profiles may contribute to the debate about gender differences in outcome after coronary revascularization. Therefore, it is extremely important that proper statistical tools are employed before reaching firm conclusions that the results of coronary artery bypass grafting are worse in women.
REFERENCES
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