Dear Editor,
We read with great interest the article by Güner et al.[1] evaluating the outcomes of isolated redo mitral valve replacement in patients with mitral valve dysfunction. The authors have addressed a very challenging clinical scenario and provided valuable information regarding the complications associated with redo mitral valve replacement.
However, we would like to raise a few methodological concerns regarding the findings.
First, there is a risk of statistical overfitting. The study is based on 13 deaths (events), and four variables were included in the analysis. Accordingly, the ratio of events per variable is 3.25. However, according to commonly accepted recommendations, the ratio of events per variable should be approximately 10.[2] Moreover, the reported wide confidence interval (CI), i.e., 3.5–114.4, indicates considerable uncertainty in the estimate. Therefore, the reported odds ratio of 20 appears to be highly unstable.
Second, the interpretation of emergency surgery as a major cause of mortality warrants caution. The study does not provide sufficient evidence to establish emergency surgery itself as the cause of death. The higher mortality rate may instead reflect greater disease severity rather than the emergency nature of the surgery. This represents potential “confounding by indication.”[3] Emergency surgery may therefore be a marker of disease severity rather than a direct cause of increased mortality.
In conclusion, this study provides valuable data on redo mitral valve replacement surgery. However, statistical overfitting and the interpretation of emergency surgery as a major cause of mortality without adequately accounting for confounding by indication suggest that the findings should be interpreted with caution. Large prospective studies with long-term follow-up are required to confirm these results.