To the Editor:
With great interest we have read the article by Douglas A. Colquhoun et al.1 regarding the effect of lower tidal volume ventilation on postoperative pulmonary complication in patients undergoing lung resection surgery with one-lung ventilation. There are two important points of concern.
First, an a priori test was used to calculate the sample size by using a two-sided Z test with unpooled variance, and the results showed that 1,315 unmatched cases in each group (total study N = 2,630) provided 90% power at an alpha = 0.05 to detect a 5% difference. However, the Z test was used to calculate the sample size, whereas the conditional logistic regression model was used to analyze the relationship of protective ventilation and pulmonary complication (main outcome), and only 762 patients matched were enrolled in the logistic regression. We think that the sample size should be calculated according to the main outcome and corresponding main statistical analysis method, rather than using a different statistical method. Therefore, the power (90%) based on the Z test and corresponding sample size may mislead readers to believe the conclusion too much. Although there are many multivariate power analysis methods, none of them is generally accepted and feasible; therefore, we suggest that the authors use univariate power analysis with logistic regression to provide accurate power.2
Second, although this study defined the criteria of one-lung protective ventilation as both tidal volumes equal to or less than 5 ml/kg predicted body weight and median positive end-expiratory pressure greater than or equal to 5 cm H2O, the generally accepted definition of lung protective ventilation contained (1) tidal volume equal to or less than 5 ml/kg predicted body weight, (2) positive end-expiratory pressure greater than or equal to 5 cm H2O, and (3) lung recruitment maneuvers.3 In addition, recruitment maneuvers are an important component of lung protective ventilation to reduce postoperative pulmonary complications.
The authors declare no competing interests.