To the Editor:
We read with great interest the article “Intraoperative Mechanical Ventilation and Postoperative Pulmonary Complications after Cardiac Surgery” by Mathis et al.1 We appreciate the authors’ great work. The lung-protection ventilation bundle and its component of driving pressure have a strong correlation with the decrease of postoperative pulmonary complications, but several concerns remain.
First, the definition of postoperative pulmonary complications does reduce the comparability between studies. A recent consensus, cited also by this article, points out that considering the common pathologic pathway, perioperative pulmonary complications should include atelectasis, pneumonia, acute respiratory distress syndrome, and aspiration pneumonia,2 these indicators are easy to achieve in clinical practice, especially in cardiac surgery with a higher monitoring level. Some of the indicators selected by the authors, including reintubation and prolonged initial postoperative ventilator duration longer than 24 h, might be partially attributed to the patient’s circular instability and consciousness disorder, not just the pulmonary complications themselves. Moreover, these endpoints are somewhat like the consensus definition of respiratory failure under mechanical ventilation, a more serious condition requiring respiratory support2 ; it is conceivable that the actual incidence of postoperative pulmonary complications may be underestimated. Different definitions may lead to different results, the inconsistency of endpoint criteria might be solved by further sensitivity analysis.
Second, the cut-off point selection of the lung-protective ventilation bundle and its components is empirical and selective in this article, this may lead to a nonoptimal clinical choice. Moreover, nonsignificant statistical relationship of tidal volume less than 8 ml/kg (according to predicted body weight) and positive end-expiratory pressure (PEEP) greater than or equal to 5 cm H2O with occurrence of postoperative pulmonary complications may also be attributed to the hasty choice. It might be more appropriate to conduct an exploratory study to analyze the lung-protective ventilation components and the optimal combination in the first step; a previous study showing a PEEP of 5 cm H2O and median plateau pressure of 16 cm H2O or less was associated with the lowest risk of postoperative respiratory complications.3
Third, according to this article, the probability of postoperative pulmonary complications is higher at both poles of body mass index (BMI) classes (underweight and high-class obesity), and the distribution of pulmonary complications with BMI was unlikely to be linear, but rather binomial, distribution. This may be explained by the accompaniment of malnutrition with being underweight and with severe obesity being prone to atelectasis—both classes are associated with increasing postoperative pulmonary complications.4,5 Additionally, BMI is associated with increasing intraabdominal pressure and decreasing pulmonary compliance.5 For example, driving pressure is more difficult to maintain at 16 cm H2O in severe obesity compared to a normal BMI with the same tidal volume and PEEP. This may lead to a bias in the distribution of protective ventilation across different BMI ranges. Eventually, the interpretation of regression results might be affected by the aforementioned factors. Moreover, in a recent study, airway closure happens with an impressive incidence in patients with obesity, lead to an overestimation of driving transpulmonary pressure.6 This complicates the interpretation of the findings in patients with obesity. However, in the subgroup analysis, the lung-protective ventilation bundle showed the same protective effect at all BMI levels, alleviating the aforementioned considerations to some extent.
The authors declare no competing interests.