To the Editor:
In their retrospective observational analysis of patients undergoing one-lung ventilation during thoracic surgery, Colquhoun et al. did not detect an independent association between a low tidal lung protection ventilation regimen and a composite of postoperative pulmonary complications.1 However, there are two additional factors that may have an impact on this finding.
First, the right lung is larger than the left lung. Normally, the left lung receives a smaller tidal volume than the right lung. By using the same tidal volume for both the left and right lungs, it is possible that a tidal volume that is protective for the right lung may be excessive for the left lung. Without accounting for this difference, it is possible that the left lung did not accrue the full benefit of low tidal volume ventilation and might have had greater injury.
Second, respiratory rate is one of the key variables through which mechanical ventilation may injure a patient.2–4 Reduction of respiratory rate and tidal volume ameliorates lung inflammation and injury. An increased respiratory rate may contribute to the development of dynamic hyperinflation and intrinsic positive end-expiratory pressure with multiple respiratory and hemodynamic consequences. It is well established that ventilator-induced lung injury is largely caused by the cyclic overstretch and/or collapse of alveoli. With a higher respiratory rate, there will be more such cycles and thus, more accumulated damage. Colquhoun’s patients with protective ventilation had higher mean respiratory rates with larger SDs. It is possible that more patients in this group had sufficiently high respiratory rates which may have negated benefits that otherwise might have been seen with protective ventilation.
If there is a future study, it would be helpful to use different left and right lung tidal volumes, as well as ensure no differences in respiratory rate between protective ventilation and no protective ventilation patients. Such a study will require a large number of subjects and a very careful prospective study design.
The author declares no competing interests.