In response.

Abstract

In response to Zhang et al.1 that we failed to assess outcomes, including safety of the recruitment maneuver (RM) and postoperative outcomes,2 while we agree that in the final analysis any maneuver performed during surgery should be directed at improving patient outcomes and is a sound suggestion for future research, the safety and beneficial effects of the RM have already been demonstrated in numerous studies in humans with healthy3 and injured lungs,4 including during thoracic surgery.5 The aim of the current study was simply to describe the physiologic effects of setting an optimal positive end-expiratory pressure (PEEP) compared with standardized PEEP on gas exchange and lung mechanics. Zhang et al.1 also comment that static compliance may not be the best option for setting the PEEP because of alveolar heterogeneity. We agree and as we described in the Methods section, we used dynamic compliance, not static compliance, to set the optimal PEEP. The best dynamic compliance identifies the optimal PEEP in patients with healthy6 and injured lungs7,8 using the PEEP level after an RM that prevents alveolar collapse while minimizing overdistention as the definition of optimal PEEP. In addition, Zhang et al.1 suggest that the higher arterial carbon dioxide tension in the optimal PEEP group may be caused by alveolar overdistention. This is correct when comparing patients with identical carbon dioxide production and alveolar ventilation, but this was not the case in our study. In contrast, some of our results suggest no differences or even less alveolar overdistention in the optimal PEEP group. No differences were found in airway pressures or the ratio of alveolar dead space to alveolar tidal volume. The higher respiratory system compliance and lower, but not significant, expiratory resistance suggest higher end-expiratory lung volume (EELV) in the optimal PEEP group. Together with the lower tidal volume in the study group, this context results in lower strain, as strain may be viewed as the ratio between delivered tidal volume and the volume to which tidal volume is distributed (EELV).9 This fact implies less overdistention. Finally, we agree with Zhang et al.1 that with PaO2 >150 mm Hg, differences between groups are not clinically important. What does matter, however, is the reason behind the higher PaO2 because optimal PEEP prevents recollapse better than that following standardized PEEP. Moreover, increasing EELV (lower alveolar collapse) minimizes mechanical stress by reducing cyclic opening and collapse. Less postoperative acute lung injury may be anticipated with reduced alveolar strain and stress. Carlos Ferrando, MD, PhD Javier Belda, MD, PhD Anesthesiology and Critical Care Department Hospital Clinico Universitario of Valencia Valencia, Spain [email protected]