To the Editor
We read with interest the review by Güldner et al.1 on intraoperative protective ventilation. The authors provide an excellent and thorough summary of the mechanisms of ventilation-induced lung injury and the role of lung-protective ventilation. They further review several interventions such as low tidal volumes, positive end-expiratory pressure (PEEP), and lung recruitment maneuvers (RM) and analyze their respective contribution to reducing stress and inflammation of lung tissue. For this purpose, they provide up-to-date information on the clinical trials aimed at optimizing intraoperative mechanical ventilation since the year 2000. In general, and despite unavoidable limitations of such trials, most of these studies support active lung protection during general anesthesia.
However, we are critical of the authors’ recommendations as to how to implement such intraoperative lung-protective ventilation strategies. We believe that the recommendations by the team of these well-published authors are too strongly skewed toward the results of the recently published PROVHILO study,2 which many of the current contributors coauthored. It appears that ample clinical and experimental results in conflict with their own findings are either neglected or unduly overridden. While addressing a highly relevant hypothesis, this study comparing a high with a low PEEP strategy for intraoperative mechanical ventilation suffers from several drawbacks related to the protocol design and therefore fails to provide an appropriate answer to the primary study questions. First, the RM applying 30 to 35 cm H2O of inspiratory pressure for three consecutive breaths only must be considered ineffective as it remains below both the known effective recruitment pressures and the minimal duration required for these inspiratory pressures to recruit the collapsed lungs of most patients under general anesthesia.3,4 This implies that many lungs never reached the desired recruited state, which in this study was unfortunately not confirmed by appropriate diagnostic means such as oxygenation or lung ultrasound.4,5 Indeed, the modest increase in compliance by 9 to 11 ml/cm H2O observed in the high PEEP group can easily be explained by a redistribution of ventilation toward hypoventilated areas and not by an effective recruitment of anesthesia-induced atelectasis. The comparable incidence of postoperative atelectasis in the high and the low PEEP groups (12%) further supports the suspicion of a lack of lung recruitment in the PEEP group. Second, the level of PEEP after the RM was not individualized. The rationale of choosing one single arbitrary level of 12 cm H2O clearly contradicts the authors’ explicit opinion stating that “certainly the level of PEEP should be chosen according to the patient’s particular characteristics.” In this context, it is remarkable that up to this day no study has evaluated the effects of such an individualized PEEP. We believe that this is essential since an unnecessarily high level of PEEP—such as 12 cm H2O in healthy lungs—could certainly foster many of its known side effects such as hypotension or the need for vasoactive support (as also reported by the PROVHILO investigators) rather than its equally known beneficial effects. Another important confounder of this study is the liberal use of fluids in amounts remarkably larger than current standards recommend.6 This fluid overload may account for the overall high incidence of postoperative pulmonary and nonpulmonary complications (40 and 50%, respectively), which are remarkably higher than could be expected for patients of the respective risk category.7
We believe that the above bias asks for caution when recommending a ventilation strategy based on low tidal volumes and low or next to no PEEP while discouraging the use of lung recruitment before sufficient evidence warrants such recommendations. The therapeutic advice given by Güldner et al. would in fact result in a higher mortality—a harder endpoint than merely the postoperative complications—if the recent evidence from a large series of almost 30,000 patients were taken into account.8 Finally, the authors also recommend that future studies should test single interventions in order to be able to extract relevant information for clinical practice more easily. If such single intervention were PEEP, we believe it is time to abandon the useless high versus low PEEP debate as there is now enough evidence demonstrating the shortcomings of such a sterile design. It is about time to evaluate the effects of an individualized level of PEEP adjusted to pathophysiologically meaningful endpoints either in conjunction with or without lung RMs under low tidal volume ventilation.
The authors declare no competing interests.