Tidal Volume, Positive End-expiratory Pressure, and Postoperative Hypoxemia: Reply

Suleiman et al.1 assert that driving pressure and mechanical power (because of increased respiratory rate in low tidal volume groups) may be more important determinants of perioperative lung injury and postoperative pulmonary complications than tidal volume and positive end-expiratory pressure (PEEP). We used a factorial cluster design to assign 2,860 patients to four combinations of tidal volume and PEEP. There were no differences in oxygenation during recovery or in pulmonary complications among the groups. We therefore concluded that any combinations of tidal volume between 6 and 10 ml/kg and PEEP between 5 and 8 cm H2O are comparably safe in relatively healthy patients having general anesthesia for orthopedic surgery.2As expected, both driving pressure and mechanical power differed among our four treatment groups (table 1). For example, mechanical power increased approximately 1.2 J/min with each increase in tidal volume and/or PEEP, ranging from 10.7 to 14.4 J/min with the biggest intergroup difference being 1.3 J/min. Driving pressure increased 2 to 3 cm H2O with higher tidal volume (table 1).Mechanical power is affected by tidal volume and respiratory rate. We therefore evaluated mechanical power as a predictor of our primary outcome, the oxygen saturation measured by pulse oximetry to fraction of inspired oxygen (Spo2/Fio2) ratio. The relationship was significant, with each Joule per minute increase in mechanical power being associated with –2.12 (95% CI, –2.49 to –1.74; P < 0.0001) reduction in Spo2/Fio2 ratio, an amount that is not remotely clinically meaningful.At least in healthy lungs, postoperative oxygenation and complications are similar at any combination of tidal volume 6 to 10 ml/kg and PEEP of 5 to 8 cm H20, which is roughly twice as high as the threshold for a change in intraoperative mechanical power that is associated with increased risk for postoperative respiratory failure.3Dr. Gama de Abreu received consultation fees from Ambu (Ballerup, Denmark), Lungpacer (Vancouver, Canda) and Medtronic (Lisbon, Portugal). The other authors declare no competing interests.