Mechanical Power and the Association with Postoperative Impaired Oxygenation and Pulmonary Complications in Orthopedic Patients: Post Hoc Analysis of a Cluster Factorial Randomized Trial.

backgroundMechanical power-the energy transferred to the respiratory system per unit time-has been proposed as an overall indicator of ventilator-related harm. The authors therefore tested in this post hoc analysis of trial data whether intraoperative mechanical power normalized to predicted bodyweight is associated with impaired oxygenation in the postanesthesia care unit (PACU) or in the ward, postoperative pulmonary complications (PPCs), and length of postoperative hospital stay (LoS).

methodsThe original trial randomly assigned patients having orthopedic surgery to sequential factorial clusters of intraoperative ventilation with tidal volumes of 6 versus 10 ml/kg predicted bodyweight and a positive end-expiratory pressure of 5 versus 8 cm H 2 O, providing a wide range of exposure to mechanical power. The authors calculated the time-weighted mechanical power normalized to the predicted bodyweight for each patient included in the underlying trial and evaluated its association with the time-weighted average oxygen saturation measured by pulse oximetry/fraction of inspired oxygen ratio (SF-TWA) during the first hour of PACU stay, SF-TWA in the ward, PPCs, and LoS using a multivariable linear mixed model. They accounted for repeated surgeries and adjusted for demographic and intraoperative characteristics.

resultsThe authors included 2,860 surgeries performed in 2,582 patients. Patients had a mean ± SD age of 63 ± 14 yr, were 53% female and 83% White; had a mean ± SD body mass index of 31 ± 7 kg/m 2 ; and were mainly American Society of Anesthesiologists (Schaumburg, Illinois) Physical Status III (72%). Mean ± SD mechanical power normalized to the predicted bodyweight was 0.20 ± 0.06 J/min/kg, and mean ± SD SF-TWA in PACU was 353 ± 41. A 0.1 J/min/kg increment in mechanical power normalized to the predicted bodyweight was associated with a reduction in SF-TWA in PACU by -11 (95% CI, -14 to -8; P < 0.001), a reduction in SF-TWA in wards by -8 (95% CI, -11 to -5; P < 0.001), and 55% higher odds of PPCs (odds ratio, 1.55; 95% CI, 1.05 to 2.27; P = 0.026). Mechanical power normalized to the predicted bodyweight was unrelated to LoS (odds ratio, 1.01; 95% CI, 0.97 to 1.05; P = 0.68). Models including peak or driving pressures explained nearly the same amount of variance in postoperative oxygenation (marginal R2 , 0.207) as the model including mechanical power normalized to the predicted bodyweight (marginal R2 , 0.210).

conclusionsHigher intraoperative mechanical power was associated with impaired postoperative oxygenation and pulmonary complications in patients undergoing orthopedic surgery. Driving pressure had a comparable strength of association with postoperative oxygenation.