Lung Mechanics of the Obese Undergoing Robotic Surgery and the Pursuit of Protective Ventilation

Abstract

Protective mechanical ventilation characterizes a strategy aimed at preventing lung overdistension (volutrauma), derecruitment (atelectrauma) and dysfunctional inflammation (biotrauma). It is usually implemented with physiological tidal volumes, for which there is strong evidence of outcome benefits, and lung expansion including positive end-expiratory pressure (PEEP) and recruitment maneuvers, with persisting controversy. Protective ventilation has been mostly studied in critical care, despite the known effects of intraoperative ventilatory settings on postoperative pulmonary outcomes 1 . Current laparoscopic robotic surgery techniques challenge the anesthesiologist to optimize mechanical ventilation in conditions where patient's physiological complexity (e.g., obesity) frequently compounds with surgical physiological burden (pneumoperitoneum, unphysiological Trendelenburg position) 2 . Unfortunately, objective data are scarce to guide clinical practice in these procedures.In this issue of Anesthesiology, the article by Tharp et al. on 91 patients with body mass index (BMI) ranging from normal to >40 kg/m 2 undergoing laparoscopic robotic surgery brings valuable data to the field 3 . The authors report significantly worse lung mechanicscompliance, driving pressures and transpulmonary pressures -with increased BMI at different surgical stages. Lungs of severely obese patients were twice more rigid than those of patients with normal BMI. Tharp et al. estimated optimal PEEP from lung mechanics measurements, and showed this to be substantially higher than the applied PEEP in most patients in the different surgical stages and BMI categories despite consistent use of currently proposed lung protective ventilation strategies 3 . Indeed, most patients presented mechanical evidence of atelectasis suggesting that they were at risk for hypoxemia and atelectrauma.Airway pressures are required to generate at least three processes during mechanical ventilation: airway flow, lung expansion and chest wall expansion. This implies that total airway pressure applied by the ventilator is not entirely spent on the lungs, i.e., is not equivalent to lung stretch or stress. In fact, during ventilation of a healthy non-obese under general anesthesia, ~60-70% of the airway pressure is distributed to the lungs, and ~30-40%