LAUSR

In this issue of Anesthesiology, Grieco et al. bring evidence that pneumoperitoneum and the Trendelenburg position impose a dangerous stress on the respiratory system of morbidly obese patients undergoing robotic gynecologic surgery. In 22% of patients after Trendelenburg positioning, severe expiratory flow limitation and airway closure were observed, with airway opening pressures ranging between 17 and 32 cm H 2 O. The authors warn against the danger of using intraoperative pressure control ventilation, which could produce severe alveolar hypoventilation in patients with airway opening pressures greater than 15 cm H 2 O. More generally, the recent development of roboticassisted surgery is, in obese patients, a serious challenge for the anesthesiologist. Severe and morbid obesity critically affect respiratory physiology. In awake obese patients lying in the supine position, the active contraction of the diaphragm and intercostal muscle opposes active forces against the crushing weight of thoracic and abdominal fat, thereby preserving end-expiratory lung volumes and maintaining lung aeration. After anesthetic induction and muscle relaxation, diaphragm and rib cage respiratory muscles become passive and the lungs are fully subjected to the overwhelming pressure of the abdominal, mediastinal, and subcutaneous adipose tissue (fig. 1). Several physiologic respiratory disorders result: a precipitous fall in transpulmonary pressure in dependent lung regions, atelectasis of the posterior segments of the lower lobes, decrease in end-expiratory lung volume, airway closure, reduction of respiratory compliance, and increased airway resistance. Decreased arterial oxygenation results from increased venous admixture and pulmonary shunt, as attested by the increase in alveolar-arterial gradient of partial pressure of oxygen. As shown in figure 2, most of respiratory disorders worsen exponentially with the body mass index. When body mass index is above 40 kg/ m, functional residual capacity is more than halved and expiratory reserve volume restricted by two thirds. As a consequence, intraoperative tidal ventilation occurs at very low lung volumes if positive end-expiratory pressure (PEEP) is not enough to re-establish expiratory reserve volume. Noncartilaginous small airways collapse at the end of expiration, resulting either in cyclical opening and closure during tidal ventilation or, if peak inspiratory pressure does not exceed the opening pressure, in persistent closure. In bariatric laparoscopic surgery, two specific conditions worsen obesity-related respiratory disorders and increase the anesthetic risk. The first is pneumoperitoneum used to facilitate surgical exposure. Intraperitoneal insufflation of carbon dioxide increases the abdominal pressure by 50%. In morbidly obese patients, the abdominal pressure is chronically elevated, reaching 10 mmHg in basal conditions (twice the normal value). After pneumoperitoneum, it increases to 15 mmHg, a high pressure that displaces the diaphragm cranially, increases volume of atelectasis in dependent lung regions, reduces functional residual capacity, decreases respiratory compliance, and increases airway resistance. All these respiratory disorders are partially reversed by PEEP, beach position, and recruitment maneuver. Interestingly, pneumoperitoneum is associated with an improvement in arterial oxygenation, likely resulting from a shift of pulmonary blood flow from lower to