During anesthesia and paralysis, morbidly obese patients, whose lung volumes are already reduced, are at greater risk of severe impairment of respiratory mechanics and gas exchange than are normal-weight patients. Pelosi et al. hypothesized that lung collapse and alterations in respiratory mechanics might be a function of an increased body mass index and intraabdominal pressure. In nine normal and in nine morbidly obese postoperative abdominal surgery patients, they measured lung volumes, respiratory system elastance, and intraabdominal pressure at positive end-expiratory pressure (PEEP) 0 and 10 cm H2O to determine whether PEEP might improve respiratory mechanics and oxygenation in the morbidly obese patients.
The normal participants were studied in the recovery room 20 min after surgery, whereas the obese patients were transported to the intensive care unit, where the entire protocol was conducted after they had reached respiratory and hemodynamic stability. During the study protocol, all patients were anesthetized with diazepam, paralyzed with pancuronium bromide, and mechanically ventilated. The research team measured gas exchange; elastic and flow-resistive properties of the respiratory system, lung, and chest wall; and end-expiratory lung volume during the last 5–10 min of each of the two different PEEP levels, which were applied in random order and maintained for at least 25–30 min. Intraabdominal pressure readings were obtained using intrabladder catheters.
The obese patients had lower lung volumes, higher intraabdominal pressures, higher lung and chest wall elastance measurements, and lower oxygen pressures than did normal subjects. Increasing PEEP to 10 cm H2O reduced elastance and improved oxygenation in obese patients but not in normal patients. Further studies would be helpful to define optimal levels of tidal volumes (not investigated in this study) and PEEP to keep the lung open in these at-risk patients, and to define the body mass index levels at which PEEP or larger tidal volume would be effective in maintaining normal respiratory mechanics and oxygenation.