Excess body weight is not independently associated with outcome in mechanically ventilated patients with acute lung injury

Background: Despite an epidemic of obesity among adults, the effect of excess body weight on outcome from critical illness is not well studied. Objective: To examine the association between excess body weight and outcome in mechanically ventilated patients with acute lung injury. Design: Secondary analysis of participants in trials of therapy for acute lung injury. Setting: 10 U.S. medical centers that participate in the National Heart, Lung, and Blood Institute Acute Respiratory Distress Syndrome Network. Patients: 902 mechanically ventilated patients who were enrolled in randomized, controlled trials of therapy for acute lung injury. Intervention: Assignment to higher (12 mL/kg of predicted weight) or lower (6 mL/kg of predicted weight) tidal volume ventilation strategies with specified weaning protocols. Some patients also received ketoconazole, lisofylline, or placebo by factorial design. Measurements: Mortality rate, rate of unassisted ventilation by day 28, and number of ventilator-free days. Results: Indirect causes of lung injury, including trauma, were more common in obese patients. Overweight and obese patients had higher peak and plateau airway pressures before enrollment because of higher set tidal volumes. After risk adjustment, excess body weight was not associated with death, achievement of unassisted ventilation, or number of ventilator-free days. This lack of effect persisted with categorical or continuous measures of body mass index (BMI). We found no significant interaction between ventilator protocol assignment and BMI category. Conclusions: After risk adjustment, overweight and obese patients with acute lung injury have outcomes similar to those of patients with normal BMI. The lack of interaction between ventilator protocol assignment and BMI suggests that patients with normal, overweight, or obese BMI benefit from lower tidal volume ventilation for acute lung injury.