Couture EJ, et al. - Researchers tested the validity of their hypothesis that the functional residual capacity (FRC) would be higher after a five-minute period of positive pressure ventilation compared with spontaneous ventilation at zero inspiratory pressure in awake morbidly obese volunteers. Switching spontaneous ventilation at zero inspiratory pressure to positive pressure resulted in an increase in the FRC in awake morbidly obese volunteers. No measurable effect of the beach chair (BC) position on the FRC was noted when compared with supine (S) positioning. The reverse Trendelenburg (RT) position resulted in an optimal FRC.

• A prospective crossover randomized trial design was used.
• In a randomized order, obese subjects underwent a combination of one of three positions, supine (S), beach chair (BC), and reverse Trendelenburg (RT), and one of two ventilation strategies, spontaneous ventilation at zero inspiratory pressure (ZEEP-SV) or with positive pressure (PP-SV) set to an inspiratory pressure of 8 cmH₂O, positive end-expiratory pressure of 10 cmH₂O, and fraction of inspired oxygen of 0.21.

• This study included 17 obese volunteers with a mean (standard deviation; SD) body mass index of 50 (8) kg·m^-2.
• Significantly higher mean (SD) FRC was observed in the three positions (S, BC, RT) using PP-SV vs ZEEP-SV [2571 (477) vs 2215 (481) mL, respectively; mean difference, 356; 95% confidence interval (CI), 209 to 502; P < 0.001].
• Significantly higher mean (SD) FRC was seen in the RT vs BC position [2483 (521) vs 2338 (469) mL, respectively; mean difference, 145; 95% CI, 31 to 404; P=0.01], while no difference was seen between S and BC [2359 (519) mL vs 2338 (469) mL, respectively; mean difference, 21; 95% CI, -93 to 135; P=0.89].