Single-breath washouts in a rotating stretcher

Vital capacity single-breath washouts using 90% O-2-5% He-5% SF6 as a test gas mixture were performed with subjects sitting on a stool (upright) or re cumbent on a stretcher (prone, supine, lateral left, lateral right, with or without rotation at end of inhalation). On the basis of the combinations o f supine and prone maneuvers, gravity-dependent contributions to N-2 phase III slope and N-2 phase IV height in the supine posture were estimated at 1 8% and 68%, respectively. Whereas both He and SF, slope decreased from supi ne to prone, the SF6-He slope difference actually increased (P = 0.015). N- 2 phase III slopes, phase IV heights, and cardiogenic oscillations were sma llest in the prone posture, and we observed similarities between the modifi cations of He and SF6 slopes from upright to prone and from upright to shor t-term microgravity. These results suggest that phase III slope is partiall y due to emptying patterns of small units with different ventilation-to-vol ume ratios, corresponding to acini or groups of acini. Of all body postures under study, the prone position most reduces the inhomogeneities of ventil ation during a vital capacity maneuver at both inter- and intraregional lev els.