The gravity dependence of phases III (IIIa and IIIb), IV, and V of simultaneously performed He-bolus and N2-resident gas single-breath washout curves was studied in different body positions by the technique of 180 degrees body inversion between inspiration and expiration. Phase IIIa was mainly determined by nongravitational factors. Phase IIIb was influenced by gravitational, as well as nongravitational, factors. The former were more important with the bolus method in both lateral decubitus positions and the latter with the N2 method in the prone and supine positions. Phases IV and V were mainly gravity dependent. The difference in gravity dependence between the He and N2 methods appeared to be correlated with the vertical interregional concentration gradients of both gases; indeed the vertical gradient was larger for the 133Xe bolus inhaled at residual volume (which is comparable to the He-bolus distribution) than for the 133Xe residual volume-to-total lung capacity ratio (which is comparable to the N2-resident gas distribution). The greater gravity dependence in the lateral decubitus positions than in the supine or prone postures was related to the larger vertical interregional concentration difference as well as to the more pronounced sequential ventilation in the former positions. Finally the negligible effect of gravity on phase IIIa, its moderate effect on phase IIIb, and its predominant effect on phases IV and V were in agreement with the increased sequential filling and emptying due to gravity near residual volume.

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