Cranio-caudal distribution of inspired gas and perfusion in supine man

We measured the cranio-caudal distribution (A/B) of slowly inspired gas (VI) and of perfusion (Q) at different lung volumes in 8 supine subjects. When supine closing capacity (CC) exceeded supine FRC, A/B of VI was greater than unity and decreased at higher lung volumes (VL). When CC less than FRC, A/B of VI less than or equal to 1.0 and showed no VL dependence. When abdominal girth/height ratio (Ag/Ht) exceeded 0.50, supine CC was greater than upright CC and A/B of VI was greater. In contrast, A/B of Q greater than 1.0 at all VL and was not related to (FRC--CC). The results suggest that cranio-caudal distribution of inspired gas is influenced by airway closure in the dependent paradiaphragmatic lung regions and that the latter is enhanced in the presence of abdominal obesity. Perfusion distribution is preferential to lung apices, relatively volume independent, and not influenced by airway closure.