Influence of size of emboli on extravascular lung water

We examined the influence of the size of emboli on the vascular volume (QL) and extravascular volume (Qev) accessible to ³HOH during a single pass through an isolated dog lung lobe using the double indicator-dilution method with ¹²⁵I-human serum albumin as the vascular indicator. As successively more beads of a given diameter (58, 548, or 3,175 μm) were introduced into a lung lobe, a linear relationship between QL and Qev was obtained as they both decreased. The slope of the graph of QL vs. Qev with progressive embolism was directly proportional to the bead diameter. This suggested an approach for estimating the total vascular volume in vessels smaller than the diameter of the beads before embolization, referred to as Qm. If it is assumed that most of the transvascular diffusional exchange of ³HOH occurs in vessels smaller than the smallest beads (mainly capillaries) and that vessel obstruction does not change the ratio of Qev to the perfused capillary volume, the slope of the plot of QL vs. Qev is an estimate of the fraction, Qm/QL, of the total vascular volume in vessels smaller than the bead diameter. In the dog lung lobes studied, Qm/QL was ~ 0.64 for 58-μm vessels, 0.75 for 548-μm vessels, and 0.82 for 3,175-μm vessels. The results suggest that, with occlusion of vessels ≥ 58 μm, ³HOH does not diffuse significantly into unperfused regions. The results are also consistent with the concept that the capillary volume is at least one-half of the lobar vascular volume and that more than one-half of the noncapillary volume in the lobe is in vessels > 0.5 mm diam.