EFFECTS OF FLOW HETEROGENEITY ON THE MEASUREMENT OF CAPILLARY EXCHANGE IN THE LUNG

The effects of flow heterogeneity on the measurement of transcapillary escape of small molecules for pet-fused in situ sheep lungs were eval uated. Lungs were studied at five flows (1.5-5.0 l/min) ranging from z one 1 to zone 3 conditions. At each flow, multiple indicator-dilution curves were collected using C-14-labeled urea (U) or butanediol (B) as the diffusing tracer, and radiolabeled 15-mu m microspheres were inje cted. The lungs were removed, dried, sectioned, weighed, and counted f or microsphere radioactivity. Flow heterogeneity, expressed as relativ e dispersion, decreased with increasing flow, from 0.838 +/- 0.179 (me an +/- SD, n = 8) to 0.447 +/- 0.119 (n = 6). We applied homogeneous f low models of capillary exchange to compute permeability-surface area product (PS) and a related parameter, (DS)-S-1/2, for diffusing tracer s. (D is effective diffusivity of capillary exchange.) PS and (DS)-S-1 /2 increased to a maximum with increasing flow, but the ratio of (DSU) -S-1/2 to (DSB)-S-1/2 remained constant. A new model incorporating flo w heterogeneity and recruitment (the variable recruitment model) was u sed. The variable recruitment model described the effects of flow on c apillary recruitment, but incorporating heterogeneity into the computa tion did not alter (DS)-S-1/2 values from those computed assuming homo geneous flow.