Effects of different-size microemboli on lung fluid and protein exchange

We examined the effects of embolization with different-size glass-bead microemboli on pulmonary transvascular fluid and protein exchange in the sheep lung lymph fistula preparation. Embolization with either 200- or 500-micron-diameter glass beads caused comparable increases in pulmonary vascular resistance, which were sustained for the duration of the study. The 200-micron beads increased pulmonary lymph flow (Qlym) and did not affect the lymph-to-plasma protein concentration ratio (L/P), whereas injection with 500-micron beads increased Qlym and decreased L/P. The latter changes were comparable to those observed after an increase in pulmonary microvascular pressure induced by left atrial hypertension, suggesting that the 500-micron beads increase the Qlym by raising the microvascular hydrostatic pressure. In contrast, the 200-micron beads increased the transvascular clearance of proteins to a greater extent, since L/P did not decrease as Qlym increased. These findings suggest that lung vascular permeability increases after embolization with smaller (200-micron) but not with larger (500-micron) emboli. The increased permeability after embolization with small beads may be due to transmission of the permeability-increasing substances to the downstream capillaries via the collateral pulmonary arteries. This would not occur after embolization with larger emboli if these obstructed the pulmonary arteries upstream from branching points of the collateral arteries. The ultrafiltration of protein-poor plasma after embolization with the 500-micron beads may reflect increased fluid filtration in unobstructed microvessels due to increase in the microvascular hydrostatic pressure.