COMPARISON OF DEXTRAN AND HEMATOCRIT EFFECTS IN THE PULMONARY MICROCIRCULATION

We have compared the effects of an increase in perfusate viscosity induced by dextran and by hematocrit (Hct) on segmental vascular resistance in the pulmonary circulation and the effect of flow on microvascular resistance in lungs perfused with dextran and red blood cells. We isolated and perfused lungs of 39 neonatal rabbits weighing 670 +/- 250 g. To determine the effect of dextran, group 1 (n = 8) lungs were perfused with both 5% and 20% dextran 70 solutions (Hct, approximately 25%); to determine the effect of Hct, group 2 (n = 5) lungs were perfused alternately with 5% and 40% Hct solutions (5% dextran 70). In group 1 and group 2 lungs, experiments were done at constant flow and pressure. Group 3 and group 4 (each n = 4) lungs were perfused with 5% and 20% dextran 70, respectively (0 Hct); group 5 (n = 5), with 10% dextran 70; group 6 (n = 7), with 5% dextran 500; and group 7 (n = 6), with 20% dextran 70 (Hct, approximately 5%). All lungs were perfused in zone 3; airway and left atrial pressures were 6 and 8 cm H2O, respectively. To partition the circulation, we measured pressures in the pulmonary artery and left atrium continuously and pressures in 20-50-mu-m arterioles and 20-50-mu-m venules by micropuncture. We found that an increase in both the concentration and molecular weight of dextran increased perfusate viscosity and the resistance in all three longitudinal vascular segments: arteries, microvessels, and veins. However, Hct-induced increases in perfusate viscosity resulted in an increase in arterial and venous resistances alone, with no increase in microvascular resistance. In group 1 and group 7 lungs, which were perfused with 20% dextran 70 but with varying Hcts, a greater than 50% increase in flow rate resulted in a greater decrease in microvascular resistance in group 1 lungs that had a higher Hct. Our results indicate that dextran-induced viscosity effects are present in all three vascular segments of the pulmonary circulation, whereas Hct effects are found mainly in arteries and veins greater than 50-mu-m in diameter. Microvascular resistance is altered by flow rate and may also be influenced by dextran-red blood cell interactions.