Comparison of dextran and hematocrit effects in the pulmonary microcirculation.

Abstract

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 20o dextran 70 solutions (Hct, -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, -5%). All lungs were perfused in zone 3; airway and left atrial pressures were 6 and 8 cm H20, respectively. To partition the circulation, we measured pressures in the pulmonary artery and left atrium continuously and pressures in 20-50-,um arterioles and 20-50-gm 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%o 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 ,um in diameter. Microvascular resistance is altered by flow rate and may also be influenced by dextran-red blood cell interactions. (Circulation Research 1991;68:1108-1116 T he pulmonary circulation is comprised of a complex network of blood vessels, with each successive segment (arterial, microvascular, and venous) demonstrating unique pressure-flow characteristics. The differences in the responses of these three vascular segments to a variety of stimuli are, in part, due to the intrinsic vasomotor properties of the blood vessels as well as their passive viscoelastic properties. In addition, the microrheological behavior of blood may vary in the three segments of the pulmonary vasculature, leading to different hemodynamic events in the three vascular segments. The interactions of plasma constituents with the various circulating cells in blood are complex. In general, our knowledge of blood rheology is derived from in vitro studies using cone-plate viscometers and microtube viscometry. However, results from in vivo studies appear to be i...

Airway mechanics and lung tissue viscoelasticity: effects of altered blood hematocrit in the pulmonary circulation

“…There is a particular lack of knowledge as to how the rheological properties of the blood affect the lung tissue viscoelasticity, despite the clearly established fact that the blood composition determines the mechanical properties of the pulmonary vasculature (13,14,21). Higher levels of hematocrit (Hct) at a given shear rate result in an elevated viscosity of whole blood (26), thereby raising the vascular resistance in an exponential fashion and altering the shape of the pulmonary vascular resistance-blood flow curve (13,14,20). Furthermore, the viscoelastic properties of the blood depend greatly on the level of Hct (22).…”

Red Blood Cell Aggregation and Microcirculation in Rat Cremaster Muscle

“…Gustafsson et al [6,7], using an isolated dog hind limb, concluded that RBC aggregation is of minor importance for blood viscosity in vivo. However, other results based on studies using hyperaggregating dextrans (Dex) in mus cle [8,9] or lungs [10] or on a comparison of the effects of low-and high-molecular-weight Dex [11], strongly sug gested that red cell aggregation may affect resistance to flow in vascular beds. In addition, increased RBC aggre gation is often associated with pathological states such as diabetes [12,13], venous insufficiency [14,15], myocar-dial infarction [16], or nephrotic syndrome [17] and is considered an important factor in hemorheological disor ders [18].…”

Pulmonary Hemodynamics

“…Although these studies were confined to the canine model, studies in humans suggest that hematocrit is indeed an important determinant of pulmonary blood flow and vascular resistance (Rosenthal and Fyler, 1974). Other studies have confirmed these findings (Wang et al, 1985) and have defined the segment in which hematocrit has most of its effect to be the capillary segment in the adult dog (Julien et al, 1985) and the arteries, microvessels, and small veins in the neonatal rabbit (Raj et al, 1989;Raj and Anderson, 1991).…”