Microvascular hemodynamics during hemodilution and hemoconcentration

Measurements of intravascular pressure, red blood cell (RBC) velocity, and microvessel hematocrit (Hct(micro)) were made in arterioles and venules of the cat mesenteric microvasculature during systemic hemodilution (cell-free plasma) and hemoconcentration (packed cells). For a range of systemic hematocrits (Hct(sys)) from 5 to 67%, changes in volumetric flux of red cells (Q̇(RBC)) were derived from the product of microvessel bulk flow and Hct(micro). During hemodilution, a heterogeneous response of changes in Q̇(RBC) was found with larger distributing arterioles (43-54 μm) exhibiting a monotonic fall, whereas increases in Q̇(RBC) above control were found in smaller arterioles that were indicative of a potential enhancement of oxygen delivery. Although the dilution response of all arterioles and venules averaged for all calibers of vessels demonstrated a decline in Q̇(RBC), alterations of Hct(micro) suggested a lessening of the disparity between Hct(sys) and Hct(micro), which was indicative of a more efficient utilization of the remaining circulating RBC volume. In response to hemoconcentration, a decrease in Q̇(RBC) also occurred, which, in concert with the dilution data, suggested that Q̇(RBC) was maximized for a range of 28 < Hct(sys) < 46%. From measurements of the arteriovenous pressure drop across mesenteric modules, regional resistance was found to exhibit a relative plateau as Hct(sys) was increased above its control value. This behavior was attributed to a decrease in vascular hindrance of the principal resistance vessels and an invariance of blood viscosity at the capillary level due to RBC redistribution and the attendant viscous behavior of blood.