Microvascular hemodynamics during systemic hemodilution and hemoconcentration.

Measurements of intravascular pressure, red blood cell (RBC) velocity, and microvessel hematocrit (Hctmicro) 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 (Hctsys) from 5 to 67%, changes in volumetric flux of red cells (QRBC) were derived from the product of microvessel bulk flow and Hctmicro. During hemodilution, a heterogeneous response of changes in QRBC was found with larger distributing arterioles (43-54 microns) exhibiting a monotonic fall, whereas increases in QRBC 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 QRBC, alterations of Hctmicro suggested a lessening of the disparity between Hctsys and Hctmicro, which was indicative of a more efficient utilization of the remaining circulating RBC volume. In response to hemoconcentration, a decrease in QRBC also occurred, which, in concert with the dilution data, suggested that QRBC was maximized for a range of 28 less than Hctsys less than 46%. From measurements of the arteriovenous pressure drop across mesenteric modules, regional resistance was found to exhibit a relative plateau as Hctsys 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.