Effect of systemic hematocrit on volume flow and cell free layer width in arterioles

Abstract

The effect of hematocrit on effective viscosity is well known for in vitro systems but the effect in vivo is less clear due to the formation of a cell free layer (CFL) at the vascular wall that may influence vascular tone. The CFL and volumetric flow rate (Q) were assessed in rat cremaster arterioles (20–40 μm) during systemic hemodilution and hemoconcentration. Systemic hematocrit (Hctsys) was increased stepwise by 5% from 40 to 75% during hemoconcentration and decreased by 5% from 40 to 20% during hemodilution. The CFL was measured with a high speed video camera and custom built software. Velocity and vessel diameter measurements were obtained from the video files using a dual‐window cross correlation technique and calipers. The CFL values were 5.4± 0.2, 3.1±0.1 and 0.9±0.1 μm at 20, 40 and 75% Hctsys, respectively. The Q values were 14.5, 9.0±0.6 and 4.9±0.4 nl/sec at 20, 40 and 75% Hctsys, respectively. Overall, Q decreased 66% from 20 to 75% Hctsys. The vessel diameter did not change significantly as Hctsys was reduced from 40% to 20% and from 75 to 40%. The arterial pressure was 17% higher at 40% and 32% higher at 75% as compared to 20% Hctsys. The flow change was inversely proportionate to Hctsys change when corrected for differences in arterial pressure. Apparently the change in CFL width did not induce vascular responses in these vessels due to simultaneous changes in other factors.