Plasma viscosity regulates capillary perfusion during extreme hemodilutionin hamster skinfold model

Effect of increasing blood viscosity during extreme hemodilution on capilla ry perfusion and tissue oxygenation was investigated in the awake hamster s kinfold model. Two isovolemic hemodilution steps were performed with 6% Dex tran 70 [molecular weight (MW) = 70,000] until systemic hematocrit (Hct) wa s reduced by 65%. A third step reduced Hct by 75% and was performed with th e same solution [low viscosity (LV)] or a high-molecular-weight 6% Dextran 500 solution [MW = 500,000, high viscosity (HV)]. Final plasma viscosities were 1.4 and 2.2 cP (baseline of 1.2 cP). Hct was reduced to 11.2 +/- 1.1% from 46.2 +/- 1.5% for LV and to 11.9 +/- 0.7% from 47.3 +/- 2.1% for HV. H V produced a greater mean arterial blood pressure than LV. Functional capil lary density (FCD) was substantially higher after HV (85 +/- 12%) vs. LV (3 8 +/- 30%) vs. baseline (100%). PO2 levels measured with Pd-porphyrin phosp horescence microscopy were not statistically changed from baseline until af ter the third hemodilution step. Wall shear rate (WSR) decreased in arterio les and venules after LV and only in arterioles after HV. Wall shear stress (WSR x plasma viscosity) was substantially higher after HV vs. LV. Increas ed mean arterial pressure and shear stress-dependent release of endothelium -derived relaxing factor are possible mechanisms that improved arteriolar a nd venular blood flow and FCD after HV vs. LV exchange protocols.