LAUSR

BACKGROUND It has previously been reported that chronic hypoxia increases blood viscosity. The increase is usually attributed to polycythemia-induced increases in hematocrit. However, the effect of acute hypoxia in humans on blood viscosity is unknown. OBJECTIVE Therefore, the purpose of this study was to determine the effect of acute hypoxia, independent of changes in hematocrit, on blood and plasma viscosity. METHODS Nine healthy volunteers breathed room air for 30 min, followed by 30 min of breathing 15% oxygen. Blood samples were collected at the end of both the normoxic and hypoxic conditions. Blood viscosity, plasma viscosity, and hematocrit were measured in each sample. RESULTS The mean±SD hemoglobin oxygen saturation significantly (P <  0.05) decreased from 98±1% during normoxia to 87±2% during hypoxia. Hematocrit was essentially identical for the two conditions (42.1% vs. 42.0%). Blood viscosity was not significantly different for the two conditions with a mean of 2.89±0.17 cP during normoxia and 2.83±0.19 cP during hypoxia. Likewise, plasma viscosity was not significantly different for the two conditions with a mean of 1.19±0.04 cP during normoxia and 1.19±0.05 cP during hypoxia. CONCLUSION Such results suggest that acute normobaric hypoxia, independent of changes in hematocrit, does not increase blood or plasma viscosity.