Blood pressure and blood viscosity are not correlated in normal healthy subjects

The relationship between blood viscosity, hematocrit (Hct), and mean arterial blood pressure (MAP) was studied in a healthy population of 91 men and 66 women with an average age of 30.6 ± 8.0 years, from the city of Victoria de Durango (1800 m elevation). In women and men, Hct values were 42.4% ± 2.9% and 47.2% ± 2.3%, blood viscosities were 4.5 ± 0.7 and 6.1 ± 1.0 cP, and MAP was 83.0 ± 6.8 and 88.0 ± 6.1 mmHg, respectively. The correlation between blood viscosity and Hct was linear and positive (r(2) = 0.48) and identical to that of previous studies reported in the literature when men and women are taken as a single group. Separating the data by gender yielded positive, linear correlations (r(2) = 0.18 and 0.10, respectively) with identical slopes, however blood viscosity for men was 1.2 cP greater than in women (P = 0.02). MAP and blood viscosity (and Hct) were not statistically associated when men and women were analyzed separately and were weakly positively correlated (r(2) = 0.08, P < 0.02) when treated as a group. The present results suggest that studies that show a positive correlation between MAP and blood viscosity (and Hct) do not differentiate data according to gender, or involve populations that do not compensate for increased blood viscosity and potentially increased shear stress.