[Blood viscosity is a chronic adjustment factor of arterial vasodilatation in humans]

Arteriolar geometry and blood viscosity, the two main factors determining vascular resistance were studied in 44 normotensive subjects and 106 hypertensive patients at the brachial circulation level. Vascular resistance was calculated as the ratio between mean blood pressure and mean arterial flow determined by pulsed Doppler. Blood viscosity at 240 sec-1 was measured with a coaxial cylinder viscometer: Compared to controls, hypertensive patients had higher vascular resistance (161 + 8 vs 124 + 9 mmHg.ml-1.sec, P less than 0.05) and higher viscosity (4.75 + 0.05 vs 4.50 + 0.07 mPasec, P less than 0.01). Vascular resistance was negatively correlated to blood viscosity both in control (r = 0.48; P less than 0.001) and in hypertensive (r - 0.39, P less than 0.001) groups. Arteriolar equivalent radius (a) calculated from Poiseuille's equation from the relation a = (8 mu/pi R) 1/4 was positively correlated to viscosity in the two regressions were statistically different (P less than 0.001) indicating that at higher viscosity levels, arteriolar equivalent radius had lesser increase in normotensive than in hypertensive groups. Thus chronic increase in blood viscosity is accompanied by a small vessels vasodilatory phenomenon which seems less efficient in normotensive than in hypertensive subjects and might contribute in part to the mechanisms of vascular resistance elevation observed in hypertension.