[Paradoxal diminution of resistance of the large arteries in human essential hypertension].

Segmental pressure drop and longitudinal resistance of brachial artery were evaluated in 11 normotensive (NT) and 19 hypertensive (HT) subjects of similar age by measuring arterial diameter (D), blood velocity (V) and flow (Q) with pulsed Doppler and blood viscosity (mu) at 96 sec-1 with a coaxial cylinder viscometer. Mean pressure drop (delta P) per unit of length of artery was calculated according to the relation delta P = 4 tau/D, where tau is the mean wall shear stress evaluated on the basis of a Poiseuille velocity profile by the formula: r = vV/D. Brachial artery resistance per unit of length was deduced as the (delta P/Q) ratio. (table; see text) In the overall normotensive and hypertensive group, (n = 30), pressure drop (delta P) was negatively correlated with mean blood pressure (r = -0.55, P less than 0.01). Despite increased viscosity hypertensive patients have a decreased resistance of large arteries whose mechanisms include a reduction in shear stress close to the endothelium and a dilation of the conduit artery; these two abnormalities contribute to decrease the viscous force of the circulating blood column against the arterial wall.