NONINVASIVE DETERMINATION OF SHEAR-RATE DISTRIBUTION ACROSS THE ARTERIAL LUMEN

In vitro experiments have shown that the shear stress exerted by flowi ng blood on the endothelial surface affects the morphology of the vasc ular wall and the release of vasoactive substances and growth factors by that wall. It is believed that the caliber of a vessel adjusts to t he local shear stress to maintain a specific value of the shear stress . The local shear stress follows from local shear rate by multiplying shear rate by the local blood viscosity. The present article describes a method in which ultrasound techniques are used to assess transcutan eously the time-dependent wall shear rate in vivo in arteries. This me thod is applied to the assessment of wall shear rate in the common car otid artery of volunteers, presumed to be healthy, in two age categori es (young age group, 20 to 30 years old, n=8; old age group, 60 to 70 years old, n=6). Although the peak shear rate in the young age group i s markedly higher than in the old age group, the mean shear rate avera ged over a cardiac cycle has the same value of 210 s(-1) for both grou ps, corroborating earlier observations that mean shear rate and, hence , mean shear stress are maintained at a particular value. Conversion o f the observed shear rates to shear stresses, assuming a blood viscosi ty of 3.5 mPa.s for both age groups, gives shear stresses of approxima tely 0.7 Pa. This is a factor of two lower than the shear stresses est imated from the relation between volume flow and artery caliber (1.5 P a).