THE ROLE OF FLUID-MECHANICS IN THE LOCALIZATION AND DETECTION OF ATHEROSCLEROSIS

Fluid dynamics research over the past twenty years has contributed imm ensely to our knowledge of atherosclerosis. The ability to detect loca lized atherosclerotic plaques using noninvasive ultrasonic methods was advanced significantly by investigations into the nature and occurren ce of velocity disturbances created by arterial stenoses, and diagnosi s of carotid bifurcation disease using a combination of ultrasonic ima ging and Doppler measurement of blood velocity is now quite routine. S ince atherosclerotic plaques tend to be localized at sites of branchin g and artery curvature and since these locations would be expected to harbor complex flow patterns, investigators postulated that fluid dyna mics might play an initiating role in atherogenesis. Several fluid dyn amic variables were proposed as initiating factors. Investigations wer e undertaken during the 1980s in which fluid dynamic model experiments with physiologic geometries and flow conditions were employed to simu late arterial flows and in which morphometric mapping of intimal thick ness was performed in human arteries. Correlations between fluid dynam ic variables and intimal thickness revealed that atherosclerotic plaqu es tended to occur at sites of low and oscillating wall shear stress, and these observations were reinforced by studies in a monkey model of atherosclerosis. Concomitantly, it was realized that arteries adapt t heir diameters so as to maintain wall shear stress in a narrow range o f values around 15 dynes/cm2, findings which were based both on observ ations of normal arteries and on animal studies in which flow rates we re manipulated and arterial diameter adaptation was measured. Currentl y, a working hypothesis for the role of fluid dynamics in atherogenesi s is that intimal thickening is a normal response to low wall shear st ress, and this intimal thickening can develop into an early atheroscle rotic plaque under certain circumstances such as excessive low density lipoprotein concentrations in blood.