Noninvasive evaluation of wall shear stress on retinal microcirculation in humans

PURPOSE. To evaluate wall shear stress (WSS) on retinal microcirculation noninvasively. METHODS. Retinal vessel diameter (D) and mean centerline blood velocity (V-max,V- (mean)) were measured in the retinal arterioles and venules at first- and second-order branches in 13 subjects using laser Doppler velocimetry (LDV). Retinal blood flow (RBF) and wall shear rate (WSR) were calculated using these two parameters. Blood viscosity at the calculated shear rate was also measured using a cone-plate viscometer. WSS was calculated as the product of the WSR and the blood viscosity. RESULTS. In the first-order branches, the averaged D, V-max,V- mean, RBF, and WSRmean were 108 +/- 13 mu m, 41 +/- 10 mm/s, 11 +/- 4 mu L/min, and 1539 +/- 383 s(-1) in the arterioles and 147 +/- 13 mu m, 23 +/- 3 mm/s, 12 +/- 4 mu L/min, and 632 +/- 73 s-1 in the venules, respectively. The apparent blood viscosities at the measured shear rates were 3.5 +/- 0.3 centipoise (cP) in the arterioles and 3.8 +/- 0.4 cP in the venules. Therefore, the averaged WSS was 54 +/- 13 dyne/cm(2) in the arterioles and 24 +/- 4 dyne/cm(2) in the venules. The WSS in the second-order arterioles was significantly lower than that in the first-order branches (P = 0.002), but the WSS in the first-order venules was similar to that in the second-order venules. CONCLUSIONS. The authors demonstrated that the WSS in the retinal vessels could be evaluated noninvasively in humans using LDV and cone-plate viscometry. This system may be useful for further clinical investigation of the role of shear stress in the pathogenesis of various retinal disorders.