Flow studies in a rigid model of an aorto-renal junction A case for high shear as a cause of the localization of sudanophilic lesions in rabbits

Abstract

Flow studies were made in a rigid model of an aorto-renal junction in an attempt to explain two observations made in our laboratory in cholesterol-fed rabbits. First, the area of the lesion was proportional to the area of the orifice it surrounded, and hence probably to the flow into the branch. Second, lesions developed proximal as well as distal to blind stumps of a renal artery in nephrectomized rabbits. The branch had a sharp entrance, and came off at 90°. The area ratio of the branch and distal aorta to the proximal aorta was 1.27 and steady flow at Reynolds numbers of 570–1070 was studied both with dye injections and with a hot-film anaemometer. The flow ratio was varied from 0.16 to 0.58. Secondary motions were observed in the main channel and in the side branch at all flow rates. Boundary layer separation occurred on the outer wall of the main branch and on the proximal wall of the side branch. The thickness of the boundary layer separation in the main branch depended on the flow rate ratio. Downstream from the junction the velocity profiles were highly asymmetric in both the renal artery and the distal aorta. In the plane of the junction, peak velocities were skewed towards the inner wall. The velocity profiles perpendicular to the plane of the junction were less disturbed. The calculated shear stress on the inner wall of the main branch within one diameter of the junction was much higher than that calculated for the corresponding Poiseuille flow. The magnitude of the shear stress, and its extension downstream from the junction increased as the flow into the branch increased. This correlated with previous observation by Cornhill and Roach [1] in the cholesterol-fed rabbit, that atherosclerotic lesions develip in regions of high shear If the branch was occluded, no vortex formed in it, and streamlines in the trunk were not affected. This is opposite to the observations of Malcolm and Roach [2] in glass models with a rounded junctional zone where a vortex formed at the entrance when the branch was occluded. Since blind stumps develop proximal as well as distal lesions in the cholesterol-fed rabbit [3], the entrance geometry is probably very critical.