To claritfy the detailed in-vivo hemodynamics of aorto-renal bifurcation, we measured the velocity profiles and bifurcation geometry at a surgically exposed left renal artery in 10 anesthetized dogs using a 20 MHz, 80 ch. pulsed Doppler velocimeter and 30 MHz, high resolution ultrasound tomography, respectively. The angle between the aorta and the left renal artery ranged from 60° to 90°. The diameter of the left renal artery ranged from 1.5 to 3.5mm. The velocity profile in the proximal renal

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... rtery in the plane parallel to the bifurcation showed a skewed pattern toward the caudal-side wall through a cardiac cycle. Reverse flow near cranial-side wall was observed even during systole, indicating flow separation. When the probe was placed on the cranial-side wall perpendicular to the vessel, the velocity component from the cranial to caudal-side wall was observed, indicating the existence of secondary flow in that direction. At 4 diameters distal to the renal ostia, velocity profiles were almost parabolic. In conclusion, the in-vivo flow pattern at the cranial-side wall of the renal ostia, where early development of atheromatous plaque is prone to localize, was characterized by (1) low shear rate, (2) oscillation of flow and (3) separation of flow.