Arterial occlusive disease: a function of vessel bifurcation angle

A clinical observation that patients with aortoiliac occlusive disease have a high aortic bifurcation serves as the stimulus for this study. A review of 100 consecutive abdominal aortograms revealed that patients with occlusive disease had an average bifurcation angle of 38 degrees and those with normal studies or aneurysmal disease had an average angle of 52 degrees. A flow visualization system was constructed to study shear stress using eight Pyrex models with varying bifurcation angles (20 to 90 degrees). The perfusate consisted of a 35% sucrose solution with anion-exchange beads to serve as flow particles. The bifurcations were photographed at three different flow rates. The length of each tracer particle was measured to determine its velocity. Velocity profile curves were constructed and shear stress calculated by the formula shear stress = mu x dv/dr, where mu is viscosity, v is velocity, and r is radius. Results of the study suggest that more acute angles cause a greater shear stress on the inner wall of the bifurcation and decreased shear stress on the outer wall. Two theories of atheroma predilection, high shear stress and low shear stress, are applied to patients with high acute aortic bifurcations.