Flow patterns and distributions of fluid velocity and wall shear stress in the human internal carotid and middle cerebral arteries.

BACKGROUND: The aim of this study is to elucidate the relationship between the flow patterns and the preferred sites of the development of atherosclerotic lesions and cerebral aneurysms in the human ICA and MCA.
METHODS: Five isolated transparent arterial trees containing the ICA and MCA with a sufficient length of the carotid siphon were prepared from humans postmortem, and flow patterns and distributions of fluid velocity and wall shear stress in these vessels were studied in detail using flow visualization and high-speed cinemicrographic techniques.
RESULTS: In the carotid siphon that contained several acute bends, due to the impingement and deflection of the flow at the bends, a strong and complex helicoidal flow formed. As a result, the approaching velocity profile was flattened at the terminal bifurcation of the ICA, but it was sharpened at the first bifurcation of the MCA. Thus, at this latter bifurcation, fluid elements impinged on the vessel wall around the flow divider with much larger velocity than that at the preceding terminal bifurcation of the ICA. Throughout the entire arterial tree, atherosclerotic lesions were found almost exclusively in regions of low wall shear stress.
CONCLUSIONS: The carotid siphon provided a flattened approaching velocity profile at the terminal bifurcation of the ICA, making the hemodynamic stresses (pressure, tension, and shear stress) exerted on the vessel wall much lower than that at the bifurcation of the MCA where the approaching velocity profile was sharpened. This may account for the relatively low incidence of aneurysm formation at this site.