Interaction between secondary velocities, flow pulsation and vessel morphology in the common carotid artery.

The common carotid artery (CCA), one of the vessels more frequently investigated by ultrasound (US), is often modeled as a straight tube in quasi-laminar flow regimens. Experimental investigations based on a prototype multigate system show that blood velocity profiles are parabolic during diastole and early systole, and flat during the systolic peak. However, during late systole/beginning of diastole, they have an "M" shape, where the velocity near the walls is higher than in the vessel center. Moreover, the profile shape changes when the sound beam direction is moved over a given cross-section; thus, suggesting a nonaxisymmetrical velocity distribution, which contradicts the straight tube assumption. The purpose of this paper was twofold. First, the actual velocity distribution in "normal" CCAs was reconstructed. The analysis of several velocity profiles confirms that the velocity distribution is markedly asymmetrical, especially during the deceleration phase following the systolic peak. Second, a tentative explanation for such behavior is given by correlating it with the growth of secondary flows caused by the slight vessel curvature and viscous effects. This explanation is supported by the comparison between in vitro results and numerical solution of the Navier-Stokes equations in laminar pulsed-flow regimens.