The distribution of nonstenosing, asymptomatic intimal plaques in 12 adult human carotid bifurcations obtained at autopsy was compared with the distribution of flow streamline patterns, flow velocity profiles, and shear stresses in corresponding scale models. The postmortem specimens were fixed while distended to restore normal in vivo length, diameter, and configuration. Angiograms were used to measure branch angles and diameters, and transverse histological sections were studied at five standard sampling levels. Intimal thickness was determined at 15 degrees intervals around the circumference of the vessel sections from contour tracings of images projected onto a digitizing plate. In the models, laser-Doppler anemometry was used to determine flow velocity profiles and shear stresses at levels corresponding to the standard specimen sampling sites under conditions of steady flow at Reynolds numbers of 400, 800, and 1200, and flow patterns were visualized by hydrogen bubble and dye-washout techniques. Intimal thickening was greatest and consistently eccentric in the carotid sinus. With the center of the flow divider as the 0 degree index point, mid-sinus sections showed minimum intimal thickness (0.05 +/- 0.02 mm) within 15 degrees of the index point, while maximum thickness (0.9 +/- 0.1 mm) occurred at 161 +/- 16 degrees, i.e., on the outer wall opposite the flow divider. Where the intima was thinnest, along the inner wall, flow streamlines in the model remain axially aligned and unidirectional, with velocity maxima shifted toward the flow divider apex. Wall shear stress along the inner wall ranged from 31 to 600 dynes/cm2 depending on the Reynolds number. Where the intima was thickest, along the outer wall opposite the flow divider apex, the pattern of flow was complex and included a region of separation and reversal of axial flow as well as the development of counter-rotating helical trajectories. Wall shear stress along the outer wall ranged from 0 to -6 dynes/cm2. Intimal thickening at the common carotid and distal internal carotid levels of section was minimal and was distributed uniformly about the circumference. We conclude that in the human carotid bifurcation, regions of moderate to high shear stress, where flow remains unidirectional and axially aligned, are relatively spared of intimal thickening. Intimal thickening and atherosclerosis develop largely in regions of relatively low wall shear stress, flow separation, and departure from axially aligned, unidirectional flow. Similar quantitative evaluations of other atherosclerosis-prone locations and corresponding flow profile studies in geometrically accurate models may reveal which of these hemodynamic conditions are most consistently associated with the development of intimal disease.

T he flow at the carotid artery bifurcation may have significant bearing on the development and management of atherosclerosis in this major blood vessel. For over a century, hypotheses have linked flow disturbances at this and other arterial branches with the formation of atheromas. Furthermore, advances in noninvasive ultrasound studies of blood velocity in the carotid arteries have pointed to the need for a more rigorous understanding of normal physiologic flow patterns at this branch. Fluid flow patterns at branches are highly dependent on geometry, Reynolds number, and flow division ratios. In the case of human physiology, the flow is additionally pulsatile, introducing a time-dependent behavior. Second order effects which influence the flow, but to a much lesser degree, are the compliance of the arterial wall 1 and the non-Newtonian viscosity of blood. 2 Recent investigators have described an average geometry of the human carotid bifurcation based on bipianar angiograms and cadaver specimens. quantified. 4 These results indicated circumferential secondary helices in the internal carotid branch, very low wall shear stresses at the outer wall of the sinus, and relatively large shear stresses at the apex and distal end of the sinus as the flow enters the internal carotid artery. No turbulence was observed in those studies. The secondary flow appeared to be influenced most strongly by a flow division between the daughter branches and, to a lesser degree, by the upstream Reynolds number.This paper presents the results of visualization of pulsatile flow patterns in a model of the carotid bifurcation and compares our observations to the results obtained with steady flow experiments. MethodsThe model used in this study was constructed of blown glass after determining an average geometry from 57 bipianar angiograms of 22 adult subjects ranging from age 34 to 77 years. 3 The human carotid pulse was simulated by a half-sine wave with DC offset to model the acceleration and deceleration of systole, followed by a relatively steady period of positive diastolic flow ( figure 1 A). A servocontrolled shaker valve was connected in series with a constant pressure head to generate the pulsatile flow.

No abstract

No abstract