Flow Visualization and LDA Measurements in Human Carotid Artery Models

Abstract

Abstract Flow parameters including pulsatile flow, wall distensibility, the non-Newtonian flow of blood in flow separation regions and high/low blood pressure, were studied in models of a healthy carotid artery model (37° bifurcation angle between the internal and external carotid artery), a 90% stenosis in the internal carotid artery and an 80% stenosis in both the internal and external carotid arteries. The goal of the study was to analyze the flow in detail, measuring the local velocity components. This knowledge is important for the interpretation of ultrasound measurements in vivo. Elastic silicon rubber models having a compliance similar to human vessels and the same surface structure as in the biological intima were prepared from casts of human carotid arteries taken at autopsy. Studies were done at various Reynolds numbers. Flow was visualized with colored dyes (steady flow) and with a photoelasticity apparatus and birefringent solution (pulsatile studies). Flow separation regions and reattachment points could be easily localized with these methods and recorded on video tape. The local velocity was measured with a 1-, 2-, or 3-D laser-Doppler-anemometer (LDA). The flow in the unstenosed model was Re = 250. In the stenosed models, the Reynolds number decreased to Re-213 under the same experimental conditions. High velocity fluctuations with vortices were found in the stenosed models. The jet flow in the stenosis increased up to 4 m/s. With an increasing bifurcation angle, the separation regions in the external and internal carotid artery increased. Increased blood pressure (a higher Re number) led to an increase in flow separation and to high velocity shear gradients. The highest shear stresses were nearly 20 times higher than normal. The 90% stenosis created very high velocity shear gradients and high velocity fluctuations. In addition to the ratio of the stenosis, the form (geometry) of the stenosis also played a major role in determining the flow structure. Behind the stenoses large eddies were found over the whole cross section. In these separation regions, particles may stick more easily to the wall and to existing stenoses. Sharp edged stenoses will grow faster than smooth stenosis formations. In the healthy carotid artery model only a slight flow separation region was observed in the internal carotid artery at the branching cross section. The flow in the healthy carotid artery model was almost ideal, whereas in the stenosed models the flow separations regions extended far into the internal carotid artery. Figure 1 shows the axial velocity component of a healthy carotid artery at Re = 350. The velocity profiles over the cross section 10mm downstream of the bifurcation in the internal carotid artery are shown for a blood-like fluid. The Womersley parameter was α = 4.15. The phase shown in 90°. Figure 2 shows the vertical velocity component at a phase of 90°. Figure 3 shows the velocity profiles in a model with a 90% stenosis in the internal carotid artery at a phase 90°. The differences in the velocity distribution can be clearly seen. High velocity fluctuations were recorded which may lead to chemical reactions in the blood cells. We conclude that a detailed understanding of flow is necessary before vascular surgery is performed especially before artificial grafts are implanted. Models should be prepared to help to optimize such grafts and no flow parameter can be neglected especially at bends and bifurcations.