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DOES HIGH SHEAR STRESS INDUCED BY BLOOD FLOW LEAD TO ATHEROSCLEROSIS? MARGOT R. ROACH* and NORMAN B. SMITHi Atherosclerosis is one of the commonest causes of death in North America and occurs almost universally in older people. In spite ofa large amount of research, we still do not know either what causes the disease or how to prevent it. One ofthe most typical features ofatherosclerosis is that it produces localized lesions. The World Health Organization has defined it as a patchy deposition of lipid in the subendothelial layers of an artery with subsequent development of elastic fragmentation and fibrous deposition with or without necrosis and thrombosis. From our biased point ofview, unless we can find a theory that explains the patchy deposition ofthe lesions, we will not understand what causes the disease. The arterial tree consists ofa network of branching distensible vessels that transport blood with pulsatile flow. In virtually all species, most of the early lesions occur at bends and bifurcations. We believe that the flow distortions produced in these regions are more apt to explain the localization of plaques than are the small differences in the wall structure that occur there. Unfortunately, no definitive experiments have been done, or can be done with existing technology, to measure the local differences in forces or stresses produced by the blood flow in the region of bends or bifurcations. Hence we will assess the indirect evidence that we believe suggests that high shear stress may cause local damage to the wall in the regions that develop lesions. While a number ofauthors have speculated on what hemodynamic force could affect the vessel wall, tensile and compressive forces seem much less likely than shear forces. Shear, by definition, is related to the velocity gradient with respect to distance, dv/ds. In figure 1, there is high shear stress on the A wall and low shear stress on the B wall. A shearing force is applied parallel to the Work supported by Ontario Heart Foundation.»Departments of Biophysics and Medicine, University of Western Ontario, and Department of Medicine, University Hospital, London, Canada N6A 5Cl.¦(¦Department of Biophysics, University of Western Ontario.© 1983 by The University of Chicago. AU rights reserved. 0031-5982/83/2602-0320$01.00 Perspectives in Biology and Medicine, 26, 2 ¦ Winter 1983 | 287 B R Inlet Developed Flow E Separation Zone Fig. 1.—Schematic diagram ofthe velocity profiles (shown by the length ofdie arrows) in different parts ofa straight tube exiting from a reservoir and terminating in a bifurcation. surface and makes one layer slide on the other, as seen, for example, with a deck of cards. In engineering, shear is measured with a hot-film anemometer. Basically a platinum film is applied to the end of a tube and wired to a heater. The film is kept at a constant temperature that is 2-3 percent higher than that of the fluid. As the colder fluid flows past, the film is cooled (faster if the flow is faster), and the current required to heat the film to its original temperature can be calibrated to measure velocity. By measuring the velocity at known distances from the wall, the velocity gradient (dv/ds) can be determined [I]. The chief difficulty with this instrument is that the smallest available one is about 1 mm in diameter, and obviously this will cause a significant distortion in flow in even relatively large arteries. For this reason, it seems more reasonable to use large models that are hydrodynamically similar to the arteries to estimate what the shear stresses are likely to be near bends and bifurcations. Hydrodynamk Modeling In a long straight tube the flow is relatively simple. As it enters from a reservoir, the velocity profile is flat (i.e., the velocity is constant across the tube). Then, because the flow cannot slip on the wall [2] and the fluid is viscous, the center velocity gradually becomes faster and faster through the inlet zone (C-D), as shown in figure 1. Eventually the fluid reaches a steady state where there is no acceleration, and the velocity profile is constant, as shown in the D-E region of figure 1. This is known as the...


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