Hydrodynamics in Bifurcations of the Coronary Arteries with Special Reference to Atherogenesis

In order to elucidate a possible role of hydro-dynamics in atherogenesis, we have studied (i) the vessel geometries and (ii) the flow disturbances in the coronary artery bifurcations of the pig.Intact fresh hearts, each of which included the ascending aorta, were obtained at a local slaughterhouse. The molds of some coronary arterial systems were constructed by using a plastic resin, Technovit 4143-C (Kulzer & Co. G. m. b. H.), to investigate in detail the architecture of the heart vessels. The left coronary artery (LCA), after arising from the sinus of the aorta, runs a short distance in the form of the funnel-shaped convergence and gives off the left circumflex (LCX) and anterior descending (LAD) arteries. The LCX and its branches were characterized by Y-shaped bifurcations. On the other hand, the LAD and their tributaries showed distinctive features of side branches. The same features are also observed in the right coronary artery (RCA) and its branches. As the main stem of the RCA branches off the rami atrioventriculares, it curves sharply. In the case of the Y-shaped bifurcation, the parent vessel changes its cross section from a circle to a dumbbell via an ellipse when it bifurcates. The elastic hoop stress acting the wall due to a pressure load is not uniform in tubes with such a non-symmetric cross section. Therefore stress or strain concentration may be expected at the bifurcations and this kind of non-uniformity might be a factor in atherogenesis.To visualize the flow pattern in the coronary artery bifurcations, the major coronary arteries with the ascending aorta were made transparent after removing the surrounding myocardial tissues. The test fluid was used to perfuse the transparent vessels at a constant flow rate in the physiological range. Secondary flow, which has a similar pattern to the horseshoe vortex produced around a wall-based protuberance in a tube, was generated at the bifurcations except for the first LCA branching site. The lack of secondary flow at the LCA bifurcation is attributable to the appreciable decrease in total cross-sectional area after the bifurcation. This area reduction is typical in pig heart vessels but not in humans. Therefore we can expect the generation of secondary flow at the human LCA bifurcation. The secondary flow produces complicated flow field around the bifurcations in collaboration with flow separation, which is occasionally generated due to the flow division ratio between the branches and geometrical abnormalities. The disturbed flow affects wall shear stress distribution and movements of particulate elements of blood, which may lead the disease process of atherosclerosis.