Flow through a venous valve and its implication for thrombus formation

To elucidate the possible connection between the flow patterns in the pockets of venous valves and thrombus formation, detailed studies of the behavior of model particles and red cells flowing through a venous valve have been carried out using isolated transparent dog saphenous veins containing two-leaflet valves, and cinemicrographic techniques. It was found that large paired vortices, located symmetrically on both sides of the bisector plane of the valve leaflets, were present in each valve pocket under physiological flow conditions. Particles continually entered the valve pockets from the mainstream, spending long periods of time describing a series of spiral orbits of decreasing diameter, while moving away from the bisector plane, and eventually left the vortex, rejoining the mainstream. With concentrated suspensions of red cells, it was found that another smaller counter-rotating secondary vortex, driven by the large primary vortex existed deep in each valve pocket. The concentration of red cells in this secondary vortex remained appreciably lower than that in the mainstream. In such regions, fluid circulated with extremely low velocities, thus creating a very low shear field which allowed red cells to form aggregates. The results suggest that in some pathological states, the valve-pocket vortices could act as automatic traps and generators of thrombi in a fashion similar to that previously demonstrated in an annular vortex formed downstream from a sudden tubular expansion.