Fresh human aortic and artificial heart valves studied in vitro using ultramicroscope anemometry. Further results

The systolic flow phenomena distal to 3 different bioprostheses (Hancock, Carpentier Edwards, Ionescu Shiley) and to a preparation of a fresh human aortic valve was investigated in vitro by means of ultramicroscope anemometry and pressure measurements. The following hemodynamic criteria were used to compare the function of these and the previously tested valves quantitatively (6): 1. Total pressure loss produced by the valve, 2. Magnitude and shape of the region of flow separation distal to the valve. Distal to disc valves large excentric deadwater regions combined with stagnation point flow were observed (6). A flow of such a pattern is known to be thrombogenic (4). Hydrodynamically, the bioprostheses approximate the fresh human aortic valve only with respect to the absence of a thrombogenic flow pattern. The pressure loss of the bioprosthetic valves, however, is unacceptably high. Which properties of bioprostheses are responsible for their hydrodynamic disadvantage? The results show that the flow leaving the natural human aortic valve is divergent enough to remain attached to the aortic wall, whereas, in bioprostheses, the flow is either convergent or its divergency is not sufficient to avoid flow separation. Distal to bioprostheses nearly 100 mm long jets appear surrounded by a concentric deadwater region. This disturbed flow causes the high pressure loss of bioprostheses. Consequently further improvement of bioprostheses may be attained not only by enlarging the orifice diameter relative to the sewing ring diameter, but furthermore by preserving the bell-shaped design of the opened valve in order to avoid flow separation.