Comparison of ball and disc valves for mitral valve replacement

Abstract

Ball valves, because of their bulk, interfere with myocardial contraction and are responsible for the relatively high incidence of postoperative morbidity and mortality. As a consequence, a low-profile valve, such as a disc valve, interferes less with myocardial function and is less prone to complications. Resistance to forward flow over a disc or ball valve is dependent upon the size of the residual valve orifice (inflow area) and the size of the frustum (outflow area). An insertion ring of any artificial valve restricts the valve orifice. Those valves with the thickest insertion rings will have the least residual valve orifice for flow; the smaller the valve employed the greater this restriction and gradient. This accounts for the greater resistance over a Magovern valve than other prevalent types. The optimal design for a disc valve encompasses as narrow an insertion ring as possible to provide a large orifice area in addition to as large a frustum area as possible to reduce the resistance of the disc to forward flow and to allow a large free area for flow to escape between the disc and valve ring. Laboratory studies indicated that a ratio of approximately 1 to 1.5 for orifice area to frustum area was necessary to provide the least gradient. The proximity of the frustum area anatomically to the posterolateral myocardial wall further restricts the flow of blood in this area. To overcome this, the frustum area for the disc valves of various sizes provided for this additional factor. The clinical results in the use of the disc valve of our design have demonstrated that it is associated with a lower operative mortality and morbidity. The smaller cage has not interfered with myocardial contraction nor produced ventricular septal irritation. There has been a considerably lessened incidence of thromboembolism. Comparative hemodynamic studies have demonstrated a superior physiologic response following the use of the Kay-Suzuki disc valve as compared to the Starr-Edwards ball valve.