Fluid dynamics of prosthetic heart valves

Abstract

The initial intention in the development of prosthetic heart valves was to provide a substitute for the normal and it was hoped that when a prosthetic valve, whether of biological or of non-biological origin, was inserted it would behave in a manner analogous to the normal valve. Increasing experience, however, is showing that the behaviour of prosthetic valves approximates to that of the diseased valve which it is replacing rather than to the normal valve^1–3 the difference between the two being merely one of degree. Thus, all prosthetic valves at present in use are associated with a degree of stenosis and also with a degree of regurgitation, closure being determined by flow reversal rather than by other fluid dynamic forces. To understand the reasons for this type of behaviour and, therefore, to be in a position to lay down criteria for improved valves, one has to consider the behaviour of the normal, the diseased and the prosthetic valve from two points of view. Firstly, the types of flow pattern which occur in the vicinity of valves and secondly, the way in which the energy loss (that is the fall in pressure head across the valve) is determined. The latter has three components: a frictional component accounted for by the viscosity of blood and being related directly to the cardiac output; an inertive loss due to the energy required for acceleration and deceleration of blood not fully recovered, which is related to the heart rate; and a turbulent loss which is related to the square of the cardiac output.^{4, 5}