Principles of Fluid Mechanics Applied to Some Situations in the Human Circulation and particularly to the Testing of Valves in a Pulse Duplicator

Abstract

The use of a pulse duplicator to test prosthetic aortic valves or to investigate the mechanics of diseased valves after death is now well established (McMillan, Daley, and Matthews, 1952; McMillan, 1955). In testing an artificial valve, two main ques-tions have to be answered. The first concerns the mechanical strength of the valve and therefore its ability to withstand for many years pressures and flow rates comparable to those it would experi-ence in the human body. The second problem is to determine whether the flow characteristics of the prosthetic valve are suitable for clinical use. Until now the pulse duplicator has been used to answer only the first question and little attention has been paid to the second. In preliminary experiments with a new type of pulse duplicator, we experienced some difficulty in producing simultaneously with water a pressure wave form and quantity flow comparable to those produced by blood in life. So far as the authors are aware, almost all pulse duplicators hitherto reported use water as a working fluid (Davila, Trout, Sunner, and Glover, 1956; Kelley, Goodale, and Castleman, 1960; Callaghan, Wil-lans, and Cardozo, 1961). The use of glycerine-water mixtures, as reported by Marx, Baldwin, and Kittle (1959) and Starkey, Sirak, Collins and Hagan (1963), is exceptional. There are sound practical reasons for this, but the significance of the results when using water instead of blood is open to criticism. Blood is more viscous than water, and it seems that it should be easier to per-fuse water through a heart than blood. In practice, the reverse appeared to be true. This difficulty forced on us the more fundamental examination of the problem, which is the subject of this paper. TYPES OF FLUID FLOW In order to make clear the arguments that follow, it is useful to look first at some of the flow phenomena that may occur in the circulation. Fluid flow in a closed duct may be either laminar or turbulent. The properties of flow change markedly between these two states. In changing from one type of flow to the other, there is normally an unstable transition period. LAMINAR FLOW The characteristic of laminar flow is the absence of lateral mixing. A stream of dye introduced into laminar flow remains distinct. The liquid in fact moves in a series of concentric cylinders like a well lubricated expanding tele-scope, minimizing friction both against the walls of the duct and between the particles of fluid themselves. Any disturbance introduced into such a flow will be damped out.