EXPERIMENTAL INVESTIGATION OF THE MOTIONS OF THE PUMPING DIAPHRAGM WITHIN A SAC-TYPE PNEUMATICALLY DRIVEN VENTRICULAR ASSIST DEVICE

The motions of the pumping diaphragm within a sac-type pneumatically d riven ventricular assist device (VAD) were monitored non-intrusively u sing a laser Doppler anemometer (LDA) working in the back-scattering m ode. The interaction between the diaphragm and the flow, and the causa tive factors of the motions were investigated. The motions of a number of points on the diaphragm were determined by analysing the timing of interruption of the Doppler signals due to intrusion of the diaphragm on the LDA sampling volume. The position-time relationships for the i dentified diaphragm points were then interpolated into two-dimensional surfaces to reconstruct the moving patterns of the diaphragm at selec ted instants. The motion is strongly affected by the orientation of th e device with respect to gravity due to hydrostatic pressure gradients on the liquid side, which are generally large compared with those ass ociated with the fluid accelerations. Hence, during ejection, the part of the diaphragm experiencing the lowest hydrostatic pressure is alwa ys set in motion first, while during filling, the part with the highes t hydrostatic pressure moves first. Consequences of these non-symmetri cal motions on local flow behaviour are discussed, and methods of cont rolling diaphragm shape during pumping are outlined.