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 driven ventricular assist device (VAD) were monitored non-intrusively using a laser Doppler anemometer (LDA) working in the back-scattering mode. The interaction between the diaphragm and the flow, and the causative 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 identified diaphragm points were then interpolated into two-dimensional surfaces to reconstruct the moving patterns of the diaphragm at selected instants. The motion is strongly affected by the orientation of the device with respect to gravity due to hydrostatic pressure gradients on the liquid side, which are generally large compared with those associated with the fluid accelerations. Hence, during ejection, the part of the diaphragm experiencing the lowest hydrostatic pressure is always set in motion first, while during filling, the part with the highest hydrostatic pressure moves first. Consequences of these non-symmetrical motions on local flow behaviour are discussed, and methods of controlling diaphragm shape during pumping are outlined.