PRESSURE DEVELOPMENT WITHIN A SAC-TYPE PNEUMATICALLY DRIVEN VENTRICULAR ASSIST DEVICE

Intrinsic features of the pumping process of a pneumatically driven ve ntricular assist device (VAD) and the effects of different types of pn eumatic drivers upon its performance were investigated in vitro by ana lysing the pressure distributions within the device and the motions of the prosthetic valves. It was found that the stretching of the flexib le, elastic diaphragm in both late systole and diastole initiates a pr essure oscillation which directly affects the timing of the pumping pr ocess. The timing was also found to be dependant on the length and sti ffness of the cannulae which link the VAD to the model circulation sys tem. During the stretch-induced oscillation in late systole, the VAD h ousing experiences partial collapse due to fluid momentum effects, whi ch tends to increase the effective stroke volume of the device, and re duce the amplitude of the pressure oscillation. Reducing the rising (f alling) rate of driving pressures (dp(d)/dt) may not necessarily reduc e the maximum rate of change of the blood chamber pressure (dp(ch)/dt( max)) but may upset the stability of the pumping process. This is beca use a minimum dp(ch)/dt(max) exists, which is determined by the stretc h-induced oscillation. In order to minimize dp(ch)/dt(max) and to prov ide the device with a stable working condition, dp(d)/dt should match the dp(ch)/dt(max).