Ventricular assist device volume compensation using a two phase fluid.

A two-phase fluid (TPF) volume compensation chamber (VCC), using Freon as a working fluid, has been developed to reduce pressure fluctuations created in electric ventricular assist devices (VADs) during emptying and filling of the blood sac. The advantages of a TPF VCC over the currently employed flexible compliance chambers are as follows: 1) the TPF VCC has rigid external walls, eliminating the deleterious effects of fibrous ingrowth; 2) the internal diaphragm is made of Saran HB, which has a permeability two orders of magnitude less than that of butyl rubber; and 3) the TPF VCC has an external volume of 477 ml, comparable to that of the flexible compliance chambers used for electric VADs with a stroke volume of 100 ml. Experiments were conducted with the VCC attached to an electric VAD operated in a mock circulatory system. The results show that the strong temperature sensitivity of Freon vapor pressure does not compromise VAD flow output over a temperature range of 6 degrees C. Since body temperature generally fluctuates no more than 5 degrees C, this result is promising. A compromise is seen, however, in VAD power consumption. As the temperature of the VCC drops 6 degrees C, the mean pressure of the system drops 95 mmHg, causing an increase in power consumption that may require introduction of a temperature control system.