THE HEART-HEMOPUMP INTERACTION - A STUDY OF HEMOPUMP FLOW AS A FUNCTION OF CARDIAC ACTIVITY

The Hemopump is a useful left ventricular assist device. Because it is a rotary blood pump, the pump performance is not constant and is depe ndent on the cardiac cycle. We measured the static flow delivered by t he pump at varying pressure heads (Delta P) in a mock circulation. The se data are compared to the pump performance in vivo. On the basis of these results, 5 sheep were instrumented for continuous Hemopump flow measure ment as well as left ventricular and aortic pressure measureme nts. The Hemopump flow was relayed instantaneously to the pressure hea d. Low filling and ventricular failing (through intravenous administra tion of a beta-blocker) conditions were applied. The in vivo measured flows also are pressure head dependent, but the flow curve shows hyste resis resulting in a loop during each cardiac cycle. The in vivo peak flows (Delta P = 0) are similar to the in vitro data. The in vivo mean s flows (a = 50 mm Hg) are similar to the in vitro data for the lower pump speeds but are less than that at the higher pump speeds (3.74 +/- 0.55 L/min in vivo at Speed 7 versus 4.6 L/min in vitro). Low filling interrupts the Delta P-flow loop and reduces flow. In the failing ven tricle, Delta P increases and flow is reduced. The cannula leaks and r esults in aortic insufficiency (0.36 +/- 0.05 L/min) when the pump is turned off. Several conclusions have been drawn from these tests: Card iac activity is beneficial for the pump performance as well as when th e aortic pressure curve is nonpulsatile; the longer the systolic phase , the higher the pump flow; the pump should never be turned off in cli nical use; and filling is important for the pump's performance.