The Heart‐Hemopump Interaction: A Study of Hemopump Flow as a Function of Cardiac Activity

Abstract

The Hemopump is a useful left ventricular assist device. Because it is a rotary blood pump, the pump performance is not constant and is dependent on the cardiac cycle. We measured the static flow delivered by the pump at varying pressure heads (delta P) in a mock circulation. These data are compared to the pump performance in vivo. On the basis of these results, 5 sheep were instrumented for continuous Hemopump flow measurement as well as left ventricular and aortic pressure measurements. The Hemopump flow was relayed instantaneously to the pressure head. Low filling and ventricular failing (through intravenous administration of a beta-blocker) conditions were applied. The in vivo measured flows also are pressure head dependent, but the flow curve shows hysteresis 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 means flows (delta = 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 ventricle, delta P increases and flow is reduced. The cannula leaks and results in aortic insufficiency (0.36 +/- 0.05 L/min) when the pump is turned off. Several conclusions have been drawn from these tests: Cardiac activity is beneficial for the pump performance as well as when the aortic pressure curve is nonpulsatile; the longer the systolic phase, the higher the pump flow; the pump should never be turned off in clinical use, and filling is important for the pump's performance.

Simulation study of the Hemopump as a cardiac assist device

“…At each speed level, the Q-AP curve shows hysteresis, resulting in a loop that proceeds clockwise during a cardiac cycle. This phenomenon, which is due to the inertial property of the blood within the cannula and pump, was also observed previously by MEYNS et al (1996) explained by the fact that, when the pump speed is increased, more blood is expelled by the pump, and the ventricular cavity is reduced. As a result, the contribution of the heart to the pump flow is reduced, which is illustrated by the decrease in the upstroke in flow during the ejection phase at high speeds, in our simulation, the contractility and end-diastolic filling volume 70 "-~ of the left ventricle were set so that the resultant range of pressure differences across the pump was consistent with that reported by MEYNS et al (1996) in their in vivo experiment.…”

“…it has been suggested that the performance of the pump depends upon the physiological status of the heart, not only of the left ventricle but also of the right ventricle MEYNS et al, 1996;ROBERT, 2001).…”

“…Consequently, we obtain the values of the two coefficients An and Bn, in the linear equation (3) by fitting a least squares line to the corresponding data of MEYNS et aL (1996) in the range of 0-140mmHg. Figure 1 shows the seven fitted straight lines, and Table 1 lists the values of An and Bn for each of the seven pump rotation speeds.…”

“…This pump can be operated at seven different rotation speeds ranging from 17 000 (speed 1) to 26 000 revolutions min-* (speed 7), with an interval of 1500 revolutions min -1. MEYNS et al (1996) performed in vitro measurements of the steady flow-pressure difference relationships of the pump at the above seven speeds under pressure differences of up to 230 mmHg. Their data (Fig.…”

In Vitro Evaluation of an Axial Flow Pump: Mock-Pump Interaction and an Approach to Control

“…Circulatory mock temporal pressure and flow signals show similar changes, as observed during in vivo studies (26,27). There is a pressure transient observed in the aortic pressure waveform during the aortic valve closure (Fig.…”