Hemodynamic system analysis of intraarterial microaxial pumps in vitro and in vivo

Because of the lack of a sophisticated pump management system, the performance of the Hemopump in patients cannot assessed successfully. To clarify the interrelationship between an intravascular nonpulsatile pump and a pulsating ventricle, an in vitro study was set up under controlled conditions. Before these in vitro experiments, a series of in vivo experiments were performed in sheep using Hp31 cannulae. As anticipated, the resulting pulsatile pump flow was a function of the momentary pressure difference across the pump. This varying pump flow showed a significant flow loop hysteresis, indicating that the pressure difference across the pump is not the only parameter governing momentary pump flow of a rotary pump operating at constant speed in a pulsatile environment. Furthermore, flow in the Hp31 was significantly influenced by the inflow situation, blood supply, size of the ventricular cavity, and shape and position of the inflow cannula within the ventricle. Pulsatile flow conditions with good as well as impaired inflow into the pump were accordingly simulated in vitro to verify the in vivo measurements, to characterize the various inflow conditions, and to discuss methods of improved pump management. As a result of the in vivo and in vitro experiments, one can rely on the measurement of nonpulsatile in vitro flow and pressure differences across the pump to characterize the momentary pump flow for good inflow conditions into the pump. For these situations, the flow hysteresis produced, caused by fluid inertia within the pump and cannula, can be neglected. In contrast, for an impaired inflow situation, the calculated pump flow based on pressure difference measurements can be misleading. Consequently, an improved pump management system is required to adjust the pump speed, the pump performance, to any kind of impaired inflow.