Pressure Sensitivity of Axial-Flow and Centrifugal-Flow Left Ventricular Assist Devices

Continuous-flow ventricular assist devices (CF-VADs) defy normal physiologic principles associated with pulsatile flow. Despite being programmed at set speeds, pump flow can be modified by variations in the pressure differential across the pump, termed pressure sensitivity (PS). Currently, PS has been reported using steady-state closed-loop systems that are unable to provide physiologically-relevant assessment of PS or account for partially- or fully-unloaded ventricles. We report a unique model system to examine PS and its influence on efficiency of CF-VADs. A mock-circulation loop was designed that measures low and high extremes of pressure differential. Two axial-flow and two centrifugal-flow VADs were tested. Device output flow rate, preload, and afterload were measured and PS was calculated. Numerical models were implemented to simulate “fully-loaded,” “partially-unloaded,” and “fully-unloaded” cardiac cycles. Our open-loop model successfully generated pressure gradients that were lower than typical when using static, closed-loop systems. All devices exhibit highest PS during early diastole; however, average PS values of centrifugal-flow were 3× greater than axial-flow devices. The average maximum PS for the axial and centrifugal VADs under physiologic conditions was 0.08 and 0.42 L/min/mmHg, respectively. Compared to the axial-flow pumps, the two centrifugal-flow VADs in our study demonstrate increased PS at intermediate to low flow rates. Enhanced device PS allows for more effective self-regulation of device output, thus allowing a given VAD to better mimic the native heart under exercise conditions, and minimize undesirable effects, including ventricular suck-down or atrial collapse.