Physiological Control of Left Ventricular Assist Devices

Abstract

This thesis describes a physiological controller for turbodynamic ventricular assist devices (VADs). VADs are blood pumps that are implanted in patients with severe heart failure and can be used temporarily until a later heart transplantation or permanently as an alternative to heart transplantation. Typically, VADs are implanted between the left ventricle (LV) and the aorta. The flow generated by turbodynamic VADs depends on the speed of the rotor. In current clinical practice, this speed is chosen by the physician and is kept constant. Changes in the hemodynamic status of the patient can render the chosen pump speed inappropriate such that undesired and potentially harmful events like overor underpumping can occur. Therefore, it is desirable to have an automatic control algorithm that matches the flow generated by the pump to the perfusion requirement of the patient and that effectively avoids overand underpumping. A preload sensitive speed (PRS) controller is developed in order to achieve a physiological adaptation of turbodynamic VADs. This controller changes the pump speed according to the end-diastolic volume of the LV. The control law behind the PRS controller is inspired by the Frank-Starling law of the heart and thereby closely imitates the human physiology. In vitro experiments show that the behavior of the PRS-controlled VAD is very similar to the behavior of the native heart. Thus far, however, no experiments have been conducted in vivo. The introduction of an LV volume feedback in the PRS controller is necessary to compensate for unknown disturbances. However, since feedback can lead to instability, a stability analysis of the PRS controller was conducted. A simplified model of the circulation and the VAD was implemented with a wide range of parameter combinations and its stability was investigated using the Nyquist criterion. For all parameter combinations, the closed loop system proved to be stable with a minimum gain margin of 3 and an infinite phase margin. In order to obtain physiological experimental results in vitro, a hybrid mock circulation was developed. It combines a numerical model of the human cardiovascular system with a real hydraulic blood pump. The numerical-hydraulic interface between the numerical model and the pump consists of two pressure-controlled reservoirs and a flow probe, which en-