Considerations for the Design and Control of Pulsatile Rotary Total Artificial Hearts

Abstract

Heart disease remains the leading cause of death in the developed world. The scarcity of donor hearts and limited efficacy of drug therapy in patients with end-stage heart failure establish ventricular assist devices (VADs) and total artificial hearts (TAHs) for blood circulation support as a clinical necessity. Due to their small size and superior durability, rotary blood pumps (RBPs) have almost entirely replaced earlier positive displacement VADs, while the only available TAH remains the pneumatically actuated SynCardia TAH. The development of rotary TAHs is underway and promises to open up new long-term treatment options in the future; however, there is ongoing controversy with respect to the attenuated-pulse or nonpulsatile waveforms observed with RBP support, which is fuelled by the increasing occurrence of adverse events such as gastrointestinal bleeding in rotary VAD patients. While in particular rotary TAHs operated at constant speed provide pulseless (or attenuated-pulse), continuous flow perfusion, rapid impeller speed modulation to modulate the pump outflow may be a viable approach to artificially induce pulsatile arterial pressure waveforms. However, previous attempts failed to restore physiologic levels of pulsatility, which is attributed to challenges in the design and control of a device capable to generate similar pulse amplitudes and rates of change of pressure (𝑑𝑃/𝑑𝑡) as the native heart. Therefore, to work towards this objective, the primary aim of this PhD project was to investigate and identify favourable design characteristics and control strategies for rapid RBP speed modulation. First, factors limiting device performance with speed modulation in praxis were derived theoretically and evaluated on the example of the BiVACOR TAH, where the motor drive was identified as the major source of power loss during rapid impeller acceleration. It was further indicated, that flat pressure head-flow (HQ) curves of the pump may improve the device performance. Secondly, a motor geometry analysis was performed using the finite element method (FEM) to evaluate axial flux motor designs with respect to their suitability for RBP speed modulation. The focus of the analysis was on geometries which may cater for favourable hydraulic geometries and with the objective to reduce axial attractive forces and rotor inertia and increasing the efficiency. A methodology to design a motor for desired characteristics was outlined and the required slot depth and permanent magnet thickness and resulting rotor inertia to compensate for performance deterioration due to increased inner stator radii and gap lengths were determined. Subsequently, speed control strategies were evaluated with the BiVACOR device in an in vitro study. The finding that the speed profile significantly influences the overall device efficiency and its pulsatile outflow led to the development of a model framework to numerically optimise rotary TAH speed profiles to maximise the 𝑑𝑃/𝑑𝑡 and/or surplus haemodynamic energy (𝑆𝐻𝐸) generated by the device. Based on the results, a potential control strategy for pulsatile rotary TAH was outlined, and the performance envelope in terms of maximum achievable pulsatility was explored on the example of the HeartMate II. It was found that pulsatility approaching physiologic levels with 𝑑𝑃/𝑑𝑡 > 400 𝑚𝑚𝐻𝑔/𝑠 can be generated with RBP; similar results were achieved in a preliminary in vivo study with the BiVACOR TAH applying optimised speed waveforms. However, rapid speed modulation may be implemented at the expense of a substantial increase in power consumption, thus device optimisation for the specific application may be required to implement a viable long-term pulsatile RBP operating mode. The numerical framework was then finally applied to investigate the influence of hydraulic and motor characteristics on the ability of a device to generate pulsatility. Six different pumps corresponding to RBP ranging from centrifugal pumps to axial flow pumps were modelled and compared in the numerical model. The modelling approached was based on specific speed as a design variable determining the impeller type and steepness of the HQ characteristics of the pumps. It was found that the maximum pulsatility generated by pumps decreased with increasing specific speed (increasing steepness of the HQ curve), which substantiated the hypothesis that a flat HQ-curve is beneficial for rapid speed modulation. Lastly, the relative influences of motor model parameters and hydraulic efficiency on the pump were investigated, showing that the motor torque constant and hydraulic efficiency were the most influential characteristics with respect to the device performance, indicating that motor design changes to optimise maximum torque and efficiency may be worthwhile even when the sacrifice of a slightly increased rotor inertia must be made to facilitate those changes.