Assistance Ratio: An Approach to Quantify the Hydraulic Load Distribution in LVAD Therapy

Abstract

Left ventricular assist devices (LVADs) have grown in popularity for treating patients with severe heart failure. In many cases rotary blood pumps (RBPs) are used as LVADs, which are set to a fixed rotational speed in the clinical environment. The physicians rely on practical guidelines, available measurements and experience in choosing the rotational pump speed. This can lead to poor or unfavourable patient support, possibly even dangerous operating conditions. Hence, there exists great potential to implement closed-loop control of LVADs to overcome these problems. However, the choice of the control variable is not trivial. This paper presents a methodological approach to deriving a suitable control variable for closed-loop LVAD therapy. To simplify the setpoint choice for clinicians, an assistance ratio is introduced to represent the sharing of hydraulic workload between the LVAD and the native heart. Available measurements include the left ventricular and aortic root pressure, pump flow and aortic flow rate, with the latter estimated using a Kalman filter. Based on these measurements, assistance ratios based on flow, work and pulsatility are proposed. The merits of different assistance ratios are assessed based on their ability to provide insight into hemodynamic support, myocardial protection and left ventricular unloading, along with their ease of implementation and robustness of their calculation. Their implementation is tested in a hybrid mock circulatory loop and retrospectively on data from animal experiments. An assistance ratio based on flow is shown to be the most robust, whilst giving some indication of hemodynamic support. The closed-loop control using an assistance ratio therefore makes it easier to achieve different therapeutic goals. This approach has the potential to reduce interventions by the clinician and could eventually lead to a versatile definition of LVAD therapy protocols, independent of pump-type or patient.