Restoring pulsatility and peakVO2 in the era of continuous flow, fixed pump speed, left ventricular assist devices: 'A hypothesis of pump's or patient's speed?'

Physiology of Continuous-Flow Left Ventricular Assist Device Therapy

The expanding use of continuous-flow left ventricular assist devices (CF-LVADs) for end-stage heart failure warrants familiarity with the physiologic interaction of the device with the native circulation. Contemporary devices utilize predominantly centrifugal flow and, to a lesser extent, axial flow rotors that vary with respect to their intrinsic flow characteristics. Flow can be manipulated with adjustments to preload and afterload as in the native heart, and ascertainment of the predicted effects is provided by differential pressure-flow (H-Q) curves or loops. Valvular heart disease, especially aortic regurgitation, may significantly affect adequacy of mechanical support. In contrast, atrioventricular and ventriculoventricular timing is of less certain significance. Although beneficial effects of device therapy are typically seen due to enhanced distal perfusion, unloading of the left ventricle and atrium, and amelioration of secondary pulmonary hypertension, negative effects of CF-LVAD therapy on right ventricular filling and function, through right-sided loading and septal interaction, can make optimization challenging. Additionally, a lack of pulsatile energy provided by CF-LVAD therapy has physiologic consequences for end-organ function and may be responsible for a series of adverse effects. Rheological effects of intravascular pumps, especially shear stress exposure, result in platelet activation and hemolysis, which may result in both thrombotic and hemorrhagic consequences. Development of novel solutions for untoward device-circulatory interactions will facilitate hemodynamic support while mitigating adverse events. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

An Intra-Cycle Optimal Control Framework for Ventricular Assist Devices Based on Atrioventricular Plane Displacement Modeling

A promising treatment for congestive heart failure is the implementation of a left ventricular assist device (LVAD) that works as a mechanical pump. Modern LVADs work with adjustable constant rotor speed and provide therefore continuous blood flow; however, recently undertaken efforts try to mimic pulsatile blood flow by oscillating the pump speed. This work proposes an algorithmic framework to construct and evaluate optimal pump speed policies with respect to generic objectives. We use a model that captures the atrioventricular plane displacement, which is a physiological indicator for heart failure. We employ mathematical optimization to adapt this model to patient specific data and to find optimal pump speed policies with respect to ventricular unloading and aortic valve opening. To this end, we reformulate the cardiovascular dynamics into a switched system and thereby reduce nonlinearities. We consider system switches that stem from varying the constant pump speed and that are state dependent such as valve opening or closing. As a proof of concept study, we personalize the model to a selected patient with respect to ventricular pressure. The model fitting results in a root-mean-square deviation of about 6 mmHg. The optimization that considers aortic valve opening and ventricular unloading results in speed modulation akin to counterpulsation. These in silico findings demonstrate the potential of personalized hemodynamical optimization for the LVAD therapy.

Exercise physiology in left ventricular assist device patients: insights from hemodynamic simulations

Left ventricular assist devices (LVADs) assure longer survival to patients, but exercise capacity is limited compared to normal values. Overall, LVAD patients show high wedge pressure and low cardiac output during maximal exercise, a phenomenon hinting at the need for increased LVAD support. Clinical studies investigating the hemodynamic benefits of an LVAD speed increase during exercise, ended in inhomogeneous and sometimes contradictory results. The native ventricle-LVAD interaction changes between rest and exercise, and this evolution is complex, multifactorial and patient-specific. The aim of this paper is to provide a comprehensive overview on the patient-LVAD interaction during exercise and to delineate possible therapeutic strategies for the future. A computational cardiorespiratory model was used to simulate the hemodynamics of peak bicycle exercise in LVAD patients. The simulator included the main cardiovascular and respiratory impairments commonly observed in LVAD patients, so as to represent an average hemodynamic response to exercise. In addition, other exercise responses were simulated, by tuning the chronotropic, inotropic and vascular functions, and implementing aortic regurgitation and stenosis in the simulator. These profiles were tested under different LVAD speeds and LVAD pressure-flow characteristics. Simulations output showed consistency with clinical data from the literature. The simulator allowed the working condition of the assisted ventricle at exercise to be investigated, clarifying the reasons behind the high wedge pressure and poor cardiac output observed in the clinics. Patients with poorer inotropic, chronotropic and vascular functions, are likely to benefit more from an LVAD speed increase during exercise. Similarly, for these patients, a flatter LVAD pressure-flow characteristic can assure better hemodynamic support under physical exertion. Overall, the study evidenced the need for a patient-specific approach on supporting exercise hemodynamics. In this frame, a complex simulator can constitute a valuable tool to define and test personalized speed control algorithms and strategies.