Exercise physiology with a left ventricular assist device: Analysis of heart-pump interaction with a computational simulator.

Libera Fresiello,Gianfranco Ferrari,Bart Meyns,Frank Rademakers,Arianna Di Molfetta,Piet Claus,Vincenzo Lionetti

doi:10.1371/journal.pone.0181879

Abstract

Patients with a Ventricular Assist Device (VAD) are hemodynamically stable but show an impaired exercise capacity. Aim of this work is to identify and to describe the limiting factors of exercise physiology with a VAD. We searched for data concerning exercise in heart failure condition and after VAD implantation from the literature. Data were analyzed by using a cardiorespiratory simulator that worked as a collector of inputs coming from different papers. As a preliminary step the simulator was used to reproduce the evolution of hemodynamics from rest to peak exercise (ergometer cycling) in heart failure condition. Results evidence an increase of cardiac output of +2.8 l/min and a heart rate increase to 67% of the expected value. Then, we simulated the effect of a continuous-flow VAD at both rest and exercise. Total cardiac output increases of +3.0 l/min (+0.9 l/min due to the VAD and +2.1 l/min to the native ventricle). Since the left ventricle works in a non-linear portion of the diastolic stiffness line, we observed a consistent increase of pulmonary capillary wedge pressure (from 14 to 20 mmHg) for a relatively small increase of end-diastolic volume (from 182 to 189 cm3). We finally increased VAD speed during exercise to the maximum possible value and we observed a reduction of wedge pressure (-4.5 mmHg), a slight improvement of cardiac output (8.0 l/min) and a complete unloading of the native ventricle. The VAD can assure a proper hemodynamics at rest, but provides an insufficient unloading of the left ventricle and does not prevent wedge pressure from rising during exercise. Neither the VAD provides major benefits during exercise in terms of total cardiac output, which increases to a similar extend to an unassisted heart failure condition. VAD speed modulation can contribute to better unload the ventricle but the maximal flow reachable with the current devices is below the cardiac output observed in a healthy heart.

Highlights

Exercise incompetence is a hallmark of heart failure and is the result of multiple interacting cardiac and vascular impairments
Due to the sympathovagal imbalance implemented in the simulator, the peak HR reached for heart failure (HF) is 112 bpm, corresponding to 67% of the expected value
The Ventricular Assist Device (VAD) is capable to increase its flow only by +0.9 l/min at exercise. Given this limited increase of CO and the modest contribution of the VAD to it, we investigated if VAD speed modulation could provide some benefits in terms of hemodynamics during exercise

Summary

Introduction

Exercise incompetence is a hallmark of heart failure and is the result of multiple interacting cardiac and vascular impairments. VAD patients, hemodynamically stable at rest, reach only 50% of the expected exercise capacity [1]. Several clinical studies investigated why VAD therapy cannot restore exercise capacity to normal values [2,3]. What emerged so far is that VAD patients show similar limitations as heart failure patients such as autonomic nervous system impairment, chronotropic incompetence, poor perfusion of exercising muscles, right ventricular dysfunction and iron deficiency [4]. Heart failure and VAD patients show similar hemodynamics from rest to exercise, in terms of wedge pressure, pulmonary pressure and right atrial pressure elevation [5]. Some studies investigated if a VAD speed modulation during exercise could overcome this but results showed little effects on exercise performance [6] and on cardiac output [5]

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Conclusion