The role and source of respiratory heart rate variability: exploration using mathematical models of heart rate regulation and blood circulation

Respiratory heart rate variability (RespHRV), also known as Respiratory Sinus Arrhythmia, is a healthy variation of Heart Rate (HR) whereby HR increases during inhalation and decreases during exhalation. Although RespHRV has long been used as a prognostic indicator of heart health, several hypotheses have been suggested for its physiological benefits. The aim of this study was to test the hypothesis that RespHRV minimizes the work done by the left ventricle of the heart while maintaining physiological levels of carbon dioxide in the blood. To do this, we coupled a previously published mathematical model of HR control, which includes a model of the lungs and gas exchange, with two different mechanical models of blood circulation. This allowed us to calculate the work done by the left ventricle of the heart from its pressure and volume and study the effects of different models of heart contraction, which could not have been done previously. We found that although the central respiratory modulation of HR is the main source of RespHRV in our model, changes in the intrathoracic pressure also have a sizeable effect on RespHRV. The two models of heart contraction led to different responses when the effect of the intrathoracic pressure on blood flow was removed from the models, providing a potential way to test the models experimentally. We calculated the work done by the left ventricle of the heart over a whole interval or beat-by-beat and although the two work calculation methods gave a value of RespHRV for which a minimum work exists, the beat-by-beat calculations gave lower values, closer to physiological values.