Modeling cardiorespiratory coherence in exercise anticipation
Volitional motor activity is associated with a feedforward cardiorespiratory response to actual or impending movements. We have previously shown in the CRC study that the expectation of physical exercise causes a decrease in cardiorespiratory coherence that scales with the anticipated load. The present work uses a modeling approach to investigate the mechanisms that can cause a fall in cardiorespiratory coherence (CRC). We devised a Hodgkin-Huxley model of a cardiac pacemaker cell using the NEURON module. We simulated the effect of autonomic tone, sympathetic & respiratory-vagal modulation, and respiratory irregularity on pacemaker cell output by injecting efflux/influx current to model the parasympathetic/sympathetic effects, respectively. The vago-sympathetic tone was modeled by altering the direct current bias of the injected current and the respiratory-vagal effect by the periodic modulation of the injected current at a frequency of 0.2 Hz, corresponding to a respiratory rate of 12 breaths/min. Sympathetic modulation was simulated by injecting a low-frequency current close to Mayer wave frequency (0.08 Hz). We computed the coherence between the instantaneous pacemaker rate and respiratory-vagal modulation current as a model analog to experimental CRC. We found that sympathetic modulation, low vagal tone/high sympathetic tone, and respiratory irregularity can cause a decrease in CRC. We corroborated the model results with the actual data from the CRC study. In conclusion, we employ a novel approach combining insights from the experimental study and a physiologically plausible modeling framework to understand the mechanisms underlying the fall of cardiorespiratory coherence induced by the expectation of exercise. NEW & NOTEWORTHY Cardiorespiratory coherence is diminished in response to respiratory irregularity, low vagal/high sympathetic tone, and prominent low-frequency sympathetic modulation. Expectation of physical activity induces respiratory irregularity and increased sigh frequency and that contributes to diminished cardiorespiratory coherence in expectation of exercise. There is a greater fall of coherence with the non-linear (logistic) transformation of injected current, indicating the non-linear nature of cardiorespiratory interactions preceding the onset of exercise.