Sensation and control of breathing: a dynamic model.

A dynamic model of the CO2 respiratory control system is proposed, which can provide a qualitative basis for predicting breathing sensations. The discomfort index, which represents breathing sensations, is assumed to be composed of two sources: the arterial CO2 level and the respiratory motor command. The respiratory controller receives inhibitory neuromechanical and excitatory CO2 signals from the plant. The CO2 signal is enhanced by exercise stimuli. This dynamic multiplicative-type controller is used in simulations of key experiments: exercise and CO2 rebreathing with and without resistive loading. The dynamics of the discomfort index, the respiratory motor command, ventilation, and arterial CO2 concentration conform to the experimental data. The perceptual sensitivity to CO2 relative to respiratory effort is significantly correlated with the slope of hypercapnic ventilatory response. This result shows a clear linkage between ventilatory response and breathing sensations. Although it is shown that the automatic controller effectively minimizes the discomfort index for perturbations about an operating point under certain conditions, the discomfort index itself does not seem to be an underlying control principle of the proposed automatic controller model. Rather, breathing sensations may influence ventilatory responses by modifying the output of the automatic controller.