Respiratory Sensation and Control of Breathing

The observation that non-chemically mediated respiratory load compensation is dependent on a state of wakefulness suggested that the perception of the load or the conscious appreciation of the ventilatory consequences of the loading is required for respiratory motor output to increase. This led to studies of respiratory sensation using a variety of psychophysical approaches. These psychophysical studies revealed that respiratory-related physical changes are consciously appreciated and indicated that sensory information from the ventilatory apparatus does reach the cerebral cortex. This was further supported by physiological studies that demonstrated respiratory-related cortical-evoked potentials over somatosensory regions of the brain. Studies utilizing chest wall vibration support an important role for chest wall muscle spindles in mediating respiratory sensation. Our studies have also shown that voluntarily reducing the level of ventilation at a constant level of chemical drive results in a progressive proportional increase in the intensity of the unpleasant sensation of respiratory discomfort and the increase in respiratory sensation is predominantly a function of the degree to which tidal volume is reduced suggesting that limiting chest expansion or thoracic displacement is the proximate cause of the unpleasant sensation. Our observations that the sensation of dyspnea intensifies with increases in ventilation as well as when ventilation is reduced below the spontaneously adopted free breathing level can be simulated by mathematical models that suggest that respiratory drive integration depends not only on the direct effects of chemical and mechanical feedback but also on the perceptual consequences of these stimuli.