Respiratory Sensation and Control of Breathing

The observation that non-chemically mediated respiratory load compensation is dependent on a state of wake- fulness suggested that the perception of the load or the conscious appreciation of the ventilatory consequences of the load- ing is required for respiratory motor output to increase. This led to studies of respiratory sensation using a variety of psy- chophysical 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 somato- sensory 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 con- stant 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 un- pleasant 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. In the course of daily living, the body is challenged to maintain homeostasis by changes in metabolic, mechanical, and environmental conditions. Challenges posed to the respi- ratory system are greatest by diseases of the ventilatory ap- paratus that affect the mechanical properties of the lung and chest wall and the mechanical advantage of the respiratory muscles. In the face of all of these perturbations, the respira- tory system manages to make appropriate adjustments so as to maintain ventilation at appropriate levels and preserve blood gas and acid-base homeostasis. The initial focus of our research was to identify the mechanisms of the respiratory adjustments to mechanical derangements and the interaction of chemical, neuromechanical, and, behavioral systems in the regulation of breathing.