The Physiological Basis of Respiratory Sensation

Abstract

Respiratory sensation often occurs in patients suffering from acute or chronic respiratory and/or cardiac diseases, leading to dyspnoea. It is supported by the cortical integration of sensory pathways arising from the airways and lungs, respiratory muscles, and circulatory system. Their activation by direct electrical stimulation or circumstances mimicking pathological events (mechanical and less often chemical test agents) evokes cortical potentials and modifies the spontaneous EEG rhythms in cortical areas which also receive information from the skin, joints, and limb muscles. The quantification of respiratory sensation is obtained by psychophysical methods based on different theories linking the stimulus to its perception. Pathological or environmental circumstances act as triggers of the dyspnoea sensation. Ventilatory loading, elicited by dense gas breathing and mostly pathological airway obstruction, leads to an enhanced intrathoracic pressure and respiratory muscle work which in turn activate the vagal and respiratory muscle afferents. Experiments in healthy subjects testify for marked alterations of the tactile sensation and voluntary motor control to limb muscles when the respiratory system is loaded. These viscero-somatic interactions could partly support the well-known phenomenon of altered exercise performances and perception of the body image in patients suffering from chronic respiratory diseases, apart from any disturbances in respiratory gases.Respiratory sensation should be considered a nonphysiological situation because the ventilatory act at rest, as well as during exercise hyperpnoea does not necessarily involve the perception of respiratory movements. Thus, healthy subjects have to think to perceive the amplitude and periodicity of their breaths. The dyspnoea sensation occurs in pathological circumstances. It covers a wide range of perception grading from the sole permanent and uncomfortable perception of spontaneous breathing to that of suffocation. There are great interindividual scattering of dyspnoea, the same degree of pulmonary impairment referring to different levels of dyspnoea. This partly results from the fact that the perception of breathing requires the cortical and subcortical integrations of different sensory pathways from the respiratory tract, the respiratory muscles, and the circulation, which should be differently activated by the pathology.Our purpose was not to duplicate the very interesting review published in 1995 by Shea et al.[1] on the respiratory sensations in humans. Our formation of physiologist incited us to ground this paper on experimental animal data assessing the projections onto the cerebral cortex of peripheral nervous pathways arising from the cardiorespiratory system. Then, we summarized the clinical observations on the origin of respiratory sensation in humans and briefly described the methods used to quantify respiratory sensation. We reported the changes in respiratory sensation due to an increase in background airway resistance and the modifications of...