The Physiology of Respiratory Sensations

The Lungs As Sensory Organs

Chapter 6 describes the role of a variety of pulmonary receptors in the determination of minute ventilation, tidal volume, and respiratory rate. These receptors also play an important part in the production and relief of breathing discomfort.

Pulmonary stretch receptors (PSRs) appear to allow us to monitor the size of a breath. As discussed previously, the sensation of air hunger produced experimentally by acute hypercapnia is made worse when a subject is constrained to breathe with small tidal volume and is relieved by larger breaths. Presumably, the information on breath size arises from stimulation of the PSRs.

Rapidly adapting receptors (RARs) and C fibers, which are stimulated by chemicals and possibly inflammatory processes in the lungs, also have roles in ventilatory control and conceivably contribute to breathing discomfort associated with airway inflammation. Perhaps the most intriguing example of the possible role of PSRs and RARs in the formation of respiratory sensations is in patients with asthma. Individuals who experience acute bronchoconstriction in the context of asthma may develop shortness of breath even before their lung function tests demonstrate a significant abnormality in expiratory flow. At these mild degrees of bronchoconstriction, the most common sensation described is chest tightness or constriction⁸. Evidence that this sensation arises from the lungs is provided by an experiment in which the dyspnea of asthma is reduced by the inhalation of lidocaine⁹, a local anesthetic that can reduce the activity of pulmonary receptors, especially C fibers, and a study in which the sensation has been shown to persist in subjects with bronchoconstriction in whom the work of breathing has been removed by the institution of mechanical ventilation¹⁰.

Stimulation of RARs and C fibers in the lungs may also contribute to the sensation of burning of which many patients with acute bronchitis complain. Anecdotal reports of patients with pulmonary embolism, or blood clots to the lungs, who received thrombolytic therapy to dissolve the clots and experience near-instantaneous relief of their breathing discomfort as the clots dissolve, suggest that stimulation of pulmonary vascular receptors by the increased pulmonary artery pressure associated with the embolism may be the source of the breathing discomfort in this condition.