Aggravation of dyspnoea by coughing: vagal mechanisms

It is a common clinical observation that the sensation of dyspnoea is aggravated by coughing in patients with dyspnoea. Since the vagus nerve plays a significant role not only in generation of the cough reflex but also in the modulation of respiratory sensations, it is possible that the aggravation of dyspnoea with coughing may occur through vagal mechanisms. The results of our study in healthy human subjects showed that the free breathing after the breakpoint of breathholding causes an immediate relief of air hunger whereas voluntary coughing causes a delay in the relief of air hunger and a slower relief occurred during citric acid-induced coughing. Conversely, the voluntary coughing and citric acid-induced coughing per se failed to induce air hunger. Nebulized lidocaine inhalation completely abolished the cough response to citric acid inhalation causing an immediate relief of air hunger whereas airway anaesthesia improved only slightly the air hunger during voluntary coughing. We also showed that the breathing pattern during a paroxysm of coughing can be characterized by a high frequency with a relatively small tidal volume. Like voluntary coughing, panting caused a slight delay in the relief of air hunger but this delay was not affected by airway anaesthesia. Although the evidence was circumstantial, these results of human experiments suggest that the aggravation of dyspnoea during coughing may be due to the aggravation of air hunger brought about by vagally-mediated mechanisms. However, we cannot still deny the possibility that neural behaviours that sustain the rapid, shallow breathing may cause the subject to have a sensation of unrelieved air hunger, indicating the importance of a central effect.