Separation of cough and reflex bronchoconstriction by inhaled local anaesthetics

Cough and airway constriction are common features of respiratory diseases. Both can be caused by stimulation of airway nerves. We have studied the effects of airway anaesthesia on these reflexes, stimulated by inhaled capsaicin, in order to determine whether they are controlled by the same sensory nerves. Ten volunteers had capsaicin cough dose responses performed before and at 10 min after inhaling placebo (ascorbic acid in saline), and the topical anaesthetics lignocaine 40 mg, and dyclonine 8 and 4 mg. The effect of the drugs on respiratory resistance (Rrs), measured using a forced oscillation technique, was measured both before and after the inhalation of a dose of capsaicin which caused less than two coughs. Lignocaine (40 mg) and dyclonine (8 mg) caused significant reports of oral anaesthesia but only lignocaine reduced the cough response to inhaled capsaicin, increasing the log dose of capsaicin causing three or more coughs by 162%. None of the treatments altered basal Rrs or its increase after inhaled capsaicin. Thus, the cough and reflex bronchoconstriction caused by inhaled capsaicin have different sensitivities to inhaled local anaesthesia, suggesting that the effect may be mediated by different sensory pathways.