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.