Cough induced by low pH

Citric acid has been used as a tussive agent in the investigation of the co ugh refer. The mechanism of cough stimulation remains unclear. We conducted studies to compare the cough response of citric acid to an organic (acetic acid) and an inorganic acid phosphoric acid). we also compared the cough r esponse of citric acid to capsaicin, a non-acid tussive stimulus. In study I, 26 subjects inhaled equimolar concentrations of citric and acetic acid. In study 2, 22 subjects inhaled citric acid and phosphoric acid in concentr ations of similar pH. Eighteen subjects from study 2 completed a capsaicin cough challenge test. The cough response was recorded and the concentration causing two coughs, the cough threshold, calculated (D2). The correlation of cough thresholds between both citric and acetic acid (r=0.79, 95% CI 0.3 7-0.90, P<0.0001) and citric acid and phosphoric acid (r=0.68 95% CI 0.37-0 .86, P=0.0005) were significant. There was no correlation between the citri c acid and capsaicin cough thresholds. These results show that an individua l's cough response to acetic and phosphoric acid are similar to that caused by citric ic acid. Thus these acids may cause cough by a common mechanism such as disturbance of the pH of the airway surface liquid. Capsaicin does not appear to share this common pathway.