Effect of changes in inspiratory flow rate on cough responses to inhaled capsaicin

1. Twelve non-smoking subjects inhaled capsaicin at three different inspiratory flow rates: 50, 100 and 150 litres/min. Capsaicin was delivered by a breath-actuated dosimeter; inhalations consisted of 0.21–13.6 nmol of capsaicin in doubling amounts given in random order. 2. The mean number of coughs per challenge decreased with increasing inspiratory flow rate. The difference in cough numbers were significant: 7.7 (95% confidence interval 2.5–12.8) for 50 versus 100 litres/min and 10.9 (95% confidence interval 5.0–16.9) for 100 versus 150 litres/min. 3. On a separate day, a cough threshold was measured by giving increasing doses of citric acid that were inhaled at 50 litres/min. There was a positive correlation between the sensitivity to capsaicin and the cough threshold to citric acid (r = 0.69, P = 0.01), and also between the cough latencies (r = 0.67, P = 0.02). 4. The negative relationship between the cough response and the inspiratory flow rate may be caused by increased laryngeal deposition at lower inspiratory flow rates. 5. These results are compatible with a similar anatomical distribution of cough receptors for capsaicin and citric acid. 6. These results suggest that changes in inspiratory flow rate may affect the results of cough challenges.