Background Patients with chronic cough (CC) have substantially higher 24-h cough rates compared to patients with asthma (A) or healthy controls (HC). CC patients also have a more sensitive cough reflex, when measured as the ‘threshold’ dose of a tussive agent (eg, capsaicin) inducing at least two or five coughs (C2/C5), although there is substantial overlap between CC and HC. We aimed to investigate if the ‘magnitude’ of cough response to inhaled capsaicin would better differentiate CC patients from A and HC subjects.
Methods We devised a cough challenge technique, with increasing doubling concentrations of capsaicin (0.48–1000 μMol) extended beyond the C5 threshold, up to the maximum tolerated dose. At each capsaicin concentration four single-breath inhalations were administered from a dosimeter 30 s apart. After each inhalation, the number of coughs in the first 15 s was counted. Using Generalised Estimating Equations (GEE), we explored the effect of group and gender on average cough response to increasing concentrations of capsaicin.
Results We studied 20 treatment resistant CC, 18 stable mild/moderate A and 20 HC subjects, matched for age and gender; (M:F 10:10, 9:9,10:10, respectively), age [mean (±SD) 57.1 years (±15.7), 51.7 years (±13.5), 58.8 years (±13.5); p=0.17], and lung function [3.02L (±0.98), 2.96L (±1.10), 3.20L (±0.99); p=0.72]. CC (vs HC p<0.001) and females (vs males, p<0.001) cough significantly more after capsaicin (see Abstract S115 Figure 1). There was no significant difference between A and HC (p=0.407).
Conclusions Magnitude of cough responses to inhaled capsaicin, beyond the standard endpoints:
clearly discriminates chronic cough from healthy controls,
demonstrates gender differences,
may be valuable for investigation of physiological mechanisms and anti-tussive agents.
In addition to a reduced cough threshold, these findings are consistent with a failure of inhibitory mechanisms to prevent coughing to repetitive and potent stimuli in chronic cough.
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