Repeatability of cough-related variables during fog challenges at threshold and suprathreshold stimulus intensity in humans

Cough-related variables such as cough frequency, time to onset tie. the tim e until the first cough occurs) and the cough index tie. the ratio between the cough frequency and the time to onset) may be important when interpreti ng results of cough challenges for therapeutic interventions or for compara tive research purposes. Nevertheless, repeatability (or reproducibility) fo r these widely used variables has been poorly studied. In thirty normal subjects, coughing was induced by inhalation of threshold (T) and suprathreshold (1.6 x T) concentrations of ultrasonically nebulized distilled water (fog). Cough threshold was taken as the lowest fog concent ration that evoked at least one cough effort during two challenges separate d by a 30-min interval. During challenges performed at both threshold and s uprathreshold stimulus intensity, cough frequency, time to onset, and the c ough index were assessed; within-subject repeatability for these variables was subsequently evaluated. Median+/-interquartile range tough threshold value was 0.9 +/-0.5 ml.min(-1 ). During the two challenges performed to assess cough threshold, the mean/-SD values of cough frequency, time to onset, and cough index were similar (5.0+/-2.7 and 5.3+/-3.1 coughs min(-1), 32.4+/-13.3 and 32.9+/-13.6 s, an d 0.2+/-0.2 and 0.2+/-0.2, respectively). However, none of these cough-rela ted variables proved to be sufficiently repeatable, During the two challeng es performed at suprathreshold stimulus intensity, mean values of cough fre quency, time to onset, and cough index were also similar (20.0+/-9.0 and 18 .2+/-10.2 coughs.mid(-1), 13.5+/-5.8 and 12.0+/-4.62 s, and 1.7+/-1.0 and 1 .8+/-1.2); furthermore, all considered variable of suprathreshold challenge turned out to be reproducible. In conclusion, during fog challenges at threshold stimulus intensity, cough frequency, time to onset and cough index cannot reliably be used for evalu ating cough responses, However, these cough-related variables may represent useful and reliable research tools in the evaluation of suprathreshold cou gh responses.