In COPD, improvements in lung mechanics following bronchodilator, measured using the forced oscillation technique (FOT), are more sensitive than spirometry at detecting improvement in lung function following bronchodilator. The relationship between these improvements in lung mechanics and improvements in functional outcomes, such as exertional dyspnoea, following bronchodilator, in COPD is unknown.

17 COPD subjects were recruited into a double blind placebo controlled randomised cross over study. Dyspnoea was induced using a standardised six-minute walk test (6 MWT), and measured by borg score throughout the test. Measurement of respiratory system conductance (Grs), respiratory system reactance (Xrs), inspiratory capacity (IC) and spirometry were made at baseline and 1 h after a single dose of either 18 μg of tiotropium bromide plus 200 μg salbutamol, or placebo.

Subjects had a mean baseline FEV₁ of 45.5 ± 11.0% predicted. The bronchodilator induced reduction in exertional dyspnoea correlated significantly with the increase in Grs (r_s = 0.59, p = 0.01) and approached significance with FEV₁ (r_s = 0.45, p = 0.07) but not with FVC (r_s = 0.30, p = 0.24), Xrs (r_s = 0.19, p = 0.47) or IC (r_s = −0.08, p = 0.78). Increase in Grs was the best and sole predictor of reduction in exertional dyspnoea, explaining 41% of the variance. There was no additional contribution to the model from the increase in FEV₁ or IC.

Bronchodilator induced improvements in exertional dyspnoea in moderate to severe COPD are predicted by improvements in Grs, measured by FOT, independent of improvements in spirometry or hyperinflation. The findings suggest that FOT may be useful for measuring response to bronchodilator in COPD.