Dyspnoea-referenced minimal clinically important difference in the resting inspiratory capacity response to...

In COPD, the clinical significance of bronchodilator-induced changes in inspiratory capacity (IC) at rest remains uncertain. Unlike forced expiratory volume in one second (FEV₁), IC may better reflect symptom improvement, but no minimal clinically important difference (MCID) has been formally established.

To relate short-acting bronchodilator (SABD)-induced dyspnoea relief in stable COPD patients at rest with the concomitant IC changes and determine the MCID of the latter.

In 106 stable COPD outpatients undergoing routine lung function testing, spirometry and plethysmography were performed before and after SABD administration. Dyspnoea was assessed using a visual analogue scale (VAS) and a five-point Likert scale. Three approaches were used to estimate MCID, namely two anchored methods (receiver operating characteristics (ROC)-based and difference-based) and a distribution-based method.

Dyspnoea improvement was reported in 64% of patients. IC changes correlated strongly with symptom relief, while FEV₁ changes did not. ROC analysis identified an optimal IC threshold of 0.125 L (5% of baseline) with excellent predictive accuracy. The distribution-based estimate was 0.187 L, and the lowest difference-based estimate was 0.27 L. Averaging the ROC and distribution thresholds yielded a proposed MCID of 0.150 L.

IC changes after SABD administration at rest are more closely associated with dyspnoea relief than FEV₁ changes. An IC increase of 0.150 L may serve as a practical benchmark in future large-scale studies aiming at evaluation of the practical utility of IC changes in clinical practice, e.g. to inform therapeutic strategies, such as guiding the use of rescue SABD.