Perception of dyspnoea during acute changes in lung function in patients with either asthma or COPD.

The aim of the study was to evaluate the relationship between several lung function indices and perceived dyspnoea during bronchoconstriction. Acute changes in lung function were induced by inhaled histamine followed by terbutaline, in 12 asthmatics and 12 subjects with chronic obstructive pulmonary disease (COPD). A bipolar visual analogue scale (VAS), allowing subjects to report either improvement or worsening when moving off from a 'nochange' midpoint, was used to rate shortness of breath. Large swings in ratings were seen in all asthmatics and in seven out of 12 COPD subjects (high perceivers). Using linear regression of VAS rating against parallel change in lung function, on a within-subject basis, the highest degree of correlation between dyspnoea and objective response was found to involve the change in specific inspiratory resistance (sRin) in the asthmatics. In the five low perceivers, the ability to discriminate an increase in airway obstruction, estimated as the VAS/change in lung function slope, was very poor. Using a stepwise multiple regression analysis, the sensation of dyspnoea was found to be significantly related to the FEV1 and the sRin in the asthmatics, to the inspiratory vital capacity and the maximal inspiratory flow at 50% FVC (MIF50) in the COPD subjects with high perception, and to the MIF50 in the COPD subjects with low perception.