Dynamic hyperinflation during the 6-min walk test in severely asthmatic subjects

We tested the hypothesis that dynamic hyperinflation develops in severe asthmatic subjects during exercise. Changes in inspiratory capacity (IC) were measured during the 6-min walk test (6MWT) in severe asthmatic subjects compared with chronic obstructive pulmonary disease (COPD) subjects with a similar degree of bronchial obstruction. We assessed whether changes in IC were associated with changes in dyspnoea perception. 27 severe asthmatic subjects (10 males and 17 females) and 43 COPD subjects (35 males and eight females) were recruited. The two groups performed similarly in the 6MWT (p=0.90). At the end of the test, the Borg score increased significantly in both groups (mean difference: for asthmatic subjects 1.7 +/- 1.6; p<0.0001; for COPD subjects 3.1 +/- 1.9; p<0.0001). IC measured at the beginning of 6MWT was not different between groups (2.25 +/- 0.47 L in asthmatic subjects versus 2.38 +/- 0.60 L in COPD subjects; p=0.32) and decreased in both groups (mean difference: for asthmatic subjects 0.160 L; p=0.02; for COPD subjects 0.164 L; p<0.0001). However, changes in IC were significantly associated with changes in the Borg score in the COPD group (r(2)=0.17; p=0.006), but not in the asthma group (r(2)=0.06; p=0.20). In severe asthmatic subjects, IC significantly drops during the 6MWT to the same extent as COPD subjects with a similar degree of lung impairment, indicating the development of dynamic hyperinflation. Contrary to COPD, in asthmatic subjects the occurrence of dynamic hyperinflation was not associated with changes in dyspnoea perception.