CHANGES IN RESIDUAL VOLUME DURING INDUCED BRONCHOCONSTRICTION IN HEALTHY AND ASTHMATIC SUBJECTS

The increase of residual volume (RV) was investigated during acute bronchoconstriction induced in healthy subjects by methacholine (MCh) (Group 1, n = 13) and in asthmatics by MCh (Group 2, n = 21), or house-mite dust (Group 3, n = 11) during early and late airway responses (EAR and LAR), or a series of deep breaths (Group 4, n = 7). In all subjects the difference between residual volume after partial (RVp) and maximal maneuver (RV), expressed as a percentage of control FVC, increased during bronchoconstriction and was correlated with the percent increase of maximal to partial flow ratio at 50% of control FVC (M/P50) (r = 0.854, p < 0.0001). At comparable reduction of partial expiratory flow at 50% of control FVC (VP50), the decreases of FEV(1) and FVC were less in healthy than asthmatic subjects, whereas the change of FEV(1)/FVC was similar in all groups, reflecting similar change in slope of flow-volume curves. The increase of RVp was similar in all groups (range: 15 to 19%), but the increase of RV was 6 +/- 1% (SEM) in healthy subjects and significantly greater (range: 11.1 to 13.3%) in all groups of asthmatics (p < 0.02, analysis of variance [ANOVA]). The effect of deep inhalation (DI) on the airway caliber as assessed by the increase of M/P50 was higher in normal subjects than in all groups of asthmatics (p < 0.0001). A negative correlation was found between the increases of RV and M/P50 in all groups (r = -0.358, p < 0.01), suggesting that healthy subjects had a limited increase of RV because of a marked bronchodilator effect of DI. At comparable fall of FEV(1), the increase of RV was similar in all groups while greater increase of RVp and decrease of VP50 were found in healthy subjects than in asthmatics. No correlation was found between changes in RV and M/P50. These findings indicate that during acute bronchoconstriction the ability of airways to dilate after DI does influence the increase of RV. We speculate that a delayed occurrence of airway closure and/or a reduction of expiratory airflow limitation after DI may represent the mechanisms of smaller increase of RV in healthy subjects compared with asthmatics.