REGULATION OF VENTILATORY CAPACITY DURING EXERCISE IN ASTHMATICS

In asthmatic and control subjects, we examined the changes in ventilat ory capacity (VE(Cap)), end-expiratory lung volume (EELV), and degree of flow limitation during three types of exercise: 1) incremental, 2) constant load (50% of maximal exercise capacity; 36 min), and 3) inter val (alternating between 60 and 40% of maximal exercise capacity; 6-mi n workloads for 36 min). The VE(Cap) and degree of flow limitation at rest and during the various stages of exercise were estimated by align ing the tidal breathing flow-volume (F-V) loops within the maximal exp iratory F-V (MEFV) envelope using the measured EELV. In contrast to mo re usual estimates of VE(Cap) (i.e., maximal voluntary ventilation and forced expiratory volume in 1 s x 40), the calculated VE(Cap), depend ed on the existing bronchomotor tone, the lung volume at which the sub jects breathed (i.e., EELV), and the tidal volume. During interval and constant-load exercise, asthmatic subjects experienced reduced ventil atory reserve, higher degrees of flow limitation, and had higher EELVs compared with nonasthmatic subjects. During interval exercise, the VE (Cap), of the asthmatic subjects increased and decreased with variatio ns in minute ventilation, due in part to alterations in their MEFV cur ve as exercise intensity varied between 60 and 49% of maximal capacity . In conclusion, asthmatic subjects have a more variable VE(Cap) and r educed ventilatory reserve during exercise compared with nonasthmatic subjects. The variations in VE(Cap), are due in part to a more labile MEFV curve secondary to changes in bronchomotor tone. Asthmatics defen d VE(Cap), and minimize flow limitation by increasing EELV.