Maximal inspiration may produce a transient fall in airway conductance in asthmatic subjects. Since most forced expiratory maneuvers are preceded by maximum inspiration, a study was undertaken to determine how this conductance change or other effects of a maximum inspiration might alter the maximum expiratory flow rate. We measured airway conductance before and after maximal lung inflation and compared forced expiratory flow at 25% of the vital capacity (FEF25) measured from full (MEFV) and partial (PEFV) flow-volume curves in 6 asthmatic and in 6 normal control subjects. The PEFV curves were done in two ways: (1) expiration was initiated after a 5-sec breath-hold from approximately 50% of the vital capacity without prior deep inspiration (PEFVA curves), and (2) after inspiring to total lung capacity (TLC) and breath-holding for 2 sec the subjects slowly to 50% of the vital capacity, held their breath for 5 sec, and then forcibily expired (PEFVB curves). All 6 asthmatic subjects had lower FEF25 in the PEFVB curves than PEFVA curves (mean reduction 19.9%; p < O.Ol). The decrease in PEFVB flow was prevented by prior administration of isoproterenol. No significant difference was found in FEF25 between PEFVA and PEFVB curves of control subjects. In addition, no significant difference was found between FEF25 from MEFV and PEFVA curves of asthmatic subjects. We found that peak changes in airway conductance occurred approximately 15 sec after maximum inspiration in the asthmatic; since MEFV curves were performed immediately after inspiring to TLC, failure to observe a reduction in FEF25 in MEFV curves may be attributed to a delay in the development of increased bronchomotor tone. The results suggest that changes in bronchomotor tone induced by maximum inspiration may produce time-dependent changes in maximum expiratory flow.

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