Effect of Deep Inspiration on Maximum Expiratory Flow Rates in Asthmatic Subjects

Abstract

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 (PEFV_A 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 (PEFV_B curves). All 6 asthmatic subjects had lower FEF25 in the PEFV_B curves than PEFV_A curves (mean reduction 19.9%; p < O.Ol). The decrease in PEFV_B flow was prevented by prior administration of isoproterenol. No significant difference was found in FEF25 between PEFV_A and PEFV_B curves of control subjects. In addition, no significant difference was found between FEF25 from MEFV and PEFV_A 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.