Use of the Isovolume FEF25-75% to Assess Small Airway Obstruction

The mean forced expiratory flow during the middle half of the forced vital capacity (FEF25-75%) is the simplest test currently in use for detection of early, reversible airway obstruction. However, FEF25-75% often decreases paradoxically following bronchodilator challenge, while other tests of airflow improve. Review of spirometric tracings from 30 patients revealed a 67% reduction in FEF25-75% with more than 80% increase in the 1-second forced expiratory volume (FEV1). When FEF was recalculated with correction for changes in the forced vital capacity (isovolume forced expiratory flow), there was a 90% improvement following administration of bronchodilator. These findings suggest that FEF25-75% should be volume-corrected for determining response to bronchodilators.