Assessing the reversibility of airway obstruction

To determine whether changes of partial expiratory flow-volume curve (PEFV) and inspiratory capacity (IC) detect functional responses to bronchodilator in patients who do not meet the FEV1 criteria for reversibility of airway obstruction.

The effects of salbutamol (200 microg by metered-dose inhaler) on lung function were examined in 50 patients with asthma and 28 patients with COPD. Measurements evaluated were FEV1, forced expiratory flow at 30% of control FVC from maximal expiratory flow-volume curve (Vm30), forced expiratory flow at 30% of control FVC from PEFV (Vp30), and IC. On a separate occasion, a representative sample of 26 subjects inhaled placebo to determine the 95% confidence limits (CLs) of each of the parameters.

A percent and absolute increment of FEV1 above the upper CL was recorded in 28 patients. Of these, 26 had a percent and absolute increase of Vp30, 21 of Vm30, 9 of FVC, and 11 of IC above the 95% CL. Of the 50 patients who did not have an increase in FEV1 above the 95% CL, 25 had a percent and absolute increase in Vp30, 15 of Vm30, 3 of FVC, and 13 of IC above the 95% CL. On average, the percent and absolute increase Vp30 above the 95% CL significantly identified more responders than every other parameter.

Increases in maximal flow detected by PEFV and/or changes in IC may be substantially obscured by the effects of inspiration to total lung capacity required for the measurement of FEV1 in patients with chronic bronchoconstriction. Decreases in functional residual capacity (FRC) manifested by an increase of IC occur because, in patients whose FRC is dynamically determined, bronchodilatation that increases maximal flow in the tidal breathing range allows patients to breathe at lower lung volumes. Changes of FEV1 frequently fail to detect significant functional response to bronchodilators in patients with chronic airflow obstruction.