Abstract

Background The usual analysis of forced oscillometry measures respiratory resistance (Rrs) and reactance (Xrs) averaged over several tidal breaths (whole-breath analysis). Recent within-breath analyses have separated Rrs and Xrs into their mean inspiratory and mean expiratory components (inspiratory–expiratory breath analysis) but these have not been used to compare patients with asthma and those with chronic obstructive pulmonary disease (COPD). Large inspiratory–expiratory variations in Xrs at 5 Hz (ΔX5) in an individual have been used as a surrogate marker of expiratory flow limitation.

Methods Whole-breath and inspiratory–expiratory impulse oscillometry was assessed in 34 patients with asthma (49±3 years; 15 male, forced expiratory volume in 1 s (FEV₁) 69±4% predicted), 48 patients with COPD (64±2 years; 32 male, FEV₁ 59±3% predicted) and 18 normal subjects (37±2 years; 8 male).

Results Whole-breath analysis failed to discriminate between patients with asthma and patients with COPD either for all patients or for patients with FEV₁ <60% predicted. Inspiratory–expiratory analysis in patients with FEV₁ <60% predicted showed that in the COPD group mean expiratory X5 (−0.44±0.04 kPa/l/s) was greater than inspiratory X5 (−0.23±0.02 kPa/l/s, p<0.001) whereas patients with asthma did not show such changes (−0.36±0.07 kPa/l/s vs −0.26±0.03 kPa/l/s, p=0.23). Even though ΔX5 was larger in patients with COPD (0.21±0.03 kPa/l/s) than in patients with asthma (0.10±0.07 kPa/l/s), this was not significant (p=0.15).

Conclusions Whole-breath impulse oscillation system analysis failed to discriminate between patients with asthma and those with COPD. Inspiratory–expiratory X5 analysis differentiated patients with asthma from those with COPD presumably reflecting enhanced dynamic airway narrowing on expiration in COPD. Further studies are needed to confirm these differences and investigate their cause.