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 (inspiratoryeexpiratory breath analysis) but these have not been used to compare patients with asthma and those with chronic obstructive pulmonary disease (COPD). Large inspiratoryeexpiratory variations in Xrs at 5 Hz (DX5) in an individual have been used as a surrogate marker of expiratory flow limitation. Methods Whole-breath and inspiratoryeexpiratory impulse oscillometry was assessed in 34 patients with asthma (4963 years; 15 male, forced expiratory volume in 1 s (FEV 1 ) 6964% predicted), 48 patients with COPD (6462 years; 32 male, FEV 1 5963% predicted) and 18 normal subjects (3762 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 FEV 1 <60% predicted. Inspiratoryeexpiratory analysis in patients with FEV 1 <60% predicted showed that in the COPD group mean expiratory X5 (À0.4460.04 kPa/l/s) was greater than inspiratory X5 (À0.2360.02 kPa/l/s, p<0.001) whereas patients with asthma did not show such changes (À0.3660.07 kPa/l/s vs À0.2660.03 kPa/l/s, p¼0.23). Even though DX5 was larger in patients with COPD (0.2160.03 kPa/l/s) than in patients with asthma (0.1060.07 kPa/l/s), this was not significant (p¼0.15). Conclusions Whole-breath impulse oscillation system analysis failed to discriminate between patients with asthma and those with COPD. Inspiratoryeexpiratory 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.

The impulse oscillometry system (IOS) measures airflow resistance (Rrs) and reactance (Xrs), which reflect small airway resistance and compliance respectively. These parameters may be more sensitive than spirometry in COPD and severe asthma where small airway disease is predominant. We compared 28 asthmatic patients (age 50±3 yr; 10 male, FEV1 74±4% predicted), 38 patients with chronic obstructive pulmonary disease (COPD) (64±2 yr; 23 male, FEV1 64±3 %), and 18 normal subjects (37±2 yr; 5 male). Patients with asthma and COPD showed similar increases in Rrs at 5 Hz (R5= 0.53±0.03 KPa/L/s and 0.51±0.03 KPa/L/s respectively) and in Xrs magnitude (reactance at 5 Hz (X5)= −0.22±0.02 KP/aL/s and −0.24±0.02 KPa/L/s respectively) compared to normal subjects (R5=0.38±0.02 KPa/L/s and X5= −0.11±0.01 KPa/L/s). In more severely obstructed patients, (FEV1 <60% pred.) expiratory Xrs was significantly different, with greater frequency dependence of resistance ( Figure 1, Panels A,B). Integrated low frequency reactance below resonant frequency, AX, was significantly greater in COPD (4.10±0.64 KPaL/s) compared to asthma (1.97±0.35 KPaL/s). We conclude that with similarly severe obstruction, assessed by FEV1, asthma and COPD patients show different Xrs reflecting enhanced expiratory flow limitation in COPD. The greater frequency dependence of resistance in COPD, along with significantly greater Xrs at low frequencies may indicate abnormal small airway compliance.[figure1] This abstract is funded by: NHLI, London. Am J Respir Crit Care Med 179;2009:A5082 Internet address: www.atsjournals.org Online Abstracts Issue