Evaluation of impulse oscillation system: comparison with forced oscillation technique and body plethysmography

To the Editor:

We read with interest the study by Hellinckx et al. 1, comparing the techniques of impulse oscillation, forced oscillation and body plethysmography. The impulse oscillation system (IOS) was recently introduced by Jaeger (IOS Enrich Jaeger, Hoechberg, Germany) and its clinical potential is due to its relatively easy acquisition of data. Subjects perform tidal breathing in the upright position without any forced expiratory manoeuvres. This method could therefore potentially acquire broad usage, especially among patients with poor coordination. However, our limited experience with IOS suggests that its wider utilization will be problematic. Conversely, body-box plethysmography is an established method that allows for the measurement of airways resistance and spirometry and has been universally accepted as the gold standard in the clinical assessment of lung function.

There is very little data available on the comparison and, more specifically, assessment of the reproducibility and sensitivity of IOS, body-box plethysmography and spirometry. We have recently performed spirometry, IOS and body-box plethysmography in nine (six female) stable asthmatic patients aged mean±se 40±4.3 yrs. The patients underwent spirometry, IOS and body-box plethysmography on two separate days, 1-week apart. Each test was performed before and 30 min after inhalation of 12 µg Eformoterol fumarate (Oxis®, Turbohaler®, AstraZeneca) or 50 µg Salmeterol (Serevent, Accuhaler®, Allen and Hanburys) in a randomized, double-blind, double-dummy, crossover manner. Preinhalation and change from baseline (Δ) results for forced expiratory volume in one second (FEV₁), respiratory system resistance measured at 5 Htz (R_rs5) using IOS and total specific airway compliance (sG_aw) using body-box plethysmography were compared using Jaeger equipment. This allowed for the assessment of the reproducibility of each technique in recording baseline measurements in the individual subjects, and the sensitivity of each method to detect change in airway measurements in response to bronchodilators. The results showed that reproducibility between the baseline testing on the 2 days was FEV₁>R_rs5>sG_aw with Pearson correlation coefficient (r) values of 0.95, 0.75 and 0.40, respectively. Using the Δ response, the comparative sensitivity was R_rs5>FEV₁>sG_aw with mean values of −8.3%, 7.5% and 2.5% for Eformoterol and −11.5%, 5.6% and 7.2% for salmeterol. Our data revealed that IOS and body-box plethysmography did not display the same degree of reproducibility as spirometry. However, the IOS seemed to record greater changes from the baseline in the respiratory resistance in response to the bronchodilators.

We agree with Hellinckx et al. 1 that the measurements obtained with impulse oscillation system cannot be interchangeable with that of body-box plethysmography or spirometry. It is also our opinion that the reliability of the impulse oscillation system needs to be confirmed, with particular effort made to provide standardized guidelines on its usage. At present, the impulse oscillation system could be used to assess respiratory resistance in clinical research experiments. However, it is not clear whether respiratory resistance measured with the impulse oscillation system is mutually interchangeable with airway resistance assessed by body-box plethysmography. We believe that further studies are warranted before this technique could replace traditional spirometry as the gold standard research and clinical tool.