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Expiratory flow limitation detected by forced oscillation and negative expiratory pressure

¹ TBM Lab, Dipartimento di Bioingegneria, Politecnico di Milano University, Milan, Italy, ² University Dept of Medicine, University Hospital Aintree, Liverpool, UK, ³ Respiratory Function Laboratory, Dept of Respiratory Medicine, University of Athens Medical School, Sotiria Hospital, Athens, Greece.

CORRESPONDENCE: R. L. Dellacà, Dipartimento di Bioingegneria, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milan, Italy. Fax: 39 0223999000. E-mail: raffaele.dellaca{at}polimi.it

Keywords: Chronic obstructive pulmonary disease, forced oscillation technique, respiratory system reactance, within-breath impedance

Received: March 16, 2006
Accepted October 25, 2006

The within-breath change in reactance (_rs) measured by forced oscillation technique (FOT) at 5 Hz reliably detects expiratory flow limitation in chronic obstructive pulmonary disease (COPD). The present study compared this approach to the standard negative expiratory pressure (NEP) method.

In total, 21 COPD patients were studied by applying both techniques to the same breath and in 15 patients the measurements were repeated after bronchodilator. For each patient and condition five NEP tests were performed and independently scored by three operators unaware of the FOT results.

In 180 tests, FOT classified 53.3% as flow limited. On average, the operators scored 27.6% of tests flow limited and 47.6% non-flow limited, but could not score 24.8%. The methods disagreed in 7.9% of cases; in 78% of these the NEP scores differed between operators. Bronchodilation reduced NEP and _rs scores, with only the latter achieving significance. Averaging the operators' NEP scores, a threshold between 24.6–30.8% of tidal volume being flow limited by NEP produced 94% agreement between methods.

In conclusion, when negative expiratory pressure and forced oscillation technique were both available they showed good agreement. As forced oscillation technique is automatic and can measure multiple breaths over long periods, it is suitable for monitoring expiratory flow limitation continuously and identifying patients' breathing close to the onset of expiratory flow limitation, where intermittent sampling may be unrepresentative.

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