Tracking of the within-breath changes of respiratory mechanics using the forced oscillation technique may provide outcomes that characterise the dynamic behaviour of the airways during normal breathing.We measured respiratory resistance (R_rs) and reactance (X_rs) at 8 Hz in 55 chronic obstructive pulmonary disease (COPD) patients and 20 healthy controls, and evaluated R_rs and X_rs as functions of gas flow (V') and volume (V) during normal breathing cycles. In 12 COPD patients, additional measurements were made at continuous positive airway pressure (CPAP) levels of 4, 8, 14 and 20 hPa.The R_rs and X_rsversus V' and V relationships displayed a variety of loop patterns, allowing characterisation of physiological and pathological processes. The main outcomes emerging from the within-breath analysis were the X_rsversus V loop area (AXV) quantifying expiratory flow limitation, and the tidal change in X_rs during inspiration (ΔX_I) reflecting alteration in lung inhomogeneity in COPD. With increasing CPAP, AXV and ΔX_I approached the normal ranges, although with a large variability between individuals, whereas mean R_rs remained unchanged.Within-breath tracking of R_rs and X_rs allows an improved assessment of expiratory flow limitation and functional inhomogeneity in COPD; thereby it may help identify the physiological phenotypes of COPD and determine the optimal level of respiratory support.

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