Airway dynamics in COPD patients by within-breath impedance tracking: effects of continuous positive airway pressure

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 rs versus 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 rs versus V loop area (AXV) quantifying expiratory flow limitation, and the tidal change in X rs during inspiration (Δ X I ) reflecting alteration in lung inhomogeneity in COPD. With increasing CPAP, AXV and Δ X 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.