Variability of within-breath reactance in COPD patients and its association with dyspnoea

The forced oscillation technique can identify expiratory flow limitation (EFL) when a large difference in inspiratory and expiratory reactance (Delta Xrs) occurs. However, flow limitation can vary from breath to breath, and so we compared a multiple-breath Delta Xrs approach to the traditional breath-by-breath assessment of EFL. We investigated the within-and between-day reproducibility and the factors that affect the size of Delta Xrs when used as a continuous measurement over multiple breaths. In addition, we examined how multiple-breath.Xrs relates to the sensation of breathlessness. 425 moderate to very severe chronic obstructive pulmonary disease (COPD) patients and 229 controls were included. Spirometry and impedance measurements were performed on a MasterScope CT Impulse Oscillation System. Median Delta Xrs approached zero in healthy controls with little variation between measurements. COPD patients generally had higher Delta Xrs and higher variability. The COPD patients with Delta Xrs > 0.1 kPa.L-1.s(-1) were prone to be more breathless and had a higher modified Medical Research Council dyspnoea scale score. In controls, the 95th percentile of Delta Xrs was as low as 0.07 kPa.L-1.s(-1). We describe a new method to assess EFL at a patient level and propose a cut-off, mean Delta Xrs > 0.1 kPa.L-1.s(-1), as a way to identify COPD patients who are more likely to report dyspnoea.