Expiratory Flow Limitation Development Index (EDI) measured by oscillometry in COPD

Chronic obstructive pulmonary disease (COPD) patients with expiratory flow limitation (EFL) display greater hyperinflation and gas trapping. Oscillometry reactance measurements quantify EFL. Negative swings in reactance, indicating EFL onset, can develop near exhalation onset (rapid EFL) or later (gradual EFL) at lower lung volumes. We propose a new method to quantify this; the EFL Development Index (EDI). <bold>Aim:</bold> To assess the clinical utility of the EDI in COPD patients. Lung function, oscillometry and respiratory questionnaire data were collected. EFL was the maximum reactance change during tidal breathing (X5_peak-peak ≥0.59 kPa/L/s). EDI was calculated during exhalation as mean reactance divided by minimum reactance; values closer to 1 indicate more rapid EFL development. COPD patients with EFL were recruited (n = 63). Mean EDI was 0.63; values below this were categorised as EFL^gradual (n = 34) and above as EFL^rapid (n = 29). EFL was similar between groups (X5_peak-peak and ∆X5; p >0.05). EFL^rapid patients had lower forced expiratory volume in 1 second (FEV₁)/forced vital capacity (FVC) ratio values (median: 0.36 vs 0.50), higher St Georges Respiratory Questionnaire (SGRQ) impact scores (mean: 51 vs 38), higher functional residual capacity (FRC) values (median: 145% vs 110%) and higher residual volume (RV) values (mean: 161% vs 144%), all p <0.05. No difference was seen in other resistance and reactance values measured by oscillometry (all p >0.05). Rapid EFL development was associated with more severe disease, worse gas trapping with hyperinflation, and a worse quality of life. EDI may be a useful marker of EFL severity that is not detected by other oscillometry measurements.