Coving Index: A Novel, Sensitive Spirometric Marker of Small Airways Obstruction

Abstract Introduction Curvature of the expiratory flow volume loop on visual inspection of spirometry flow volumes loops (FVL) is representative of the severity of airflow limitation particularly of small airways. Despite this longstanding observation, there are few validated methods for quantifying this phenomenon. Whilst assessments such as Forced Expiratory Midflow (FEF25%-75%) have been evaluated, these measurements are subject to effort dependent variation in the Forced Vital Capacity (FVC) manoeuvre or lack validation. We sought to develop a novel method to quantify the curvature of the expiratory FVL which may be more reliable. Methods We undertook an analysis of post-bronchodilator forced spirometry assessments taken from participants with physician-assigned asthma or COPD in the NOVELTY study. All manoeuvres underwent QC assessment against ATS/ERS standards with an automated AI algorithm (ArtiQ.QC, Leuven, Belgium) and measures were taken from the “best” manoeuvre as defined by the algorithm. After evaluating several novel approaches, we derived the “Coving Index” by expressing the area under the expiratory loop relative to the area below a reference line between peak flow and FVC (Figure). We then assessed the correlations with post-bronchodilator FEF25%-75% (L/s), FVC (L) and percent predicted Forced Expiratory Volume in 1 second (ppFEV1). We also assessed the variability of Coving Index compared with the variability of FEF25%-75% to a simulated reduction in FVC by 5%. Results 7,671 spirometry manoeuvres from 3374 participants were included (60% with Asthma, 40% with COPD), mean age 58 years (SD 16), mean ppFEV1 78% (SD 25). Coving Index showed moderate correlation with FEF25%-75% for patients with asthma and patients with COPD (R2=0.575 and 0.502, respectively), and FEV1/FVC (R2=0.603 and 0.603, respectively) although was less correlated with FEV1 (R2=0.282 and 0.323 respectively). Within the asthma cohort, Coving Index showed similar variability to a simulated reduction of 5% in FVC as FEF25%-75% (median percent change of 14.1% vs 14.4%, p > 0.05). However, for the COPD cohort, coving index was less variable (median percent change of 4.9% vs 14.3%, p < 0.001). Conclusion Coving Index appears to be a reliable and sensitive measure of airflow limitation that provides quantification of the degree of curvature of expiratory mid flow. It appears to be less sensitive to variations in forced expiratory volume compared with FEF25%-75% in patients with COPD. Further evaluation of this novel marker as a predictor of lung function trajectories is warranted.