A new approach to assess COPD by identifying lung function break-points

PURPOSE: COPD is a progressive disease, which can take different routes, leading to great heterogeneity. The aim of the post-hoc analysis reported here was to perform continuous analyses of advanced lung function measurements, using linear and nonlinear regressions. PATIENTS AND METHODS: Fifty-one COPD patients with mild to very severe disease (Global Initiative for Chronic Obstructive Lung Disease [GOLD] Stages I–IV) and 41 healthy smokers were investigated post-bronchodilation by flow-volume spirometry, body plethysmography, diffusion capacity testing, and impulse oscillometry. The relationship between COPD severity, based on forced expiratory volume in 1 second (FEV(1)), and different lung function parameters was analyzed by flexible nonparametric method, linear regression, and segmented linear regression with break-points. RESULTS: Most lung function parameters were nonlinear in relation to spirometric severity. Parameters related to volume (residual volume, functional residual capacity, total lung capacity, diffusion capacity [diffusion capacity of the lung for carbon monoxide], diffusion capacity of the lung for carbon monoxide/alveolar volume) and reactance (reactance area and reactance at 5Hz) were segmented with break-points at 60%–70% of FEV(1). FEV(1)/forced vital capacity (FVC) and resonance frequency had break-points around 80% of FEV(1), while many resistance parameters had break-points below 40%. The slopes in percent predicted differed; resistance at 5 Hz minus resistance at 20 Hz had a linear slope change of −5.3 per unit FEV(1), while residual volume had no slope change above and −3.3 change per unit FEV(1) below its break-point of 61%. CONCLUSION: Continuous analyses of different lung function parameters over the spirometric COPD severity range gave valuable information additional to categorical analyses. Parameters related to volume, diffusion capacity, and reactance showed break-points around 65% of FEV(1), indicating that air trapping starts to dominate in moderate COPD (FEV(1) =50%–80%). This may have an impact on the patient’s management plan and selection of patients and/or outcomes in clinical research.