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


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 (FEV1), 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 FEV1. FEV1/forced vital capacity (FVC) and resonance frequency had break-points around 80% of FEV1, 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 FEV1, while residual volume had no slope change above and -3.3 change per unit FEV1 below its break-point of 61%.


CONCLUSIONS
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 FEV1, indicating that air trapping starts to dominate in moderate COPD (FEV1 =50%-80%). This may have an impact on the patient's management plan and selection of patients and/or outcomes in clinical research.
Respiratory Medicine and Allergology, Department of Clinical Sciences, Lund University, Lund, Sweden.
Göran
G
Eriksson
Respiratory Medicine and Allergology, Department of Clinical Sciences, Lund University, Lund, Sweden.
Linnea
L
Jarenbäck
Regional Cancer Center South, Skåne University Hospital, Lund, Sweden.
Stefan
S
Peterson
Respiratory Medicine and Allergology, Department of Clinical Sciences, Lund University, Lund, Sweden.
Jaro
J
Ankerst
Respiratory Medicine and Allergology, Department of Clinical Sciences, Lund University, Lund, Sweden.
Leif
L
Bjermer
Respiratory Medicine and Allergology, Department of Clinical Sciences, Lund University, Lund, Sweden.
Ellen
E
Tufvesson
D000368:Aged; D001985:Bronchial Provocation Tests / Q000379:methods; D019468:Disease Management; D018450:Disease Progression; D005260:Female; D006801:Humans; D008176:Lung Volume Measurements / Q000379:methods; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000379:methods / Q000706:statistics & numerical data; D009991:Oscillometry / Q000379:methods; D010993:Plethysmography, Whole Body / Q000379:methods; D029424:Pulmonary Disease, Chronic Obstructive / Q000175:diagnosis / Q000453:epidemiology / Q000503:physiopathology / Q000628:therapy; D012720:Severity of Illness Index; D012907:Smoking / Q000503:physiopathology*; D013223:Statistics as Topic; D013548:Sweden / Q000453:epidemiology
body plethysmography; break-point; impulse oscillometry; severity; single-breath carbon-monoxide diffusion test; spirometry