Rationale: Expiratory flow limitation (EFL) is defined as absence of increase in air flow during forced expiration effort compared to expiratory flow during tidal breathing as recorded by spirometry. Air trapping is expected to worsen with increase in EFL. Obesity (a chest wall disorder) may also increase EFL by an increase in IC. Objective: To evaluate the relation of magnitude of EFL to the degree of air trapping as assessed by plethysmographic lung volumes, and to assess the association of obesity with EFL in COPD. Methods: The records of patients with chronic obstructive pulmonary disease (COPD) were retrospectively reviewed. Patients with complete lung function data were included for analysis. Lung function testing was conducted according to ATS guidelines. EFL was computed as percent of the tidal volume during quiet breathing spanned (intersected) by the forced expiratory flow-volume curve. Analysis of variance was used to assess relation between EFL and plethysmographic lung volumes in patients with BMI <30 kg/m2 and those with BMI >30 kg/m2 or greater. Results: The records of 241 (140 M, 101 F) patients were available for analysis. Severity of COPD was GOLD C and D. Characteristics were: age 58 ± 11 years, BMI 28 ± 7 kg/m2, FVC (% pred) 86 ± 21, FEV1 (% pred) 68 ± 22, FEV1/ FVC (as %) 57 ± 11, TLC (% pred) 106 ± 20, RV (% pred) of 144 ± 43, FRC (% pred) 120 ± 29, and IC (% pred) 90 ± 25. Mean ± SD of EFL for the entire cohort was 51 ± 39%, 45 ± 40% in patients with BMI <30 and 68 ± 32% in those with BMI >30 or greater. We found strongest correlations between EFL and FEV1 and FVC in patients with BMI <30, and between EFL and RV and RV/TLC in patients with BMI >30. There was no correlation between EFL and IC. Conclusions: Air trapping is strongly associated with increase in EFL in patients with severe COPD, particularly in patients with BMI >30. The absence of increase in IC with BMI is due to its reduction by air trapping.
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