Both chronic airway obstruction and obesity are increasing in prevalence but the effect of their combination on pulmonary function parameters across the range of airway obstruction is unknown.

METHODS: We studied the impact of increasing body mass index (BMI) on static lung volumes and airway function in a cohort of 2,265 subjects from a large pulmonary function laboratory database who were 40-80 years of age and met GOLD spirometric criteria for COPD (post-bronchodilator FEV(1)/FVC<0.7). We also evaluated the influence of severity of airway obstruction (by GOLD criteria) on these relationships.

RESULTS: With increasing BMI in the group as a whole: functional residual capacity (FRC), residual volume (RV), expiratory reserve volume (ERV) and specific airway resistance (sRaw) decreased exponentially (all p<0.001); total lung capacity (TLC) decreased linearly (p<0.001); inspiratory capacity (IC) and IC/TLC increased linearly (p<0.001); whereas vital capacity (VC) was not significantly influenced. The effects of increasing BMI on FEV(1)/FVC and sRaw were greatest in GOLD stage III/IV (p<0.05), while increasing BMI had greater effects on IC in GOLD stage I (p<0.001).

CONCLUSION: With increasing BMI, subjects with airway obstruction had consistent reductions in lung hyperinflation, with significant improvements in IC and the FEV(1)/FVC ratio; this effect was greatest in patients with the most severe airway obstruction. These results have important implications for the clinical assessment of patients with combined obesity and airway obstruction.