Lung function and peak oxygen uptake in chronic obstructive pulmonary disease phenotypes with and without emphysema

Previous studies of associations of forced expiratory lung volume in one second (FEV(1)) with peak oxygen uptake (VO(2peak)) in chronic obstructive pulmonary disease (COPD) have not taken sex, age and height related variance of dynamic lung volumes into account. Nor have such demographic spread of spirometric measures been considered in studies comparing VO(2peak) between COPD phenotypes characterized by degree of emphysema. We aimed to assess the association of FEV(1Z-score) with VO(2peak) in COPD (n = 186) and investigate whether this association differs between emphysema (E-COPD) and non-emphysema (NE-COPD) phenotypes. Corresponding assessments using standardized percent predicted FEV(1) (ppFEV(1)) were performed for comparison. Additionally, phenotype related differences in VO(2peak) were compared using FEV(1Z-score) and ppFEV(1) as alternative expressions of FEV(1). E-COPD and NE-COPD were defined by transfer factor of the lung for carbon monoxide below and above lower limits of normal (LLN), respectively. The associations were assessed in linear regression models. One unit reduction in FEV(1Z-score) was associated with 1.9 (95% CI 1.4, 2.5) ml/kg/min lower VO(2peak). In stratified analyses, corresponding estimates were 2.2 (95% CI 1.4, 2.9) and 1.2 (95% CI 0.2, 2.2) ml/kg/min lower VO(2peak) in E-COPD and NE-COPD, respectively. The association did not differ statistically by COPD phenotype (p-value for interaction = 0.153). Similar estimates were obtained in analyses using standardized ppFEV(1). Compared to NE-COPD, VO(2peak) was 2.2 (95% CI 0.8, 3.6) and 2.1 (95% CI 0.8, 3.5) ml/kg/min lower in E-COPD when adjusted for FEV(1Z-score) and ppFEV(1), respectively. In COPD, FEV(1Z-score) is positively associated with VO(2peak). This association was stronger in E-COPD but did not differ statistically by phenotype. Both the association of FEV(1) with VO(2peak) and the difference in VO(2peak) comparing COPD phenotypes seems independent of sex, age and height related variance in FEV(1). Mechanisms leading to reduction in FEV(1) may contribute to lower VO(2peak) in E-COPD.