The role of phenotype on ventilation and exercise capacity in patients affected by COPD: a retrospective study

Abstract

Background: The idea of phenotype in chronic obstructive pulmonary disease (COPD) has evolved in the last decades, and the importance of peculiar treatment strategies has now been acknowledged. Although dyspnea and exercise limitation are hallmarks of COPD, this aspect has never been fully explored in literature in terms of disease phenotype. The aim of the present study was to explore the relevance of clinical COPD phenotypes on exercise ventilation and maximal capacity.

Methods: In this observational cohort retrospective study we analyzed the data of 50 COPD patients who underwent cardiopulmonary exercise test, categorized as emphysematous (n=29), and non-emphysematous (n=21) according to a previously validated model.

Results: We found a significant difference in terms of VE/VCO₂ slope (median values 32.4 vs 28.0, p=0.015) and VE/VCO₂ ratio at nadir (median values 37 vs. 33, p=0.004), which resulted higher in emphysematous patients, who also presented lower P_ETCO₂ values (median values 32.6 vs 35.6, p=0.008). In a subgroup of 31 tests which met the maximality criteria, emphysematous patients presented a significantly lower work rate at peak (median value 51 vs 72% predicted, p=0.016), and showed a lower peak oxygen consumption, although at the limit of significance (median values of 63 vs 85 % predicted, p=0.051).

Conclusions: This study extends our knowledge about the characterization of the COPD phenotypical expression of disease, showing that patients affected by emphysema are more prone to ventilatory inefficiency during exercise, and that this is likely to be an important cause of their overall reduced exercise capacity.

Keywords: COPD; cardiopulmonary exercise test; emphysema; exercise capacity; phenotype; ventilation.

Figure 1.

Graphical representation of the median values of VE/VCO₂ slope and intercept in the two groups during cardiopulmonary exercise test. VCO₂, Carbon dioxide output; VE/VCO₂ slope median values and interquartile range: emphysematous 32.4 (29.4-35.3), non-emphysematous 28.0 (26.1-33.2); p=0.015. VE/VCO₂ intercept median values and interquartile range: emphysematous 6.3 (3.7-7.6), non-emphysematous 5.6 (4.2-6.4); p=0.536.

Figure 2.

Graphical representation of P_ETCO₂ in the two groups during cardiopulmonary exercise test. *p<0.05. P_ETCO₂, end tidal pressure for carbon dioxide. Median values and interquartile range of P_ETCO₂ at peak: emphysematous 33 (30-35), nonemphysematous 36 (34-41); p=0.012. No significant differences between P_ETCO₂ at other time points at the Mann-Whitney Utest with Bonferroni adjustment for multiple measures.

Figure 3.

Relationship between peak exercise capacity and the carbon dioxide equivalent at nadir during cardiopulmonary exercise test. VO₂, oxygen consumption; VCO₂, carbon dioxide output; VE, Ventilation. Spearman’s r= -0.658, p<0.01, 43% of the variation explained.