Ventilatory compensation during the incremental exercise test is inversely correlated with air trapping in COPD

Background: Air trapping and gas exchange abnormalities are major causes of exercise limitation in chronic obstructive pulmonary disease (COPD). During incremental cardiopulmonary exercise testing, actual nadir values of ventilatory equivalents for carbon dioxide (V _E/VCO ₂) and oxygen (V _E/VO ₂) may be difficult to identify in COPD patients because of limited ventilatory compensation capacity. Therefore, we aimed in this exploratory study to detect a possible correlation between the magnitude of ventilation augmentation, as manifested by increments in ventilatory equivalents from nadir to peak exercise values and air trapping, detected with static testing.    Methods: In this observational study, we studied data obtained previously from 20 COPD patients who, during routine follow-up, underwent a symptom-limited incremental exercise test and in whom a plethysmography was obtained concurrently. Air trapping at rest was assessed by measurement of the residual volume (RV) to total lung capacity (TLC) ratio (RV/TLC). Gas exchange data collected during the symptom-limited incremental cardiopulmonary exercise test allowed determination of the nadir and peak exercise values of V _E/VCO ₂ and V _E/VO ₂, thus enabling calculation of the difference between peak exrcise value and nadir values of  V _E/VCO ₂ and V _E/VO ₂, designated ΔV _E/VCO ₂ and ΔV _E/VO ₂, respectively. Results: We found a statistically significant inverse correlation between both ΔV _E/VCO ₂ (r = -0. 5058, 95% CI -0.7750 to -0.08149, p = 0.0234) and ΔV _E/VO ₂ (r = -0.5588, 95% CI -0.8029 to -0.1545, p = 0.0104) and the degree of air trapping (RV/TLC). There was no correlation between ΔV _E/VCO ₂ and forced expiratory volume in the first second, or body mass index.  Conclusions: The ventilatory equivalents increment to compensate for acidosis during incremental exercise testing was inversely correlated with air trapping (RV/TLC).