Correlation of air trapping in COPD with nadir to peak increment of ventilatory equivalents at maximal effort

Introduction: Air trapping is a major abnormality contributing to effort limitation in COPD. Ventilatory equivalent for CO 2 (VE/VCO 2 ) obtained during cardiopulmonary exercise testing (CPET) often performed in these patients, is measured with attention to the nadir values obtained at the anaerobic threshold (VE/VCO 2 @nadir). Patients with COPD are often not able to develop increased ventilation pattern, designated respiratory compensation phase (RCP). Aims: We sought to investigate whether a correlation exists between the extent of air trapping and demonstrating RCP. Measuring VE/VCO 2 , we hypothesized that patients with prominent air trapping were less able to achieve higher values at peak effort (VE/VCO 2 @peak) compared to VE/VCO 2 @nadir. Methods: CPET from 20 patients, in whom simultaneous plethysmography was available were included in this retrospective analysis. VE/VCO 2 @nadir was determined as well as VE/VCO 2 @peak. ΔVE/VCO 2 was defined as the difference between VE/VCO 2 @peak and VE/VCO 2 @nadir. ΔVE/VO 2 was also determined for VE/VO 2 in a similar manner. We compared these measurements with the patients9 lung volume values obtained by plethysmography and calculated RV/TLC. Spearman9s test was used for correlation analysis. Results: Increase in ventilatory equivalents at peak effort tended to be lower and even absent with greater air trapping. We found a statistically significant inverse correlation between RV/TLC and both ΔVE/VCO 2 (r 2 = 0.333, p=0.0077), and ΔVE/VO 2 (r 2 = 0.272, p=0.0182). Conclusions: Patients with increased air trapping are less able to increase ventilation in response to metabolic acidosis such as seen during high intensity exercise.