Gas exchange efficiency slopes to assess exercise tolerance in chronic obstructive pulmonary disease

In patients with chronic obstructive pulmonary disease (COPD), the clinical use of the minute ventilation-carbon dioxide production (\(\rm{\dot V}\)_E-\(\rm{\dot V}\)_CO2) slope has been reported as a measure of exercise efficiency, but the oxygen uptake efficiency slope (OUES), i.e., the slope of oxygen uptake (\(\rm{\dot V}\)_O2) versus the logarithmically transformed \(\rm{\dot V}\)_E, has rarely been reported.

We hypothesized that the \(\rm{\dot V}\)_E-\(\rm{\dot V}\)_CO2 slope is more useful than OUES in clinical use for the pathophysiological evaluation of COPD. Then, we investigated the cardiopulmonary exercise testing parameters affecting each of these slopes in 122 patients with all Global Initiative for Chronic Obstructive Lung Disease (GOLD) COPD grades selected from our database.

Compared with the GOLD I-II group (n = 51), peak \(\rm{\dot V}\)_O2 (p < 0.0001), OUES (p = 0.0161), \(\rm{\dot V}\)_E at peak exercise (p < 0.0001), and percutaneous oxygen saturation (SpO₂) at peak exercise (p = 0.0004) were significantly lower in the GOLD III-IV group (n = 71). The GOLD III-IV group was divided into two groups by the exertional decrease in SpO₂ from rest to peak exercise: 3% or less (the non-desaturation group: n = 23), or greater than 3% (the desaturation group: n = 48). OUES correlated only weakly with peak \(\rm{\dot V}\)_O2, \(\rm{\dot V}\)_E at peak exercise, and the difference between inspired and expired mean O₂ concentrations (ΔF_O2) at peak exercise, i.e., an indicator of oxygen consumption ability throughout the body, in the GOLD III-IV group with exertional hypoxemia. In contrast, the \(\rm{\dot V}\)_E-\(\rm{\dot V}\)_CO2 slope was significantly correlated with ΔF_O2 at peak exercise, regardless of the COPD grade and exertional desaturation. Across all COPD stages, there was no correlation between the \(\rm{\dot V}\)_E-\(\rm{\dot V}\)_CO2 slope and \(\rm{\dot V}\)_E at peak exercise, and stepwise analysis identified peak \(\rm{\dot V}\)_O2 (p = 0.0345) and ΔF_O2 (p < 0.0001) as variables with a greater effect on the \(\rm{\dot V}\)_E-\(\rm{\dot V}\)_CO2 slope.

The OUES may be less useful in advanced COPD with exertional hypoxemia. The \(\rm{\dot V}\)_E-\(\rm{\dot V}\)_CO2 slope, which is independent of \(\rm{\dot V}\)_E, focuses on oxygen consumption ability and exercise tolerance in COPD, regardless of the exertional hypoxemia level and COPD grade. Therefore, the \(\rm{\dot V}\)_E-\(\rm{\dot V}\)_CO2 slope might be useful in establishing or evaluating tailor-made therapies for individual patient’s pathologies in COPD as an indicator focusing on oxygen consumption ability.