Role of CO ₂ responsiveness and breathing efficiency in determining exercise capacity of patients with chronic airway obstruction

We examined the role of CO ₂ responsiveness and breathing efficiency in limiting exercise capacity in 15 patients with chronic airway obstruction (FEV ₁ = 0.88 ± 0.25 L, mean ± SD). Responses of minute ventilation and P _0.1 (mouth pressure 0.1 s after the onset of occluded inspiration) to hypercapnia (ΔV̇E/ΔPCO ₂ , ΔP _0.1 /ΔPCO ₂ ) were measured by rebreathing, and the ratio of the two (ΔV̇E/ΔP _0.1 ) was defined as an index of breathing efficiency during hyperventilation. Exercise capacity was measured as symptom-limited, maximal oxygen consumption (V̇O ₂ max/BW) in an incremental treadmill test and also as the 12-min walking distance (TMD). All patients discontinued the treadmill test because of dyspnea, and the exericse capacity correlated with the degree of airway obstruction, although there was a wide variability among patients with comparable FEV ₁ . There were no significant correlations between the responses to CO ₂ and exercise capacity. However, there was a significant correlation between ΔV̇E/ΔP _0.1 and V̇O ₂ max/BW (r = 0.87, p < 0.001) or TMD (r = 0.78, p < 0.001), and these correlations remained significant even when the relational effects of FEV ₁ were taken out. These results support the hypothesis that airway obstruction and breathing efficiency are important, but that CO ₂ responsiveness is not a major factor in determining the exercise capacity of patients with chronic airway obstruction.