VENTILATORY INEFFICIENCY REVEALS EMERGENT PaCO ₂ EQUILIBRIUM DURING EXERCISE IN OBSTRUCTIVE LUNG DISEASE

Ventilatory "inefficiency" during exercise in obstructive lung disease-such as that resulting from increased dead space or ventilation-perfusion mismatch-is commonly interpreted as a perturbation that requires an additional increase in minute ventilation and thus in V̇E/V̇CO₂ to preserve PaCO₂ homeostasis. This observation raises a fundamental question: how could an increase in V̇E/V̇CO₂ during exercise be actively directed toward defending PaCO₂ stability if no known neural signal, governing respiration, directly encodes ventilatory "inefficiency"? We retrospectively analyzed the lung-function and anthropometric data, as well as ventilatory and gas-exchange responses obtained at rest, lactate threshold (LaT), and peak exercise, of 443 patients with obstructive lung disease who underwent cardiopulmonary exercise testing and spanning a wide range of V̇E/V̇CO₂. Relationships between PETCO₂ and V̇E/V̇CO₂, estimated V̇A/V̇CO₂, and V̇D/V̇CO₂ were examined using power-law regression. Patients were then stratified by peak V̇E/V̇CO₂, and group differences were assessed using Welch ANOVA, with effect sizes expressed as η² and Cohen's d. Across rest, LaT, and peak exercise, PETCO₂ exhibited a continuous ≈1/x relationship with V̇E/V̇CO₂ that closely paralleled PETCO₂-V̇A/V̇CO₂. Stratification by V̇E/V̇CO₂ consistently identified very large effect sizes for PETCO₂. This PETCO₂-V̇E/V̇CO₂ "phenotype" was also present in the normocapnic range. Hypercapnia was observed only in patients with low V̇E/V̇CO₂. Arterial PCO₂ obtained just at the exercise cessation retained a similar pattern, remaining inversely related to peak V̇E/V̇CO₂. The present study supports the view that PaCO₂ is emergent, and not defended, in this cohort of COPD patients. Implications for control of breathing during exercise are discussed.