Dynamic Assessment of Exercise Gas Exchange Efficiency by Breath-by-Breath Volumetric Capnography in Mild-Moderate...

A sizable fraction of dyspneic patients with only mild to moderate COPD exhibit a heightened ventilatory response to exercise relative to metabolic demands, i.e. a high ventilation ($\stackrel{.}{V}$_E)/CO₂ output ($\stackrel{.}{V}$CO₂). The lack of continuous assessment of gas exchange efficiency and estimates of arterial CO₂ partial pressure has hindered our understanding of the physiological underpinnings of this dynamic phenomenon. We compared key indices of gas exchange efficiency relative to the intra-breath CO₂ profile as a function of expired volume using breath-by-breath volumetric capnography in 30 patients (FEV₁ = 76 ± 17%) and 30 sex- and age-matched controls during incremental cycle ergometry. Wasted ventilation in the physiological dead space (V_Dphys) was calculated as the sum of airway and alveolar (_alv) dead space divided by tidal volume (V_T). Transcutaneous (_tc) readings provided estimates of arterialised PCO₂. Patients exhibited lower exercise tolerance, reporting higher dyspnoea throughout exercise (p < 0.05). Higher $\stackrel{.}{V}$_E/$\stackrel{.}{V}$CO₂ was associated with higher absolute (L) alveolar dead space (V_Dalv), but similar V_T; thus, both V_Dphys/V_T and V_Dalv/V_T were consistently higher in patients (p < 0.05). $\stackrel{.}{V}$_E/$\stackrel{.}{V}$CO₂ was elevated (≥34) in normocapnic patients (P_tcCO₂≥35 mmHg) who had a high V_Dphys/V_T (≥0.3); conversely, high $\stackrel{.}{V}$_E/$\stackrel{.}{V}$CO₂ coexisted with a lower V_Dphys/V_T only in hypocapnic subjects (p < 0.05). Higher V_Dalv and lower P_tcCO₂ were independently associated with a high $\stackrel{.}{V}$_E/$\stackrel{.}{V}$CO₂ nadir and iso-work rate dyspnoea (p < 0.001). Based on this innovative, high-density data-acquisition approach, we conclude that both wasted ventilation and alveolar hyperventilation, in a highly variable combination, contribute to excessive ventilation in dyspneic patients with mild-to-moderate COPD.