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

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 (V.E)/CO2 output (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 (VDphys) was calculated as the sum of airway and alveolar (alv) dead space divided by tidal volume (VT). Transcutaneous (tc) readings provided estimates of arterialised PCO2. Patients exhibited lower exercise tolerance, reporting higher dyspnoea throughout exercise (p < 0.05). Higher V.E/V.CO2 was associated with higher absolute (L) alveolar dead space (VDalv), but similar VT; thus, both VDphys/VT and VDalv/VT were consistently higher in patients (p < 0.05). V.E/V.CO2 was elevated (≥34) in normocapnic patients (PtcCO2≥35 mmHg) who had a high VDphys/VT (≥0.3); conversely, high V.E/V.CO2 coexisted with a lower VDphys/VT only in hypocapnic subjects (p < 0.05). Higher VDalv and lower PtcCO2 were independently associated with a high V.E/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.