A quick and easy method of measuring the hypercapnic ventilatory response in patients with COPD

SummaryBackgroundHypercapnic ventilatory response (HCVR) techniques have not previously been adequately validated in patients with chronic obstructive pulmonary disease (COPD). We have tested the hypothesis that end-tidal PCO2 may be used to test the HCVR in COPD during non-steady-state rebreathing, despite the fact that large (arterial–end-tidal) PCO2 differences (P(a–et)CO2) exist during air breathing.MethodsEight patients and 11 healthy volunteers underwent steady-state HCVR testing and non-steady-state rebreathing HCVR testing, using Pa and PetCO2.ResultsIn COPD patients, PetCO2 was lower than PaCO2 by a constant amount throughout steady-state HCVR, but equalised with PaCO2 during non-steady-state HCVR. Consequently there were no differences in HCVR slope using either method (steady-state p=0.91; rebreathing p=0.73), or HCVR intercept in rebreathing (p=0.68) whether PaCO2 or PetCO2 was used. The steady-state HCVR intercept using PetCO2 was greater than that using PaCO2 (p=0.02). In healthy volunteers PetCO2 equalised with PaCO2 during steady-state HCVR, but was progressively greater than PaCO2 during non-steady-state. Consequently, there was no difference in HCVR slope (p=0.21) or intercept (p=0.46) whether PaCO2 or PetCO2 was used. During non-steady-state there was a P(a–et)CO2 difference in slope (p=0.03) and intercept (p=0.04).ConclusionsIn COPD patients non-steady-state HCVR using PetCO2 is well tolerated, which is as accurate as PaCO2. HCVR slope may be derived using PetCO2 during steady-state testing, though there may be errors in intercept compared to use of PaCO2. In healthy volunteers PetCO2 may be used to estimate PaCO2 during steady-state but not rebreathing HCVR