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

Background: Hypercapnic 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. Methods: Eight patients and 11 healthy volunteers underwent steady-state HCVR testing and non-steady-state rebreathing HCVR testing, using Pa and PetCO2. Results: In 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). Conclusions: In 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. © 2008 Elsevier Ltd. All rights reserved.