• Annabel H Nickol, Helen Dunroy, Michael I Polkey, Anita Simonds, Jeremy Cordingley, Douglas R Corfield, and Mary J Morrell.
• Clinical and Academic Unit of Sleep and Breathing, National Heart and Lung Institute, Royal Brompton Hospital, Fulham Road, London SW3 6NP, UK. annabel@medex.org.uk
• Respir Med. 2009 Feb 1; 103 (2): 258-67.

BackgroundHypercapnic 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 PCO(2) may be used to test the HCVR in COPD during non-steady-state rebreathing, despite the fact that large (arterial-end-tidal) PCO(2) differences (P(a-et)CO(2)) 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 PetCO(2).ResultsIn COPD patients, PetCO(2) was lower than PaCO(2) by a constant amount throughout steady-state HCVR, but equalised with PaCO(2) 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 PaCO(2) or PetCO(2) was used. The steady-state HCVR intercept using PetCO(2) was greater than that using PaCO(2) (p=0.02). In healthy volunteers PetCO(2) equalised with PaCO(2) during steady-state HCVR, but was progressively greater than PaCO(2) during non-steady-state. Consequently, there was no difference in HCVR slope (p=0.21) or intercept (p=0.46) whether PaCO(2) or PetCO(2) was used. During non-steady-state there was a P(a-et)CO(2) difference in slope (p=0.03) and intercept (p=0.04).ConclusionsIn COPD patients non-steady-state HCVR using PetCO(2) is well tolerated, which is as accurate as PaCO(2). HCVR slope may be derived using PetCO(2) during steady-state testing, though there may be errors in intercept compared to use of PaCO(2). In healthy volunteers PetCO(2) may be used to estimate PaCO(2) during steady-state but not rebreathing HCVR.

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