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

Summary 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 P CO 2 may be used to test the HCVR in COPD during non-steady-state rebreathing, despite the fact that large (arterial–end-tidal) P CO 2 differences ( P (a–et)CO 2 ) exist during air breathing. Methods Eight patients and 11 healthy volunteers underwent steady-state HCVR testing and non-steady-state rebreathing HCVR testing, using P a and P etCO 2 . Results In COPD patients, P etCO 2 was lower than P aCO 2 by a constant amount throughout steady-state HCVR, but equalised with P aCO 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 P aCO 2 or P etCO 2 was used. The steady-state HCVR intercept using P etCO 2 was greater than that using P aCO 2 ( p  = 0.02). In healthy volunteers P etCO 2 equalised with P aCO 2 during steady-state HCVR, but was progressively greater than P aCO 2 during non-steady-state. Consequently, there was no difference in HCVR slope ( p  = 0.21) or intercept ( p  = 0.46) whether P aCO 2 or P etCO 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). Conclusions In COPD patients non-steady-state HCVR using P etCO 2 is well tolerated, which is as accurate as P aCO 2 . HCVR slope may be derived using P etCO 2 during steady-state testing, though there may be errors in intercept compared to use of P aCO 2 . In healthy volunteers P etCO 2 may be used to estimate P aCO 2 during steady-state but not rebreathing HCVR.