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Purpose: Respiratory pump failure in COPD patients can lead to CO₂ retention during exercise, but little is known about the factors determining CO₂-levels during exercise in COPD patients.

The aim of this study is to investigate the pattern of TcPCO₂ in COPD patients during exercise and factors driving this CO₂ response.

Patients and methods: 24 COPD patients (age 66(8) y, FEV1 43(18) %pred, TcPCO₂ at rest 37(4) mmHg, oxygen users 6/24) performed lung function and cardiopulmonary exercise test (CPET). During CPET TcPCO₂ was measured continuously and in O₂ users the CPET was performed with supplemental oxygen.

Results: At baseline, 16 patients were normocapnic (TcPCO₂ 35-45 mmHg) and 8 hypocapnic (TcPCO₂<35 mmHg). At the end of the CPET, 9 patients were normocapnic, 2 showed hypocapnia and 10 were hypercapnic. CO₂-retention (∆TcPCO₂ >4mmHg) was observed in 18 patients.

∆TcPCO₂ correlated significantly with VeMax (r=-.64; p=.004), FEV1 %predicted (r = -.53; p = .008), RV %predicted (r = .54; p = .007), RV/TLC (r = .56; p = .005), sRAW (r =.61; p = .005), sGAW (r = -.60; p = .002), and maximal TcPCO₂ (r=.63; p<.001), but did not correlate with baseline TcPCO₂ (r = -.08; p = .728). Furthermore, baseline TcPCO₂ correlated with the maximal TcPCO₂ (r = 0.67; p < .001).

Conclusion: The CO₂-response of COPD-patients during CPET is heterogeneous with 10/24 developing hypercapnia. This study revealed that the lung mechanics are the most important factor correlating with CO₂ retention during exercise while ∆TcPCO₂ was not associated with baseline TcPCO₂.