The Copmparison of Accuracy of PtcCO2 and in PetCO2 in COPD Patients

The pathophysiological characteristics of COPD is irreversible airflow restriction which may influence the gas exchange and cause the abnormal PaO2 and PaCO2. Assessment of the partial arterial pressure of carbon dioxide (PaCO2 ) is the 'gold standard' for the evaluation of the adequacy of alveolar ventilation.Method used mostly nowadays : arterial puncture which is accurate but is invasive 、painful、 non-dynamic and there is risk of infection and tissue、nerve damage. Noninvasive measurement methods (Most commonly used)： end-tidal CO2（ ETCO2）、 transcutaneous CO2(TcCO2). They all can monitor the PaCO2 dynamically、 noninvasively, but is existence of controversy of the accuracy. So the investigators want to compare the accuracy of these two noninvasive methods for assessment of PaCO2.