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.

Inclusion Criteria:

• COPD patients

Exclusion Criteria:

• hemodynamics is unstable;
• any other lung disease despite COPD；
• other diseases influencing experiments: Cognitive impairment disease、 mouth and nose trauma.

stable and acute axcerbation COPD patients