Sidestream end-tidal CO2 has been found to be closely correlated to PaCO2 especially when people have the stable hemodynamics and normal lung function. PETCO2 can basically reflect the level of PaCO2,but some investigators found that PETCO2 measurements did not reflect PaCO2 in COPD patients.There may be several reasons for this discrepancy. Due to airway obstruction and ventilation-perfusion mismatch, there is a delay in the maximum alveolar concentration of CO2 reaching the cannula, which may give lower PETCO2 values compared with PaCO2. To overcome this disadvantage, we tried to prolong their expiratory time (extend to 5-8s) in order to guarantee adequate time for alveolar CO2 reaching the cannula.our study mainly focus on two issues.First, to explore the optimal depth of tube placed when patients with COPD breathe spontaneously or receive noninvasive ventilation, and compare the differences of two condition at the same time. Secondly, to evaluate the agreement between the PaCO2 and sidestream PETCO2（5s）values in COPD patients receiving noninvasive ventilation when our extension tube locates in the optimal position, and investigate whether PETCO2（5s）can be used to monitor ventilation status dynamically.

Non-invasive positive pressure ventilation(NPPV) can significantly improve gas exchange and relieve respiratory muscle fatigue for COPD,and arterial blood gas is the gold standard to evaluate ventilation status,but the procedure is time consuming,invasive, painful and in some patients.

Sidestream end-tidal CO2 has been found to be closely correlated to PaCO2 especially when people have the stable hemodynamics and normal lung function. PETCO2 can basically reflect the level of PaCO2, but for patients with COPD, due to the presence of airway obstruction and emphysema formation, which cause ventilation-perfusion ratio(V/Q) , result to dead space-tidal volume ratio(VD/VT) and right-to-left shunting(Qs/Qt), the time of various lung units discharge of carbon dioxide is not synchronous. Some investigators found that PETCO2 measurements did not reflect PaCO2 in COPD patients. There may be several reasons for this discrepancy. Due to airway obstruction and ventilation-perfusion mismatch, there is a delay in the maximum alveolar concentration of CO2 reaching the cannula, which may give lower PETCO2 values compared with PaCO2. To overcome this disadvantage, we tried to prolong their expiratory time (extend to 5-8s) in order to guarantee adequate time for alveolar CO2 reaching the cannula.

our study mainly focus on two issues.First, to explore the optimal depth of tube placed when patients with COPD breathe spontaneously or receive noninvasive ventilation, and compare the differences of two condition at the same time. Secondly, to evaluate the agreement between the PaCO2 and sidestream PETCO2（5s）values in COPD patients receiving noninvasive ventilation when our extension tube locates in the optimal position, and investigate whether PETCO2（5s）can be used to monitor ventilation status dynamically.

Inclusion Criteria:1.patients are enrolled in accordance with 2011 international treatment guidelines of COPD.

2.patients are in a clear state of mind and can incompliance with clinicians to extend breathing only using nose and closing mouth.

Exclusion Criteria:1.Hemodynamic instability. 2.Pulmonary embolism,Pneumothorax, pleural effusion, upper airway obstruction,Chest trauma, chest wall deformities, neuromuscular diseases, lung tumor.

3.combinating acute exacerbation of bronchial asthma in this hospitalization. 4.Nose and mouth trauma, deformity, or recent surgery.

Patients with a COPD exacerbation who admitted to the respiratory department of First Affiliated Hospital of Guangzhou Medical University,Guangzhou,Guangdong,China