End Tidal Carbon Dioxide Monitoring for Evaluating Changes of PaCO2 After Noninvasive Mechanical Ventilation in COPD

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.