 Chronic obstructive pulmonary diseaseCOPD is a common disease among people. Acute exacerbation of chronic obstructive pulmonary diseaseAECOPD is the acute and aggravating phase of COPD which may lead to respiratory failure. Mechanical ventilation is a very important therapy for those patients,but traditional ventilation modes have some defects, so investigators need to find new modes to solve these problems. In this study, the investigators compare neurally adjusted ventilatory assist NAVA and PSV to observe the effects on AECOPD patients air distribution and dead space to determine if NAVA is a more appropriate ventilation mode for AECOPD patients. 