Methods: After the baseline data of spontaneous breathing was collected, 20 hypercapnic recovering AECOPD patients were placed on different mode of noninvasive positive pressure ventilation(NPPV, such as the PAV or PSV mode) randomly. For each mode, three levels (PA-, PA, PA+or PS-, PS, PS+), ) of support were applied.PS and PA are set for the patient's comfort . On the basis of these two levels, 25% increase and reduction assisted level of pressure were set both for PS and PA (PA-, PA+or PS-, PS+). At each level, the patients were ventilated at least 20 minutes until the breathing was stable. The respiratory frequency (RR), tidal volume (VT), transdiaphragmatic pressure (pdi) the pressure-time product (PTP) and root-mean-square(RMS) of EMGdi were calculated. Esophageal and gastric balloon-catheters were used to detect the intra-thoracic and abdominal pressure. Airway pressure was also measured simultaneously. EMGdi was recorded from a multipair esophageal electrode .During ventilation Airflow and ventilation were measured with pneumotachograph.
