The most significant pathophysiology change of COPD patients is persistent incompletely reversible airflow obstruction and increased lung volume. As a result, the work of breathing(WB) and neural respiratory drive (NRD)increased. Noninvasive positive pressure ventilation (NPPV) can reduce the load of respiratory muscles. Detection of NRD can be the index of monitoring for titration of optimal level of ventilator support in the future. As electromyography(EMG) is the most reliable measurement in evaluating NRD that can be used for human. Currently the standard method for evaluation of NRD with EMG is using esophageal multi-paired electrodes catheter(EMGdi) ,it is reliable but invasive .Surface respiratory EMG (sEMG）is a noninvasive measurement. Although it subjected to contamination and less sensitive, recently, advance in technology with multiple pair of surface electrodes is possible to sufficient signals for evaluation of NRD.So the investigator compare the NRD measured by EMGdi and sEMG,and consider that the correlation between them is well in different levels.