The onset and offset of the neural inspiratory time are fundamentally important measurements in studies of patientventilator interaction, where the level of assistance delivered by the ventilator is controlled by the patient's demand. The onset of neural inspiratory time can be determined in esophageal pressure, transdiaphragmatic pressure, and EMG signals. The investigator compare the onset measured by EMG, esophageal and transdiaphragmatic pressure, and consider that the correlation between them is well in different conditions.

The onset and offset of the neural inspiratory time are fundamentally important measurements in studies of patien-tventilator interaction, where the level of assistance delivered by the ventilator is controlled by the patient's demand. Patient-ventilator asynchrony evaluated using the time lag between onset of the neural inspiratory time and the onset of a mechanical ventilation signal. The onset of neural inspiratory time can be determined in esophageal pressure, transdiaphragmatic pressure, and EMG signals. The investigator compare the onset measured by EMG, esophageal and transdiaphragmatic pressure, and consider that the correlation between them is well in three different conditions.

Inclusion Criteria:

• normal cardio-pulmonary function
• without low inspiratory muscle strength
• non-smoker
• without history of the nervous system and respiratory system disease
• sever to very severe stable stage

Exclusion Criteria:

• systemic application of corticosteroids nearly 4 weeks