Inclusion Criteria:  Prolonged MV(>72 h); Written informed consent was obtained from the patients or their relatives  Exclusion Criteria:  Previous neuromuscular disease; Unrelieved pneumothorax, restricted diaphragmatic dyskinesia including abdominal high pressure, a large number of ascites Thoracic or diaphragmatic malformation; Local skin damage and infection; Indwelling a temporary or permanent pacemaker; Severe obesity(BMI>35 kg/m2) Various reasons (severe intestinal gas accumulation, structural abnormalities) lead to the failure of ultrasonic detection of diaphragm movement; Patients with an expected survival time of less than 7 days or with palliative care