Inclusion Criteria:  Age 18~75 years old Good image of TTE can be obtained Sedative mechanical ventilation （Vt 8～10ml/kg） Circulation was relatively stable before the experiment began There is no contraindication to rehydration  Exclusion Criteria:  TTE acquisition is not ideal Patients did not agree to participate in the experiment Diseases of the central nervous system，patients with increased intracranial pressure Arrhythmia patients; Patients with increased intra-abdominal pressure; Bedridden patients; Patients with venous thrombosis of lower limbs; Patients with venous foreign body of lower limbs; Patients with circulatory instability; Patients with obstructive shock; Patients with moderate or above heart valve reflux