Inclusion Criteria:  (1) pH≤7.3; (2) BE≤-8, or HCO3- < 20 mmol/L if BE was missing; and (3) PaCO2< 50 mmHg, or without chronic obstructive pulmonary disease (COPD) in discharge diagnoses if PaCO2 was unavailable.  Exclusion Criteria:  (1) age 16 or under; (2) diabetes insipidus ; (3) comorbidity of peripheral paralysis; (4) ICU stay over 100 days; (5) SBT was initiated before ICU admission , later than 48 hours after ICU admission or after a record of order for renal replacement therapy (RRT); and (6) death within the first 24 hours after ICU admission.