Inclusion Criteria:  age older than 18 years old; received V-V ECMO because of sevever respiratory failure  Exclusion Criteria:  anticoagulant contraindications; Confirmed or suspected, or previously diagnosed with heparin induced thrombocytopenia; Cerebral infarction or suspected patients; Patients with severe liver dysfunction; Allergic to heparin or argatroban; hemophilia; Unwilling or unable to complete the study.