To evaluate the pharmacokinetics of the thromboprophylactic agent enoxaparin in critically ill patients by comparing plasma anti-factor Xa activity when enoxaparin is administered either as a continuous intravenous infusion or subcutaneous bolus once daily.

To investigate possible ongoing coagulation by coagulation markers during antithrombotic therapy with standard doses of enoxaparin

Inclusion Criteria:

• Body Mass Index between 18-30 kg/m2
• Critically ill patients requiring intensive care and pharmacological thromboprophylaxis
• Expected to remain in the ICU for at least 72 h
• Written informed consent obtained from the patient or his/her legal representative.

Exclusion Criteria:

• Other indications for anticoagulant therapy than thromboprophylaxis
• Intracranial haemorrhage or central neurosurgical operation within three months prior to the admission
• Disseminated intravascular coagulation (DIC) according to the international society on Thrombosis and Haemostasis criteria
• Known heparin induced thrombocytopenia (HIT), or hypersensitivity to enoxaparin or heparin
• Treatment with enoxaparin or any other low-molecular weight heparin (LMWH) or heparin within 24 hours prior to ICU admission. If patient has received thromboprophylactic dose of LMWH within 24-72 hours before ICU admission inclusion can be done if measured anti-factor Xa level is <0.1 IU/ml at the time of admission
• Any long-term anticoagulant medication, expect low-dose aspirin
• Major bleeding within the last week unless definitively treated
• Blood platelet count <50, P-thromboplastin time (TT) <40% , international normalized ratio (INR) >1.7
• Glomerular filtration rate less than 50 ml/min/1.73 m2 estimated from serum creatinine by applying Cockcroft-Gault equation or chronic dialysis
• HIV, hepatitis B virus, or hepatitis C virus infection
• Pregnancy