This study plans to learn more about what is the best treatment to prevent blood clots in patients in intensive care units (ICU's). The investigators know that patients who are in ICU's have a higher than normal risk of getting blood clots in the veins of their arms or legs. This can be very dangerous as the clot may move into the lungs. To prevent this, the standard treatment is to give low dose heparin subcutaneously 3 times a day (usually 5000 units at each dose). In this study the investigators are randomizing patients to receive either standard of care or low dose intravenous heparin in a continuous infusion.

Macro- and micro-thrombosis both contribute significantly to morbidity and mortality in the surgical intensive care unit. Pulmonary embolism (PE) is a common and preventable cause of death in critically ill patients, with a mortality rate of up to 10%. Up to 95% of cases of PE originate from deep venous thrombosis (DVT). There are multiple pharmacologic and non-pharmacologic methods of DVT prophylaxis.The current standard of care in thromboprophylaxis in the surgical intensive care unit (SICU) at the University of Colorado Hospital is low-dose subcutaneous heparin (SCH). However, there is little evidence that this is the optimal prophylactic treatment. In fact, a database search of ICD-9 diagnoses made in 2005 suggests that the incidence of DVT in SICU patients, the majority who receive subcutaneous heparin, is approximately 7%. Surgical ICU patients are at high risk of developing DVT during their hospital stay and likely need more aggressive anticoagulation. Intravenous heparin, given at a low dose and titrated to a measurable endpoint PTT (partial thromboplastin time), may offer several benefits over the current standard of care, subcutaneous heparin. This method of treatment would offer more aggressive anticoagulation and allow dosage to be adjusted frequently based on each patient's changing coagulation status.

Inclusion Criteria:

• A signed informed consent;
• Age between 18 and 80 years
• The patient is admitted to the surgical intensive care unit at the University of Colorado Hospital

Exclusion Criteria:

• Predicated SICU stay less than 5 days;
• Pregnancy;
• Breast feeding;
• Initial platelet count < 30,000;
• Currently eligible for treatment of thromboembolism;
• Prior organ transplant;
• Cardiopulmonary bypass within previous 30 days;
• Advanced directive precluding participation;
• Already receiving pharmacologic agent for DVT prophylaxis;
• Prior diagnosis of heparin-induced thrombocytopenia;
• Heparin allergy

PI left the university in 2013. Numerous attempts were made to contact the PI and study team members, but efforts were unsuccessful. Arms are combined because no data is available regarding how participants were assigned to each study arm.

6 Months.

PI left the university in 2013. Numerous attempts were made to contact the PI and study team members, but efforts were unsuccessful. Arms are combined because no data is available regarding how participants were assigned to each study arm.

PI left the university in 2013. Numerous attempts were made to contact the PI and study team members to obtain results data, but efforts were unsuccessful.