The risk of developing a blood clot occurs in up to 60% of all critical care patients. Many times enoxaparin (or Lovenox) is given to patients who are at a higher risk of developing clots in their legs or lungs. There are two standard doses of enoxaparin that are recommended by the drug companies. These two doses have never been directly compared in trauma, general, and vascular surgery patients. The purposes of this study are:

1. to compare the development of blood clots in patients receiving 30mg twice daily of enoxaparin compared to patients receiving 40mg once daily of enoxaparin.
2. to determine if there is higher risk of bleeding complications in patients receiving 30mg twice daily of enoxaparin compared to patients receiving 40mg once daily.

Patients enrolled into the study will be randomized to receive enoxaparin, 30mg twice daily or enoxaparin, 40mg once daily. Patients will then be monitored for signs and symptoms of blood clots. At the time of discharge (or before, if medically indicated), an ultrasound test will be performed to look for blood clots in the patient's legs.

The investigators will compare incidence of blood clots formed between the 2 groups of patients to determine if one dose of enoxaparin relates to a lower rate of blood clots in critically ill patients. The investigators will also compare the incidence of bleeding complications between the 2 groups.

This is a prospective randomized single center study in human subjects.

Initiation of enoxaparin thromboprophylaxis will be done by the treatment team. Patients admitted to the trauma service will be eligible for enrollment. Research staff will approach patients or their LARs for consent prior to or at the time enoxaparin is ordered. This will include patients with traumatic intracranial hemorrhage, after the consulting neurosurgical team and primary treatment team decide to initiate thromboprophylaxis. Once enrolled, the subject will be randomized to receive either 30 mg twice daily dosing or 40 mg daily dosing of enoxaparin. However, treatment team will be responsible for deciding when to start enoxaparin treatment. Patients who are discharged without receiving enoxaparin or are started on a non-standard or therapeutic dose will be withdrawn from the study. Patient characteristics: age, sex, body mass index (BMI), medical comorbidities, Acute Physiology and Chronic Health Evaluation II score (APACHE II), injuries, and operations will be collected, coded with a unique identification letter and number combination, and entered into a database.

All patients will undergo weekly ultrasound duplex examination of the lower extremities for the presence of deep venous thrombosis. Patients who develop symptoms of pulmonary embolism including acute shortness of breath and increased A-a gradient will undergo computed tomography angiography as part of their standard care. A bleeding complication will be defined as a known bleeding episode associated with hypotension (greater than 20 mmHG drop from baseline) and the need for blood transfusion.

Current practice at OHSU is for patients age 15 and above to be admitted to the adult trauma service, and are treated with the same standard of care as adults. Currently, there are no clear data stratifying the risk of VTE in this population compared to other age groups. Therefore, patients in this age range on the adult Trauma Service will be included in the study as they are treated in the same method as our adult patient population.

Inclusion Criteria:

• Admitted patients requiring prophylactic dosing of enoxaparin (Lovenox)
• Admitted to the trauma or surgical service
• Age greater than 15 years

Exclusion Criteria:

• Unable to obtain consent from patient or authorized representative
• Presence of intracranial hemorrhage
• Receiving therapeutic dose of enoxaparin (Lovenox)
• Receiving other forms of anticoagulation
• Presence of renal failure requiring non-standard dosing regimen