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.