• The study will assess efficacy by re-evaluating thrombus at 48-72 hours as well as continued efficacy by re-evaluating the thrombus at ~30 days after study drug initiation. [ Time Frame: 72 hours and 30 days ] [ Designated as safety issue: Yes ]
  

• Pharmacokinetics (PK) will also be assessed by measuring plasma bivalirudin levels and correlation of the PD to the PK parameter will be performed. The study will also assess safety by evaluating for bleeding, as well as other adverse events. [ Time Frame: twice a week until drug discontinuation ] [ Designated as safety issue: Yes ]
  

Drug: Bivalirudin
The initial bolus dose will be 0.125 mg/kg followed immediately by a continuous infusion of 0.125 mg/kg/hour.

This will be an open-label, single-arm, dose-finding, pharmacokinetic, safety and efficacy study of bivalirudin in children 6 months to 18 years of age with deep vein thrombosis. The specific aims are as follows:

1. Pharmacodynamic analysis based on the activated partial thromboplastin time (aPTT) will be performed and the best bolus dose and initial infusion dose of bivalirudin will be determined for 3 age cohorts of children.
2. Pharmacokinetic analysis of bivalirudin will be determined based on bivalirudin plasma levels in the same 3 age cohorts.
3. Comparison of pharmacokinetics with pharmacodynamics will be performed to determine whether the aPTT is an adequate surrogate for plasma bivalirudin levels.
4. The rate of clot resolution within 48-72 hours will be determined.
5. The safety of bivalirudin will be determined by assessing for major and minor bleeding and other adverse events.

A total of 30 patients will be enrolled in this study. They will be divided into 3 groups according to their age range (6 months-age to > 5, 5 to > 12 years and 12 to >18 years). The patient must have a deep vein thrombosis (clot in an artery or vein) that has been seen on an imaging test. After receiving the study drug, the subjects will then have blood tests at specific timepoints to measure how the drug is effecting the body and how the drug is acting once it is in the body. Based upon the levels of the blood tests, the dose of the study drug may be adjusted or stopped. The subject will continue on the drug until the clot is dissolved, the subject is prescribed a different anticoagulant or the physician or parents decide to remove the child off of the study. The drug will also be discontinued if any excessive bleeding or severe side effects related to the drug are observed. A follow up imaging study will be performed on all patients 48-72 hours after the drug was started. If the clot is still present at this time, another imaging study will be done at 30 days after the drug was given. The subjects will continue to have lab tests twice weekly for about 3 weeks to monitor for the effects of the drug. After completion of the study, the patient will undergo a study exit physical exam between 20-40 days after the drug was stopped.