This pilot study is designed to determine the feasibility of conducting a randomized clinical trial comparing fondaparinux sodium (Arixtra) once daily with enoxaparin (Lovenox®) twice daily with respect to preventing deep vein thrombosis (DVT) and pulmonary embolism (PE) after bariatric surgery in obese patients.

In order to determine the feasibility of such a trial we need to determine the necessary sample size. In order to determine the necessary sample size, we need to obtain estimates of the incidence of DVT and the prevalence of PE using MRV in each treatment group. Descriptive statistics and 95% confidence intervals will be calculated to determine the incidence of DVT and the prevalence of PE using MRV in obese patients who have received treatment to prevent DVT and PE associated with bariatric surgery.

Anti-factor Xa levels will be used to help determine if treatments are reaching appropriate levels.

Inclusion Criteria:

1. Men or women 18 years or older.
2. Body mass index (BMI) of 35kg/m2 or greater.
3. Undergoing laparoscopic bariatric surgery, i.e., laparoscopic sleeve gastrectomy, laparoscopic Roux-en Y gastric bypass or laparoscopic duodenal switch.

Exclusion Criteria:

1. History of previous deep vein thrombosis.
2. History of previous pulmonary emboli.
3. History of documented clotting/coagulation disorder.
4. History of cancer.
5. Weight < 50 kg or > 200 kg or unable to fit in MRI scanner.
6. Presence of metallic foreign bodies
7. Recent history of smoking (within the last year).
8. History of venous stasis disease.
9. History of obesity hypoventilation syndrome.
10. Patients who are unable to lay flat for extended periods of time or are claustrophobic.
11. Patients who have a pacemaker, an implanted defibrillator or certain other implanted or electronic or metallic devices (implanted medical or metallic devices, shrapnel, or metal).
12. History of hypersensitivity reaction to anticoagulation products.
13. History of HIT (Heparin Induced Thrombocytopenia.
14. History of Renal Insufficiency (Creatinine Clearance < 50).
15. Active clinically significant bleeding.
16. Acute bacterial endocarditis.
17. BMI > 60.
18. Patients with metallic foreign body or implant (unable to have an MRV study).