Approximately two thirds of the adult population in developed countries is categorized as over-weight or obese (BMI>30). In spite of worldwide increasing awareness, obesity is a major health concern. In the presence of numerous diets, medical therapies, and robust research, bariatric surgery remains the most effective means of weight reduction in morbidly obese patients (BMI>40, or BMI>35 with co-morbidities). However, bariatric surgery harbors a relatively high risk for postoperative complications; of them, venous thromboembolic events (VTE) are not common, but potentially lethal. Taken together with the propensity of morbidly obese patients to develop VTE, perioperative thromboprophylaxis is mandatory.

To date, low molecular weight heparins (LMWH) are most commonly used for VTE prophylaxis in the aforementioned population. Due to the pharmacologic properties of LMWH and the characteristics of surgically treated obese patients, the optimal dose that is to be utilized for VTE prophylaxis in this population remains unclear. Assessment of anti-FXa levels in the patients' plasma can be used in order to monitor LMWH activity. Levels of 0.2-0.5 U/ml have been proposed by some authors for VTE prophylaxis.

Few studies have compared different dosing regimens of enoxaparin (between 30mg-60mg q/12h) for VTE prophylaxis in the population undergoing bariatric surgery; nevertheless, these were small non- randomized trials, containing numerous methodological weaknesses. Hence, the optimal regimen of enoxaparin to be used for the prevention of VTE in the discussed population remains unclear.

The aim of the present study is to evaluate plasma levels of anti-FXa activity, comparing two most commonly used enoxaparin prophylactic regimens (40mg vs 60mg q/24h) in a large and homogenous cohort of sleeve gastrectomy patients. Although universally used by bariatric surgeons, the pharmacologic efficacy of these regimens has not been evaluated in patients undergoing bariatric surgery.

The following steps shall be performed in the process of recruiting a patient for the study:

Preoperatively:

• A baseline blood sample for Anti-FXa levels shall be obtained.
• Randomization of the patient to one of the two treatment groups

Postoperatively:

• All patients will be treated with Sequential Compression Devices, massive I.V hydration and encouraged to ambulate early.
• Patients will receive three injections of enoxaparin, according to the treatment group they were assigned to, The first injection given on the morning after surgery. Four hours after the third injection of enoxaparin a blood sample will be taken from the patients and assessed for anti-FXa levels.

A control group of 5 patients, randomly elected, will be recruited. This group will not be treated with enoxaparin. These patients will be treated with sequential compression devices and massive I.V hydration, an accepted treatment, proven effective for the studied population. The goal is to assess the anti-FXa levels in these patients, representing the baseline levels in patients undergoing laparoscopic sleeve gastrectomy.

Furthermore, the preoperative, operative and postoperative management of these patients will be identical to that of the treatment groups and on the third day post-operation a blood sample from these patients will be retrieved and assessed for anti-FXa levels.

Inclusion Criteria:

• Adult patient undergoing laparoscopic sleeve gastrectomy.
• The patient undergoes the surgery in the surgical wing of the Tel-Aviv Sourasky Medical Center.
• The patient has received full information regarding the studies nature, has agreed to participate and has given informed consent (documented by a signed informed consent form).

Exclusion Criteria:

• Patients with a previous Venous Thromboembolic Event.
• Patients requiring an IVC filter.
• Patients with known thrombophilia due to coagulation factor disorders (i.e factor V leiden).
• Patients with a bleeding disorder
• Patients with renal failure.