**4.1 Predisposing factors to venous thromboembolism in bariatric surgery**

Morbid (BMI>50) and truncal obesity are identified as major predisposing factors for VTE. Sedentary lifestyle, increased abdominal pressure and excessive weight resting on the inferior vena cava drainage attribute to the increased risk. Additional risk factors include advanced age, history of previous VTE, immobilization, venous insufficiency and stasis, smoking, estrogen- containing oral contraceptives and hormone replacement therapy, hypercoaguable state, hypoventilation syndrome and anastomotic leakage. According to current literature, obesity interferes in intrinsic and extrinsic coagulation pathways, as well as in the anticoagulant mechanism, leading to a hypercoagulating state. Plasma concentration of fibrogen, von Willebrand, t-PA, PAI-1 and factor VII are significantly elevated in obese patients, while platelet aggregation is promoted due to leptin. There is evidence that treatment of morbid obesity can reverse partially some of the above abnormalities, as weight loss is associated with significant reduction in fibrogen, t-PA, PAI-1 and improvement of deficiency of antithrombin III.

Perioperative factors contributing to VTE include extend of surgical trauma, operative duration, length of postoperative immobilization and the use of general versus regional anesthesia. The risk of developing VTE depends on the type of major abdominal surgical procedure. Mukherjee et al. reported lower incidence of VTE among bariatric surgery patients (0.35%), while VTE rates were higher in patients undergoing nephrectomy, hepatectomy, colorectal resection, splenectomy, gastrectomy, pancreatectomy and esophagectomy. This lower rate may reflect strict adherence of bariatric surgeons to VTE prophylaxis guidelines relative to other surgical specialties.

More specifically, in laparoscopic bariatric surgery, reverse Trendelenburg position and pneumoperitoneum are associated with venous stasis of lower extremity and impaired venous return due to the compression of iliac veins and inferior vena cava. Furthermore several studies show the development of a hypercoagulable state during laparoscopy. Conversely, the risk of VTE during laparoscopy could be compensated by lower degree of surgical injury, early mobilization and reduced postoperative acute-phase response. Podnos et al. in a review of 3464 cases of GBP demonstrated that although the difference was not statistically significant, the incidence of PE was lower in laparoscopic group rather than the open group. In absence of randomized controlled studies, the evidence remains inconclusive as to the relative risk of VTE after laparoscopic bariatric surgery.
