Alterations in the gastrointestinal (GI) tract anatomy (pH, transit time, and absorptive surface), food intake, body weight, and adipose tissue after bariatric surgery can significantly affect the absorption and pharmacokinetics of oral anticoagulant medications. There is little published evidence for dose changes associated with anticoagulant medications after bariatric surgery. Warfarin can be particularly challenging, as vitamin K levels and composition of the gut microbiota affect serum levels, both of which significantly change after bariatric surgery. Furthermore, absorption mainly occurs in the distal stomach and proximal small intestine, which would be unavailable after GI bypass procedures. The aim of this study was to identify trends in warfarin dosing after bariatric surgery.
All patients chronically on warfarin anticoagulation prior to Roux-en-Y gastric bypass (RYGB) or sleeve gastrectomy (SG) were retrospectively identified. Indications for anticoagulation, history of bleeding or thrombotic events, perioperative complications, and warfarin dosing data were collected.
A total of 53 patients (RYGB n=31, SG n=22) on chronic warfarin therapy were identified (56.6% female, 54.4±11.7 years of age). There were 34.0% with prior venous thromboembolic event, 43.4% with paroxysmal or permanent atrial fibrillation, and 5.7% with mechanical cardiac valves. Warfarin was stopped an average of 5.7±1.4 days prior to operation. Three patients had intraoperative bleeding events and 6 required red blood cell transfusions post-operatively. The average daily dose of warfarin preoperatively was 7.2±2.4mg, and decreased by an average of 1.9±2.1mg per day at one month (26% reduction), 1.3±1.8mg at 6 months (18% reduction), 0.95±1.6mg at 12 months (13% reduction), and 0.66±1.9mg at 36 months (7% reduction). At 12 and 36 months, 64% and 66% of patients, respectively, remained at mean daily warfarin dose less than their preoperative level. In the first postoperative month, there was a trend toward a greater reduction in average daily dose following RYGB compared to SG (2.07 mg [28%] vs. 1.62 mg [24%] reduction); however at 12 months, the RYGB had a smaller overall change from preoperative dose levels (0.85 mg [11%] vs. 1.09mg [16%] reduction.
In the immediate postoperative period, warfarin should be resumed at approximately 25% less than preoperative dosage. Lower dose requirement within the first month after bariatric surgery is followed by a trend toward increased warfarin dose requirements. Since dose requirements change constantly over time, frequent INR monitoring is recommended post operatively. The reduction in dose may be dependent on the bariatric surgical procedure performed.