Study Confirms Warfarin Management Strategies After Gastric Bypass Surgery
October 05, 2015
April Smith, Pharm.D., BCPS
As the obesity epidemic in the United States continues to worsen, the number of bariatric surgical procedures is climbing. It is therefore feasible that many practitioners at some point will participate in the care of a bariatric surgery patient. For several years, the laparoscopic Roux-en-Y gastric bypass (LRYGB) has been the most common bariatric surgery, and it is the procedure thought to have the most effect on the disposition of drugs postoperatively. However, little is known about the implications of these surgical procedures on the pharmacokinetic disposition of drugs, especially narrow therapeutic index drugs such as warfarin. The proportion of bariatric surgery patients on chronic warfarin therapy represents a very small percentage of patients, but the consequences of postoperative coagulopathy can be grave. Because no consensus exists on how to handle warfarin dosing post-bariatric surgery, most patients are discharged on the same regimen they took preoperatively, and follow-up is left to the discretion of the originally prescribing physicians, who may be unfamiliar with the potential pharmacokinetic challenges post-bariatric surgery. Few studies have examined the impact of bariatric surgery on warfarin dosing requirements, international normalized ratios (INRs), and coagulopathy-induced postoperative sequelae.
The study by Steffen et al. in the latest issue of Pharmacotherapy1 is consistent with the findings of both Schullo-Feulner and colleagues2 and Irwin and colleagues.3 Warfarin dose reductions of around 20%–35% were necessary in these LRYGB cohorts in the first several weeks after surgery and returned to almost preoperative requirements by 6 months postoperation. Bechtel and colleagues4 reported increased readmission rates for hemorrhage within the first 30 postoperative days in warfarin users (15.8%) compared with non-warfarin users (1.2%). The Steffen et al. study helps confirm the need for vigilant monitoring of patients taking warfarin in the first several weeks after LRYGB and provides guidance on the level of dose reduction that can be anticipated. Weekly monitoring of INR and subsequent dose adjustment of warfarin are particularly critical within the first 4–6 weeks postoperation and can likely return to monthly monitoring thereafter. Future studies should aim to include laparoscopic sleeve gastrectomy (LSG) patients because this procedure represents a significant proportion of today’s bariatric surgery patients (equal to LRYGB). A study similar to that of Steffen et al. that includes both LSG and LRYGB patients would make a significant contribution to the literature to determine the potential mechanisms of altered drug pharmacokinetics by procedure type.
Steffen and colleagues discuss several potential explanations for postoperative warfarin sensitivity, including the dramatic dietary reduction in vitamin K–containing foods in the typical postoperative bariatric surgery diet, particularly within the first 30 days and out to 6 months. Because vitamin K is fat soluble, drastic weight loss within the first 6 months after surgery may reduce its storage. Changes in gut microbiota after LRYGB may also affect vitamin K production. Altered absorption or metabolism of warfarin is plausible as well, but less likely given that more than one study, including this one, has shown that after the initial significant dose reduction requirement postoperation, weekly doses of warfarin return to 80%–100% of baseline by 6 months.1-4
Read the full article in Pharmacotherapy.1
References:
- Steffen KJ, Wonderlich JA, Erickson AL, Strawsell H, Mitchell JE, Crosby RD. Comparison of warfarin dosages and international normalized ratios before and after Roux-en-Y gastric bypass surgery. Pharmacotherapy 2015;35:876–80.
- Schullo-Feulner AM, Stoecker Z, Brown GA, Schneider J, Jones TA, Burnett B. Warfarin dosing after bariatric surgery: a retrospective study of 10 patients previously stable on chronic warfarin therapy. Clin Obes 2014;4:108–15.
- Irwin AN, McCool KH, Delate T, Witt DM. Assessment of warfarin dosing requirements after bariatric surgery in patients requiring long-term warfarin therapy. Pharmacotherapy 2013;33:1175–83.
- Bechtel P, Boorse R, Rovito P, Harrison TD, Hong J. Warfarin users prone to coagulopathy in first 30 days after hospital discharge from gastric bypass. Obes Surg 2013;23:1515–9.
Dr. Smith in an Assistant Professor of Pharmacy Practice at Creighton University and has been the clinical pharmacist on the multidisciplinary bariatric surgery team at CHI Immanuel Medical Center since 2009. She is an active member of the American Society of Metabolic and Bariatric Surgery (ASMBS) and ACCP. Dr. Smith has published and presented on chronic disease state and comprehensive medication management in post-bariatric surgical patients as an invited speaker at ASMBS national meetings for the past four years, as well as at the ACCP Annual Meeting and multiple regional surgical symposia. She currently serves on the ASMBS Integrated Health Clinical Issues and Guidelines Committee and the ASMBS Membership Committee.
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