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Use of a Modified Dosing Weight for Heparin Therapy in a Morbidly Obese Patient

Clinical Pharmacy Specialist in Primary Care, Kaiser Permanente, Englewood, CO

Andrea M Wessell, PharmD BCPS CDE

Assistant Professor, Departments of Pharmacy and Clinical Sciences and Family Medicine, Colleges of Pharmacy and Medicine, Medical University of South Carolina, Charleston, SC

Terrence E Steyer, MD

Assistant Professor, Department of Family Medicine, College of Medicine, Medical University of South Carolina

Reprints: Dr. Schwiesow, Kaiser Permanente, Englewood Medical Office, 2955 S. Broadway, Englewood, CO 80110-1526, fax 303/788-1011, sarah.j.schwiesow{at}kp.org

OBJECTIVE: To report a case of successful anticoagulation using a modified dosing weight (DW) for unfractionated heparin (UFH) therapy in a morbidly obese female.

CASE SUMMARY: A 54-year-old morbidly obese (182.4 kg, 155 cm) white female presented to the emergency department with tachycardia, shortness of breath, and chest pain, and was diagnosed with a pulmonary embolism. Anticoagulation with UFH was initiated. A modified DW of 120 kg was obtained from the average of the actual body weight (ABW) and ideal body weight (50 kg). We selected this modified DW to account for heparin's altered volume of distribution in an obese patient and avoid potentially supratherapeutic activated partial thromboplastin times (aPTTs) using ABW and subtherapeutic aPTTs using DW. Therapy was initiated with a bolus dose of 9600 units (80 units/kg x 120 kg) and continuous infusion rate of 2160 units/h (18 units/kg/h x 120 kg). This infusion rate was maintained throughout the course of heparin therapy and was successful in maintaining therapeutic aPTTs.

DISCUSSION: Proper diagnosis and rapid initiation of therapy prevent mortality in patients with PE. Although weight-based heparin nomograms provide standardization through initial bolus and continuous infusion recommendations, many do not address dosing in morbidly obese patients. Several retrospective studies have evaluated actual, dosing, and ideal body weights for heparin therapy in obese patients; however, none has evaluated modified DW. In our patient, successful anticoagulation was objectively confirmed.

CONCLUSIONS: Further investigation is necessary to determine the optimal DW for UFH in morbidly obese patients presenting with acute thrombosis.

Key Words: anticoagulation, heparin, morbid obesity, pulmonary embolism

Published Online, March 1, 2005. www.theannals.com, DOI 10.1345/aph.1E300