Significant Bleeding in Elderly Patient with Atrial Fibrillation Due to Probable Interaction between Dabigatran and Amiodarone

Objective: To present the case of an elderly patient who developed significant bleeding while receiving a standard dose of dabigatran with concomitant amiodarone therapy. Case Summary: A 77-year-old man was admitted for upper gastrointestinal bleeding that presented as coffee ground emesis. A coronary artery bypass graft had been performed 4 weeks earlier, which was well tolerated, except that his postoperative course was complicated by atrial fibrillation, for which amiodarone was initiated; dabigatran was added subsequently. Upon his admission to the hospital, dabigatran was stopped. The patient's bleeding was stabilized and dabigatran 150 mg twice daily was restarted after amiodarone was discontinued. The laboratory was not equipped to measure thrombin clotting time or ecarin clotting time; therefore, the team relied on the measurement of international normalized ratio (INR) and activated partial thromboplastin time to guide therapy based on pharmacokinetic and pharmacodynamic studies of therapeutic dabigatran concentrations. Based on laboratory values the following morning, the team maintained dabigatran 150 mg once daily. Subsequent INR and partial thromboplastin time values were within the desired range. He remained stable, without further evidence of bleeding even after anticoagulation with dabigatran was restarted for atrial fibrillation. Discussion: Dabigatran received FDA approval in October 2010 to reduce the risk of stroke and systemic embolization in patients with nonvalvular atrial fibrillation. It has a favorable monitoring profile compared to warfarin. The manufacturer acknowledges the potential for amiodarone to increase the effective concentration of dabigatran because of its inhibition of P-glycoprotein. An objective causality assessment using the Horn Drug Interaction Probability Scale revealed that the interaction was probable. Conclusions: A probable interaction suggests that providers should use caution with the coadministration of amiodarone with dabigatran; dose adjustment in the setting of concomitant administration in elderly patients with atrial fibrillation should be considered.