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Published on: February 26, 2013
Characteristics of the amiodarone-warfarin interaction during long-term follow-up
Yun Lu1, Katie A Won, Brenda J Nelson
1Department of Pharmacy, Hennepin County Medical Center (HCMC), Minneapolis, MN 55415, USA. yun.lu@hcmed.org
Insights
The amiodarone-warfarin interaction frequently leads to elevated International Normalized Ratios (INRs) above 5, particularly within the first 12 weeks of combined therapy. This necessitates careful monitoring and warfarin dosage adjustments in patients receiving both medications.
Area of Science:
- Pharmacology
- Clinical Pharmacy
- Cardiology
Background:
- Amiodarone is a potent antiarrhythmic agent often prescribed for long-term management of cardiac arrhythmias.
- Warfarin is a widely used oral anticoagulant requiring careful monitoring to maintain therapeutic efficacy and minimize bleeding risk.
- The potential for drug-drug interactions, particularly with amiodarone, necessitates thorough investigation to ensure patient safety.
Purpose of the Study:
- To characterize the amiodarone-warfarin interaction during long-term anticoagulation therapy.
- To identify the incidence and timing of International Normalized Ratios (INRs) exceeding 5 in patients on combined amiodarone and warfarin.
- To assess the frequency of warfarin dosage adjustments required due to this interaction.
Main Methods:
- Retrospective review of medical records from an anticoagulation clinic (April 1998-March 2003).
- Inclusion criteria: patients >18 years, primary anticoagulation clinic use, and combined amiodarone-warfarin therapy for ≥1 month.
- Primary endpoint: occurrence of INRs >5; Secondary endpoint: warfarin dosage change frequency.
Main Results:
- 70 patients met inclusion criteria; 7 initiated amiodarone before warfarin.
- Of 2434 INRs, 43% were therapeutic, 34% below, and 23% above target.
- 4% of INRs were >5, with a relative risk of 1.366 (p=0.005) for concurrent therapy. Elevated INRs were most frequent within the first 12 weeks.
Conclusions:
- Elevated INRs (>5) are common during the initial 12 weeks of combined amiodarone-warfarin therapy.
- Warfarin dosage reduction is frequently required to manage these elevated INRs.
- No significant changes in INR or drug dosages were observed beyond the initial 12-week period.
Purpose:
Characteristics of the amiodarone-warfarin interaction during long-term follow-up were studied.
Methods:
Medical records from patients seen in the anticoagulation clinic at the Hennepin County Medical Center between April 1998 and March 2003 were retrospectively reviewed. Patients were included if they were older than 18 years, used the anticoagulation clinic as their primary clinic for anticoagulation therapy, and were receiving combined amiodarone and warfarin therapy for at least one month. The primary study endpoint was the occurrence of International Normalized Ratios (INRs) of >5 at any time during combined warfarin-amiodarone therapy. The secondary endpoint was the frequency of warfarin dosage changes.
Results:
A total of 70 patients met study inclusion criteria. Of these 70, 7 had amiodarone started before warfarin initiation. Of the 2434 INR values analyzed, 43% (n = 1043) were in the target therapeutic range, 34% (n = 820) were below target range, and 23% (n = 571) were above target range. A total of 102 INR values (4%) were above 5. The relative risk of having an INR of >5 for patients on concurrent warfarin and amiodarone versus those on warfarin alone was 1.366 (p = 0.005). INRs of >5 were most common during the first 12 weeks of combined therapy, with no subsequent large peaks evident.
Conclusion:
Among patients treated in an anticoagulation clinic, INR values of >5 were most common during the first 12 weeks of combined therapy with amiodarone and warfarin and necessitated reduction in warfarin dosage. No other notable changes in INR or amiodarone or warfarin dosage occurred throughout the remainder of the 80-week study period.
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