# Flecainide Toxicity From Clinical Pharmacology Perspectives

## TL;DR

An 84-year-old woman experienced flecainide toxicity due to a significantly longer drug half-life than expected, highlighting the importance of monitoring drug levels.

## Contribution

The study demonstrates a clinically significant discrepancy between labeled and actual flecainide half-life in an elderly patient.

## Key findings

- The patient's flecainide half-life was 56.8 hours, five times longer than the labeled 11.0 hours.
- Toxicity symptoms resolved after discontinuation and normalization of drug levels.
- Therapeutic drug monitoring is critical for safe antiarrhythmic drug use in vulnerable patients.

## Abstract

We report a case comparing the measured half-life of flecainide with the half-life stated on the label. An 84-year-old woman presented with symptoms of anorexia and exertional dyspnea. She had undergone mitral and aortic valve replacements and excision of the membranous septum in the atrium for mitral and aortic stenosis and cor triatriatum. She was regularly administered 100 mg/day flecainide for paroxysmal atrial fibrillation. A previous electrocardiogram (ECG) showed a regular sinus rhythm. However, upon admission, the ECG revealed a heart rate of 94 bpm and an accelerated idioventricular rhythm originating from the left ventricle. Flecainide toxicity was suspected, leading to the discontinuation of flecainide treatment. The following day, the serum flecainide concentration was 1,348 ng/mL, exceeding the therapeutic window of 200-1,000 ng/mL. After discontinuing flecainide, the accelerated idioventricular rhythm ceased, and a regular sinus rhythm temporarily returned. We measured blood drug concentrations several times; our calculated half-life was 56.8 h, approximately five times longer than the half-life of 11.0 h stated on the package insert. To ensure safe and effective therapy with antiarrhythmic drugs, prioritizing therapeutic drug monitoring and carefully monitoring pharmacokinetics is important, particularly during the elimination phase.