The efficacy and safety of oral (up to 400 mg in 3 h) and intravenous regimens (up to 150 mg in 10 min) of flecainide acetate were compared in the acute conversion of atrial fibrillation to sinus rhythm. Acute conversion was defined as conversion occurring within 5 h (oral) or within 30 min (intravenous regimen). Following classification in recent onset (duration less than 24 h) atrial fibrillation (n = 27) and chronic (greater than 24 h) atrial fibrillation (n = 13), patients were randomly assigned to one of the two regimens. In the group of patients with recent onset atrial fibrillation, 10 out of 14 (oral treatment) and 10 out of 13 (intravenous treatment) responded acutely. Approximately half of responding patients converted after the first oral dose or within the infusion time. In contrast, no patient with chronic atrial fibrillation showed conversion on flecainide. No serious adverse effects were encountered with the regimens used, not even in patients concomitantly using digitalis or verapamil. Thus, patients with recent onset atrial fibrillation can safely be converted to sinus rhythm using oral or intravenous regimens of flecainide.
Flecainide is well absorbed after oral administration. Following rapid infusion of flecainide, there is a short distribution phase. Plasma levels of flecainide are proportional to dose. Both QRS durations and flutter wave intervals are lengthened after flecainide administration. We report an abnormal pharmacokinetic response to oral as well as intravenous flecainide in a patient who was treated with flecainide for several episodes of atrial flutter. In contrast to the usual pattern, this patient did not show a significant increase in QRS duration, flutter wave interval, or plasma concentration. We could not explain this abnormal pharmacokinetic response, but excluded a rapid and complete renal clearance, an excessively high rate of metabolization, and an abnormal binding mechanism as possible mechanisms. We concluded that flecainide may produce abnormal pharmacokinetic mechanisms, preventing appearance of unbound substance in plasma, thereby preventing occurrence of electrophysiological effects.
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