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Introduction: Oral and intravenous (IV) flecainide are recommended as first line therapy for pharmacological cardioversion of recent-onset atrial fibrillation (AF) in patients without known relevant structural heart disease. In the present open-label, dose-escalation study, the feasibility of using flecainide acetate inhalation solution (FlecIH) for acute conversion of recent-onset AF to sinus rhythm (SR) was evaluated. Hypothesis: We hypothesized that FlecIH quickly gives rise to plasma concentrations sufficient to rapidly restore SR in patients with recent-onset AF. Methods: Patients (n=95) with symptomatic AF (for ≤ 48 hours) were enrolled and self-administered FlecIH using a breath-actuated nebulizer (30 mg [n=10], 60 mg [n=20], 90 mg [n=21], 120 mg [n=17], and 120 mg in a formulation containing saccharin [n=27]). Blood samples were collected for flecainide plasma concentrations, electrocardiograms were obtained, cardiac rhythm with a 4-hour Holter and vital signs were monitored, and adverse events (AEs) were recorded. Patients who did not convert to SR were offered alternative treatment per the investigator’s discretion. Results: Conversion rates increased with dose and maximum plasma concentrations (C max ) of flecainide. At the highest dose, 45% of patients converted to normal SR. Patients with C max > 300 ng/mL had a conversion rate of 53% whereas those with C max < 200 ng/mL had a conversion rate of 33%. The median time to conversion was 3.5 min after FlecIH administration. AEs were typically mild and transient. Commonly reported AEs associated with inhalation of flecainide included: cough, throat pain, and throat irritation; at the highest dose with the formulation containing saccharin, these AEs were reported for 37%, 11%, and 4% of patients, respectively. Cardiac AEs consistent with those observed with oral and IV flecainide and considered serious were uncommon and included 2 post-conversion pauses and 1 bradycardia, and 1 atrial flutter with 1:1 atrioventricular conduction; none required treatment and all resolved without sequelae. Conclusions: FlecIH was well tolerated. Inhalation of FlecIH at the 120 mg dose yielded therapeutic plasma levels and conversion rates within the range reported for oral and IV administration.