BACKGROUND Atrial fibrillation (AF) may have different clinical features in its early phase.

OBJECTIVE The purpose of this study was to compare the characteristics and clinical outcomes of early-phase AF with later-phase AF using a large multicenter prospective registry (CODE-AF [COmparison study of Drugs for symptom control and complication prEvention of Atrial Fibrillation]).

METHODS Patients enrolled between June 2016 and March 2021 were divided into 2 groups based on AF duration: (1) newly diagnosed (AF duration ≤90 days); and (2) previously diagnosed (AF duration >90 days). Baseline characteristics and clinical outcomes were compared.

RESULTS Among the 10,001 study participants (mean age 67.0 ± 14.5 years; 64% men), 22% were defined as newly diagnosed and 78% as previously diagnosed. Newly diagnosed patients had fewer comorbidities and more unhealthy social behaviors. Despite lower prescription rates of oral anticoagulant, direct oral anticoagulants were more frequently used. The newly diagnosed group also had a higher composite clinical outcome risk within 90 days (adjusted hazard ratio 1.81, 95% confidence interval 1.30-2.53, P <.001) and revealed a higher risk of all bleeding and heart failure admission within 90 days. No significant differences remained between the groups over 36-month follow-up.

CONCLUSION Patients with early-stage AF were younger and had fewer comorbidities. Although there was a higher risk of heart failure admissions and minor bleeding, the risks of death, stroke, and major bleeding were not significantly increased. Structured monitoring and management during the initial months are essential to address these risks.