Atrial fibrillation (AF) is a common complication following myocardial infarction (MI), yet the prognostic implications may vary depending on when AF occurs. This retrospective cohort study published in the American Journal of Cardiology analyzed 3,390 patients with MI, stratified into non–ST-segment elevation MI (NSTEMI) and ST-segment elevation MI (STEMI), to assess the impact of AF timing on long-term outcomes. 

AF was categorized as early (during index hospitalization), late (after discharge), or absent. Clinical, echocardiographic, and angiographic variables were collected, and outcomes were evaluated using Kaplan–Meier methods and multivariable time-dependent Cox proportional hazards models.

AF occurred in 451 patients (13.3%), including 249 with early AF and 202 with late AF. Kaplan–Meier analyses demonstrated significant differences in long-term major adverse cardiovascular events (MACE), a composite of recurrent MI, repeat revascularization, stroke, and all-cause mortality, according to AF timing (p<0.001), with late AF showing the highest event rates. In adjusted analyses, late AF was independently associated with increased MACE risk in both NSTEMI (hazard ratio [HR] 1.5; 95% CI 1.0–2.2; p=0.044) and STEMI (HR 2.4; 95% CI 1.1–5.3; p=0.032). 

Early AF did not show a consistent independent association with long-term MACE after adjustment. Pairwise comparisons confirmed higher risk with late AF compared with no AF, while differences between late and early AF were not statistically significant in STEMI. Older age and diabetes mellitus were identified as independent predictors of late AF.

These findings indicate that AF occurring after discharge is associated with poorer long-term outcomes following MI. Temporal classification of AF may support risk stratification, although further validation is required.