Introduction:
The study aimed to compare the characteristics, treatment, and 12-month outcomes of patients after myocardial infarction (MI) and with left ventricular ejection fraction (LVEF) < 40%, with or without atrial fibrillation (AF).

Material and methods:
The analysis involved 10,222 surviving patients who were enrolled in the ongoing, prospective Polish Registry of Acute Coronary Syndromes (PL-ACS). The major adverse cardiac events (MACE) involved death, non-fatal MI, rehospitalization due to heart failure (HF), and stroke within 12 months after MI.

Results:
Patients with AF were older (73.7 [13.1] vs. 68.3 [16.4], p < 0.001) and had a greater prevalence of previous MI (37.3% vs. 32.4%, p = 0.0007), percutaneous coronary intervention (PCI) (27.5% vs. 23.6%, p = 0.002), and HF hospitalization (34.6% vs. 19.7%, p < 0.001). In patients with AF, coronary angiography (87.3% vs. 92.5%, p < 0.001) and PCI (76.8% vs. 82.7%, p < 0.001) were performed less frequently. Patients with AF had a higher rate of all-cause mortality (26.8% vs. 17.4%, p < 0.001), HF hospitalization (35.0% vs. 25.3%, p < 0.001), stroke (5.5% vs. 2.1%, p < 0.001) and MACE (56.2% vs. 42.8%, p < 0.001). In the multivariate analysis, AF was revealed to be associated with a higher all-cause mortality (odds ratio 1.20, 95% confidence interval 1.03–1.40, p = 0.018).

Conclusions:
Patients after MI with LVEF < 40% and with AF, compared to those without AF, had worse clinical characteristics, were less frequently subjected to coronary angiography and PCI during hospitalization, and had significantly less favorable 12-month outcomes.