BACKGROUND: New-onset atrial fibrillation (AF) is reported to increase the risk of death in myocardial infarction (MI) patients. However, previous studies have reported conflicting results and no data exist to explain the underlying cause of higher death rates in these patients.

METHODS AND RESULTS: All patients with first acute MI between 1997 and 2009 in Denmark, without prior AF, were identified from Danish nationwide administrative registers. The impact of new-onset AF on all-cause mortality, cardiovascular death, fatal/nonfatal stroke, fatal/nonfatal re-infarction and noncardiovascular death, were analyzed by multiple time-dependent Cox models and additionally in propensity score matched analysis. In 89 703 patients with an average follow-up of 5.0 ± 3.5 years event rates were higher in patients developing AF (n=10 708) versus those staying in sinus-rhythm (n=78 992): all-cause mortality 173.9 versus 69.4 per 1000 person-years, cardiovascular death 137.2 versus 50.0 per 1000 person-years, fatal/nonfatal stroke 19.6/19.9 versus 6.2/5.6 per 1000 person-years, fatal/nonfatal re-infarction 29.0/60.7 versus 14.2/37.9 per 1000 person-years. In time-dependent multiple Cox analyses, new-onset AF remained predictive of increased all-cause mortality (HR: 1.9 [95% CI: 1.8 to 2.0]), cardiovascular death (HR: 2.1 [2.0 to 2.2]), fatal/nonfatal stroke (HR: 2.3 [2.1 to 2.6]/HR: 2.5 [2.2 to 2.7]), fatal/nonfatal re-infarction (HR: 1.7 [1.6 to 1.8]/HR: 1.8 [1.7 to 1.9]), and non- cardiovascular death (HR: 1.4 [1.3 to 1.5]) all P<0.001). Propensity-score matched analyses yielded nearly identical results (all P<0.001).

CONCLUSIONS: New-onset AF after first-time MI is associated with increased mortality, which is largely explained by more cardiovascular deaths. Focus on the prognostic impact of post-infarct AF is warranted.