| Summary: | <h4>Objectives</h4>Our aim was to assess the association between use of hormone replacement therapy (HRT) and risk of new-onset atrial fibrillation (AF) after myocardial infarction.<h4>Design, setting and participants</h4>We used Danish nationwide registers of hospitalizations and prescriptions to identify all women admitted with myocardial infarction in the period 1997 to 2009 and with no known diagnosis of AF. Their use of overall HRT and HRT categories was assessed. Multivariable Cox proportional hazards analysis was used to calculate the risk of new-onset AF first year after discharge, comparing use of HRT to no use.<h4>Main outcome measures</h4>New-onset atrial fibrillation.<h4>Results</h4>In the period 1997 to 2009, 32 925 women were discharged alive after MI. In the first year after MI, new-onset AF was diagnosed in 1381 women (4.2%). Unadjusted incidence rates of AF decreased with use of HRT (incidence rate 37.4 for use of overall HRT and 53.7 for no use). Overall HRT was associated with a decreased risk of AF (HR 0.82, 95% confidence interval [CI] 0.68-1.00). The lowest risk of AF was found in women ≥80 years old for use of overall HRT and vaginal estrogen (HR 0.63, CI 0.42-0.94, and HR 0.58, CI 0.34-0.99, respectively). Decreased risk of AF with use of overall HRT and HRT categories was also found in other age groups.<h4>Conclusions</h4>Use of HRT is associated with a decreased risk of new-onset AF in women with myocardial infarction first year after discharge. The underlying mechanisms remain to be determined. Unmeasured confounding might be one of them.
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