Warfarin treatment and risk of myocardial infarction — A cohort study of patients with atrial fibrillation treated in primary health care

Objective To study the risk of myocardial infarction (MI) in patients with atrial fibrillation (AF) treated in primary health care with warfarin or acetylsalicylic acid (ASA, aspirin). Methods The study population included subjects (n = 12,283) 45 years or older diagnosed with AF who were treated in 75 primary care centres in Sweden between 2001 and 2007. MI was defined as a hospital stay for MI during 2001 through 2010 registered in the Swedish Patient Register. Associations between warfarin or ASA treatment and incident MI were explored using Cox regression analysis, by estimating hazard ratios (HRs) and 95% confidence intervals (95% CIs). Adjustment was made for age, socio-economic factors and cardio-vascular co-morbidity. Results Persistent treatment (“per protocol” treatment) with warfarin alone was present among 32.4% of women and 37.4% of men, and with ASA alone among 30.0% of women and 28.1% of men. The fully adjusted HRs for MI, compared to those with no antithrombotic treatment, with warfarin treatment for women were 0.26 (95% CI 0.16–0.41) and for men 0.28 (95% CI 0.20–0.39); and the corresponding HRs for those treated with ASA were for women 0.57 (95% CI 0.37–0.87), and for men 0.44 95% CI (0.31–0.63). The fully adjusted HR for MI when comparing patients with warfarin treatment to those with ASA treatment was for women 0.46 (95% CI 0.27–0.80), and for men 0.58 (95% CI 0.38–0.89). Conclusions Warfarin seems to prevent MI among AF patients in a primary healthcare setting, which emphasizes the importance of persistent anticoagulant treatment in those patients