Assessment of female sex as a risk factor in atrial fibrillation in Sweden: nationwide retrospective cohort study

OBJECTIVE To determine whether women with atrial fibrillation have a higher risk of stroke than men. DESIGN Nationwide retrospective cohort study. SETTING Patients with a diagnosis of atrial fibrillation in the Swedish hospital discharge register between 1 July 2005 and 31 December 2008. Information about drug treatment taken from the Swedish drug register. PARTICIPANTS 100,802 patients with atrial fibrillation at any Swedish hospital or hospital affiliated outpatient clinic with a total follow-up of 139,504 years at risk (median 1.2 years). We excluded patients with warfarin at baseline, mitral stenosis, previous valvular surgery, or who died within 14 days from baseline. MAIN OUTCOME MEASURE Incidence of ischaemic stroke. RESULTS Ischaemic strokes were more common in women than in men (6.2% v 4.2% per year, P<0.0001). The univariable hazard ratio for women compared with men was 1.47 (95% confidence 1.40 to 1.54), indicating a 47% higher incidence of ischaemic stroke in women than in men. Stratification according to the CHADS(2) scheme showed increased stroke rates for women in all strata. After multivariable adjustment for 35 cofactors for stroke, an increased risk of stroke in women remained (1.18, 1.12 to 1.24). Among patients with "lone atrial fibrillation" (age <65 years and no vascular disease), the annual stroke rate tended to be higher in women than in men, although this difference was not significant (0.7% v 0.5%, P=0.09). When low risk patients with CHADS(2) scores of 0-1 were stratified according to their CHA(2)DS(2)-VASc scores, women did not have higher stroke incidence than men at CHA(2)DS(2)-VASc scores of 2 or less. CONCLUSION Women with atrial fibrillation have a moderately increased risk of stroke compared with men, and thus, female sex should be considered when making decisions about anticoagulation treatment. However, women younger than 65 years and without other risk factors have a low risk for stroke, and do not need anticoagulant treatment.