The purposes of this study are to assess the prevalence of patients with a low risk of stroke or systemic embolism based on a CHA2DS2-VASc score among patients who have an AF-related thromboembolism and to identify the clinical and laboratory risk factors associated with thromboembolism in AF patients with a CHA2DS2-VA score of 0 or 1.

Both European and American guidelines recommend the use of the CHA2DS2-VASc risk score which has been validated as useful risk stratification for stroke prediction in AF from several independent cohorts. The guidelines have suggested that, in non-valvular AF, OAC could be omitted for males with a CHA2DS2-VASc score of 0 and for females with a score of 1 given the true low risk of ischemic stroke in that population. The American guideline has still suggested anti-platelets therapy could be recommended in patients who only have one additional risk factor for stroke. The risk of ischemic stroke in Asian people, however, has been known to be quite different from that in Western people, especially in low risk (CHA2DS2-VASc score of 0 or 1) patients based on the CHA2DS2-VASc score. In a nationwide study from Taiwan, the annual risk of ischemic stroke was 1.21% in AF patients with a CHA2DS2-VASc score of 0 and 2.16% in AF patients with a CHA2DS2-VASc score of 1, which were much higher than that reported from Western countries. Another study which enrolled 9727 Hong Kong AF patients from a hospitalized cohort, the annual stroke rate was as high as 2.41% among 395 patients with a CHA2DS2-VASc score of 0. Furthermore, a population-based study in an East Asian cohort of 22 million people found an increased risk of stroke in younger patients (i.e., 30-55 years) with AF who are not recommended for prevention of thromboembolism by current guidelines.Thus, stroke risk among Asian patients with AF and a CHA2DS2-VASc score of 0 or 1 might be higher than that seen among Caucasians, and OAC should be recommended in such patients for effective stroke prevention.

In addition, literature review suggests that female sex as an independent risk factor of stroke is still controversial, because some studies demonstrated that the odds ratio or hazard ratio of female to male sex for thromboembolism is not significant. Indeed, the analysis of a J-RHYTHM registry revealed that female sex was not a risk for thromboembolism in the Japanese cohort. Therefore, the novel risk stratification for stroke prevention in AF patients who have CHA2DS2-VASc score of 0 or 1 in men or CHA2DS2-VASc score of 1 or 2 in female should be needed in Asian population in order to define true low risk patients in the low risk population based on the CHA2DS2-VASc scoring system.

Inclusion Criteria (Experimental Group):

• Patients who experienced the stroke or systemic embolism between 2013 and 2016
• Patients who had AF before the occurrence of the stroke or systemic embolism
• Patients whose CHA2DS2-VASc score of 0 or 1 in men or CHA2DS2-VASc score of 1 or 2 in female at the time of the the stroke or systemic embolism
• Willing and able to provide informed consent
• Age greater than or equal to 18 years

Inclusion Criteria (Control Group):

• Patients who did not experience the stroke or systemic embolism between 2013 and 2016
• Patients who had AF
• Patients whose CHA2DS2-VASc score of 0 or 1 in men or CHA2DS2-VASc score of 1 or 2 in female
• Willing and able to provide informed consent
• Age greater than or equal to 18 years

Exclusion Criteria (Both experiment and control groups)

• Patients who do not meet all of the above listed inclusion criteria
• Patients with more than mild mitral valve stenosis or prosthetic mitral valve

Atrial Fibrillation(AF) patients defined as a low risk group based on a CHA2DS2-VASc score