The most common causes are age, high blood pressure, and diabetes, and heart failure and coronary artery disease. In this issue of the Korean Circulation Journal, Kim et al,6) Figure 2 of this study shows how Hb levels influence the occurrence of AF. Similarly, the higher the body mass index (BMI), the higher the incidence of cardiovascular disease (CVD). And the U-shape pattern that the incidence of CVD increases even in patients with a relatively low BMI is also similarly applied in this study.7) There is a difference in cut off value for men and women,8) but this is thought to be due to a physiological difference depending on gender. And it shows a U shape pattern in common. In the case of anemia, it varies according to the cause of the disease, but the more severe it is, the higher the incidence of CVDs including AF can be expected. However, it is interesting to note that in the case of polycythemia, the incidence of AF increases as the level of Hb increases. The mechanism of the disease has not been revealed, but it is generally thought that the increased viscosity of the blood, shear stress, hypoxia and inflammation, etc. affect the incidence of AF.2)3)7)9)10) Since this study was analyzed through retrospective claim data, there is a limitation in that it cannot take into account both the patient's drug prescription and the included confounding bias. Therefore, it seems that the basic animal and cell level experiments should be conducted along with prospective studies for each factor.

And with the occurrence of AF, the part that we should be interested in is the occurrence of stroke. The reason why the occurrence of AF is important is the effect of subsequent strokes. It is well known that the incidence of ischemic events is high in patients with polycythemia, and anti-platelets have been used to prevent this. However, if AF occurs in patients with Hb more than the cut off value shown in this study, it is not known whether the risk of stroke is increased compared to patients who do not. This is because all patients are already using anticoagulation or antiplatelet according to the CHA₂DS₂-VASc score. Therefore, it is believed that a more systematic study based on prospective data is necessary.

This study is meaningful in predicting the risk of AF through Hb data that can be easily obtained in clinical practice. However, in patients taking anticoagulation, changes in Hb cannot have a significant meaning with only one test, and changes often occur according to the occurrence of events such as bleeding. Therefore, it is considered that a prospective study is also needed to determine how changes of Hb level according to sequential measurements affect the occurrence of AF and stroke.