2.6 Histological examination
The left atrium was fixed in 4% paraformaldehyde, embedded in paraffin, cut into 2 μm-thick sections, and stained with Masson’s trichrome. Images were obtained using an optical microscope (BZX710; Keyence). The ratio of atrial fibrosis distribution was calculated by dividing the total area of fibrosis (defined as the amount of collagen deposition stained with aniline blue) by the total tissue area.