2.1. Patients and study protocol
We retrospectively studied 268 consecutive patients who underwent a primary mitral valve repair for treating MR from January 2008 to December 2017 in Osaka City General Hospital, Osaka, Japan. We excluded patients: 1) who were treated without mitral annuloplasty (n = 10), 2) who underwent intraoperative conversion to mitral valve replacement (n = 3) and 3) who had a history of paroxysmal or chronic AF/AT (n = 83). Consequently, we enrolled 172 patients as a final cohort in this retrospective study. The clinical, echocardiographic and follow-up data were collected from the medical records and the echocardiographic database. The end-diastolic aorto-mitral angle was additionally measured from the digital echocardiographic images at the pre-operative and post-operative periods. Patients were generally followed up at our outpatient clinics 3, 6 and 12 months after their surgeries and yearly thereafter. The post-operative AA event was defined as paroxysmal or persistent AF or AT that occurred after the hospital discharge, irrespective of the presence or absence of symptoms. AA events that occurred before discharge were not considered post-operative AA events. The protocol of this retrospective study was approved by the institutional review board of the Osaka City General Hospital, with a waiver of individual consent.