Embolic stroke of undetermined source (ESUS) accounts for approximately 25% of all ischemic strokes, with patent foramen ovale (PFO) considered a major mechanism behind ESUS. When specific anatomic criteria are met, transcatheter PFO occlusion is regarded as the gold standard for secondary stroke prevention. However, previous studies have shown that percutaneous PFO closure may increase the risk of new-onset atrial fibrillation and atrial flutter (AF), arrhythmias that elevate the risk of embolic stroke. This raises concerns about performing PFO closure. Indeed, the most common clinically significant complication after catheter-based PFO closure is AF. Available data indicates that AF typically occurs in the early period following occluder implantation, with peak incidence around the 14th day post-closure, followed by a decline after the 45th day. Although AF is considered the most frequent adverse complication after percutaneous PFO occlusion, its incidence remains relatively low, ranging from 3% to 7.4%, depending on the method used for arrhythmia screening, though this remains unclear. In this study, investigators will prospectively monitor patients who have experienced ESUS and are indicated for PFO closure. They will be screened for the occurrence of AF following percutaneous PFO closure using a 3-week ECG Holter monitor (from day 7 to day 28 post-closure) and again 1 week (one year post-closure) after the procedure. The study aims to determine the true incidence of new-onset AF following catheter-based PFO occlusion and to identify the risk factors associated with its development.