Objective   To evaluate the electrocardiographic (ECG) characteristics before and after transcatheter closure of atrial septal defect (ASD) in patients over 40 years of age.     Methods   Each of 305 ASD patients underwent 12-lead ECG once at 24 h before and after closure procedure. Nineteen patients with cardiac arrhythmia confirmed preoperatively underwent 24-hour ambulatory ECG monitoring. ECG features were observed and analyzed.     Results   The QRS axis declined from (79.1±36.1) degrees to (76.4±36.8) degrees (P0.05). The incidence of normal ECG increased and right bundle branch block significantly decreased after transcatheter closure(P0.01). The number of the right heart dilatation decreased but no difference was found (P0.05). The incidence of sinus bradycardia increased significantly (P0.01) and was related to age

diameter of defect

and size of closure (P0.05).  Seven patients developed atrial fibrillation. Significant difference was found in age

diameter of defect

and size of closure between patients with atrial fibrillation and those without atrial fibrillation (P0.01). There was no significant change in other patients with arrhythmia (P0.05).    Conclusion   Volume load of the right ventricle is decreased after transcatheter closure of atrial septal defect in patients over middle age. New arrhythmia can form while some persistent arrhythmia may remain unchanged within short period of time after transcatheter closure.