Background: In recent Danish nationwide register-based study, adults with small, unrepaired atrial septal defects (ASD) have increased risk of pneumonia, atrial fibrillation (AF) and stroke. Moreover, they revealed higher mortality than the background population.

Objective: In this nationwide study, we evaluate the hidden burden of atrial and ventricular arrhythmias in adult patients with a small, unrepaired ASD without a previous diagnosis of AF.

Methods: All Danish patients, aged 18-65, diagnosed between 1953 and 2011 with an unrepaired ASD and no documented AF were invited for 7 days Holter-recording, echocardiography and 6 min walk test. The first 48 hours Holter-recording was completely analysed, while only AF was screened for throughout all 7 days. Furthermore, the entire patient group were characterised using the unique Danish registries.

Results: A total of 151 patients (mean age 32 years) were included. Approximately 80% of the patients had spontaneous closure of their defect. Despite this, occult arrhythmias were frequent. The most common arrhythmia was supraventricular tachycardia (n=24, 16%) with non-sustained atrial arrhythmias in 21 patients and AF in two patients. A considerable number of patients had non-sustained ventricular tachycardia (n=12, 8%). Patients with ASD and tachyarrhythmias had increased right ventricular to left ventricular diastolic area in echocardiography and higher age when compared with ASD patients without arrhythmias.

Conclusion: Adult patients with small, unrepaired ASD have a hidden burden of both atrial and ventricular tachyarrhythmias. The mechanism likely relates to the residua of previous right-heart volume overload and incomplete reverse remodelling. Our results support guidelines recommending continued follow-up of patients with small, unrepaired ASD.