Asymptomatic arrhythmias seen in patients with ACHD may be associated with development of symptoms and increased risk of death, and are more common in particular lesions or repairs. Bradyarrhythmias or tachyarrhythmias may occur, with some requiring treatment even when asymp- tomatic. For example, sinus node dysfunction is common in patients with atrial switch repairs of transposition of the great arteries (TGA), whereas complete heart block is seen in patients with con- genitally corrected transposition of the great arter- ies (CCTGA) or late after atrioventricular septal defect (AVSD) repair, especially in those patients with transient postoperative heart block.S3.4.1-1 S3.4.1-3 Some of these events have occurred as late as 15 years after surgery. The atrioventricular node is typically displaced inferiorly in AVSD, which is associated with relative hypoplasia of the left Atrial tachyarrhythmias are common in atrial switch repairs of TGA, Fontan repairs, and Ebstein anomaly.S3.4.1-5 S3.4.1-7 Thus, baseline and periodic screening for asymp- tomatic arrhythmias with ambulatory electrocar- diographic monitoring is advised to ensure that asymptomatic arrhythmias that would warrant a change in therapy are not present,S3.4.1-8 acknowl- edging the limitations of monitoring over short periods of time. Any symptoms of arrhythmia should prompt investigation to establish an accu- rate diagnosis and direct subsequent therapy. 3.4.2. Ionizing Radiation Principles Recommendation for Ionizing Radiation Principles Referenced studies that support the recommendation are summarized in Online Data Supplement 6. COR LOE Recommendation I B-NR Strategies to limit and monitor radiation exposure are recommended during imaging of patients with ACHD, with studies not involving ionizing radiation chosen whenever Recommendation-Specific Supportive Text 1. 