CMR may be indicated for assessment of ventricular size and function, systemic and venous baffle obstruction and leaks, and valvular function. Baseline study is recommended with periodic follow-up CMR, with frequency of repeat imaging determined anatomic and physiological findings. CCT may be used if CMR is not feasible and to evaluate cross-sectional imaging status post-stent therapy. If CCT is used instead of CMR imaging, the frequency should be weighed against radiation exposure. 6-minute exercise test or CPET, depending on clinical indication. ACHD indicates adult congenital heart disease; CCT cardiac computed tomography; CMR, cardiovascular magnetic resonance imaging; CPET, cardiopulmonary exercise test; d-TGA, dextro-transposition of the great arteries; ECG, electrocardiogram; and TTE, transthoracic echocardiogram. CLINICAL STATEMENTS AND GUIDELINES There is a biologically plausible explanation that may include longer atrial tachycardia cycle lengths in the context of extensive atrial sutures/scar that could favor rapid (eg, 1:1) ventricular conduction, a reduction in stroke volume with faster heart rates attributable to poor atrial transport, and myocardial ischemia despite the absence of CAD attributable to an inefficient coronary circulation supplying the systemic ventricle.S4.4.1.1-12 Efforts to maintain sinus rhythm or atrial pacing (and not simply rate control) should be the initial strategy of management, acknowledging that patients may rarely tolerate permanent atrial tachycardia when attempts to maintain sinus rhythm have failed. Atrial arrhythmias predominantly involve tissue of right atrial origin which, because of the surgi- cal anatomy, is found primarily in the pulmonary venous atrium, making access for catheter abla- tion challenging. 4.4.1.2. Transposition of the Great Arteries With Arterial Switch Recommendations for d-TGA With Arterial Switch Referenced studies that support recommendations are summarized in Online Data Supplement 46. 