Atrial isochronal late activation mapping keeps the diaphragm alive

•For patients with congenital heart disease, significant atrial low voltage areas may occur that can be the substrate for reentrant atrial arrhythmias. While atrial flutter may occur in these patients, it is important to consider the full differential of supraventricular tachycardias.•Up to 25% of patients with D-transposition of the great arteries (D-TGA) have atrial tachyarrhythmias, which includes cavotricuspid isthmus dependent atrial flutters but are also at risk for other atrial reentrant arrhythmias due to Mustard and Senning procedures.•Isochronal Late Activation Mapping (ILAM) can be performed during sinus rhythm and identifies deceleration zones often present in low voltage areas of the atrium or ventricle. Atrial ILAM’s can identify areas critical to atrial flutter reentry sites.