Idiopathic Atrial Flutter With Tachycardia-Induced Cardiomyopathy Detected at a Routine Infant Health Checkup: A Case Report.

Idiopathic atrial flutter (AFL) is an uncommon supraventricular tachyarrhythmia in early infancy. When sustained, AFL may lead to tachycardia-induced cardiomyopathy (TIC), a potentially reversible cause of ventricular dysfunction after rhythm control. We report a 3-month-old boy in whom severe TIC was detected incidentally at a routine infant health checkup despite minimal symptoms. The examining pediatrician noted persistent age-inappropriate tachycardia and referred the infant urgently. On admission, the ventricular rate fluctuated between 160 and 240 beats/min, likely reflecting variable atrioventricular conduction. Chest radiography showed marked cardiomegaly. Echocardiography revealed a severely dilated left ventricle (left ventricular end-diastolic dimension 41 mm; Z-score +7.3) with diffuse hypokinesis and markedly reduced systolic function (left ventricular ejection fraction approximately 20%), consistent with advanced TIC. Adenosine triphosphate transiently induced atrioventricular block and unmasked typical flutter waves, confirming AFL. Synchronized direct-current cardioversion (1 J/kg) immediately restored sinus rhythm, followed by standard heart failure therapy. Rapid reverse remodeling was documented quantitatively, with regression of cardiomegaly and normalization of ventricular size and function. The infant was discharged on Day 25 and remained well without recurrence at follow-up. This case highlights that AFL in infancy can be clinically subtle while causing profound TIC and underscores the importance of careful heart rate assessment during infant health supervision.