Case Report: A 19-year-old male presented with a three-year history of tachycardia and occasional lightheadedness but no limitations in activities. He denied substance use and took no medications. His vital signs and physical exam were age-appropriate except for a heart rate of 118 bpm. Initial EKG appeared to show only sinus tachycardia, though with slightly rightward P- and R-wave axes of 99 and 97 degrees, respectively. Echocardiogram demonstrated normal cardiac anatomy but global systolic and diastolic dysfunction with an ejection fraction of 45%. He underwent patch ambulatory rhythm monitoring (ZioPatch®) that revealed a heart rate range of 22 – 226 bpm with frequent non-conducted P-waves resembling second-degree Mobitz II atrioventricular (AV) block; the longest ventricular pause was 2.7 seconds due to 4:1 block (see Figure 1). Evaluation for etiologies of cardiomyopathy including labs and genetic screening was unrevealing. Cardiac MRI showed a small pericardial effusion, mild left ventricular dilation with moderate systolic dysfunction, and mild right ventricular systolic dysfunction; there was no abnormal late gadolinium enhancement to suggest fibrosis consistent with myocarditis. The patient was started on an ACE-inhibitor for American Heart Association Heart Failure Stage C, with a plan for eventual pacemaker placement. However, a follow up ZioPatch® showed a maximum heart rate of 238 bpm suggesting ectopic atrial tachycardia as the predominant underlying rhythm with continued intermittent AV block resembling Mobitz II. The patient was taken to the electrophysiology lab, where endocardial voltage mapping revealed an ectopic atrial focus in the base of the left atrial appendage (see Figure 2). Radiofrequency ablation resulted in prompt termination of his incessant atrial tachycardia with immediate restoration of normal sinus rhythm. Six months later, his cardiomyopathy had completely resolved, and his rhythm remained normal sinus with no AV block. Discussion: Ectopic atrial tachycardia, when sustained, is a rare cause of cardiomyopathy in children and young adults. The diagnosis is usually identified on EKG by abnormal morphology and axis of the P-wave, a very subtle finding in our patient with slightly rightward P-wave axis. While sustained ectopic atrial tachycardia can be associated with first-degree or Mobitz I (Wenckebach) AV block, likely due to high vagal activation in response to the incessant tachycardia, it is extremely rare to see higher-grade AV block as our patient demonstrated. Strictly speaking, while our patient’s AV block appeared to be Mobitz II, it did not meet formal criteria for this, as it requires the baseline rhythm to be sinus. This AV block immediately resolved once the ectopic focus was successfully ablated. This case highlights an important reversible cause of cardiomyopathy in children and young adults that, when appropriately recognized by clinicians, has the potential for full recovery.

Figure 1

Patch ambulatory rhythm monitoring (ZioPatch®) demonstrating frequent non-conducted P-waves resembling second-degree Mobitz II AV block. Used with permission from iRhythm Technologies.

Figure 1

Patch ambulatory rhythm monitoring (ZioPatch®) demonstrating frequent non-conducted P-waves resembling second-degree Mobitz II AV block. Used with permission from iRhythm Technologies.

Figure 2

Bi-atrial voltage mapping of impulse activation in the patient’s heart, with red representing the earliest activation. This demonstrates an ectopic focus with the earliest activation near the base of the left atrial appendage. The red dots represent the location of ablation. SVC=superior vena cava, IVC=inferior vena cava, RA=right atrium, LA=left atrium, LAA=left atrial appendage.

Figure 2

Bi-atrial voltage mapping of impulse activation in the patient’s heart, with red representing the earliest activation. This demonstrates an ectopic focus with the earliest activation near the base of the left atrial appendage. The red dots represent the location of ablation. SVC=superior vena cava, IVC=inferior vena cava, RA=right atrium, LA=left atrium, LAA=left atrial appendage.