Irregularly irregular AVNRT misdiagnosed by smartwatch

A fifty-one-year-old woman presented with an irregularly irregular rhythm noted on a smartwatch. EP study revealed a narrow complex A-on-V tachycardia with significant cycle-length variability. Pacing maneuvers were consistent with atrioventricular nodal re-entry tachycardia (AVNRT), and we performed slow pathway modification. While AVNRT can have some degree of variability - particularly on warm up - the amount of irregularity here was striking, and much more than we have ever encountered or than has been previously reported in the literature.