A 'Regularly Irregular' tachycardia: what is the diagnosis?

We report the case of a 46-year-old female presented to the emergency room with sustained palpitations. Her ECG showed a narrow QRS regularly irregular tachycardia. This tachycardia was immediately terminated by a single dose of adenosine, and sinus rhythm was restored. Diagnosis of atrial tachycardia, orthodromic reciprocating tachycardia, and atrial nodal reentrant tachycardia (AVNRT) are discussed. An electrophysiological study was performed for further evaluation, and our final hypothesis was AVNRT with triple nodal pathways. A single application of radiofrequency energy in the posterior septum near the coronary sinus ostium effectively eliminated the tachycardia.