Beyond the Classical Approach: Successful Sequential Ablation of Fast-Slow and Slow-Fast Atrioventricular Nodal Reentrant Tachycardia. An Illustrative Case

Atrioventricular nodal reentrant tachycardia (AVNRT) is a frequent mechanism of regular narrow-QRS supraventricular tachycardia and is usually supported by dual atrioventricular nodal physiology. Typical slow-fast AVNRT is the most common pattern, whereas atypical fast-slow forms are less frequent and may require a more detailed electrophysiological evaluation. This case describes a 72-year-old woman with long-standing palpitations that became progressively more frequent and symptomatic despite antiarrhythmic treatment. A 12-lead electrocardiogram documented a narrow-QRS supraventricular tachycardia at 172 beats/min with RP interval longer than PR interval, and structural heart disease was excluded by transthoracic echocardiography. During the electrophysiological study, a spontaneous clinical tachycardia with a cycle length of 330 ms and ventriculoatrial interval of 198 ms was observed. Ventricular pacing maneuvers excluded an accessory pathway and supported the diagnosis of atypical fast-slow AVNRT. Radiofrequency ablation close to the coronary sinus ostium terminated the tachycardia. Subsequently, atrial stimulation induced a second tachycardia with a cycle length of 430 ms and simultaneous atrioventricular activation, compatible with typical slow-fast AVNRT. A second target located 6 mm from the initial ablation site was identified and treated with radiofrequency energy, resulting in noninducibility without atrioventricular conduction compromise. This case highlights the diagnostic and therapeutic relevance of recognizing coexisting AVNRT mechanisms and supports individualized sequential ablation when immediate recurrence or a different inducible circuit is demonstrated.