Atrioventricular nodal reentrant tachycardia (AVNRT), the most common regular supraventricular tachycardia, has fascinated cardiac electrophysiologists for several decades. The reentrant circuit consists of at least two pathways, which are essentially atrio-nodal connections with different electrophysiological (EP) properties, known as fast and slow pathways in reference to their conduction times. Three forms of AVNRT, slow/fast, fast/slow and slow/slow, have generally been recognized based on the conduction time and the anatomical location of each reentrant pathway. Another intriguing form, which features a variable AH/HA ratio and may readily be misdiagnosed as atrial tachycardia, is also described in this chapter. In addition, several other interesting phenomena observed during EP studies are elucidated, and a few controversial issues regarding the anatomical boundaries of the AVNRT circuit are addressed. Finally, transcatheter mapping and ablation techniques, currently used in EP laboratories, are highlighted and practical approaches are delineated.

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