Unusual Mechanism of PR Interval Variation and Nonreentrant Supraventricular Tachycardia as Manifestation of Simultaneous Anterograde Fast and Slow Conduction Through Dual Atrioventricular Nodal Pathways

Noninvasive recordings in a 69-year-oId woman showed t wo distincl PR in tervals of about 0.21 and 0.58 s, suggestive of dual AV nodal conduction. Various unusual mechanisms of transition from short to long and from long to short conduclion interrals and phenomena of concealed conduction were due to the presence of two functionally separated intranodul pathways. Refractoriness of the slow pathway was associated with bradycardia. Episodes of tachycardia exhibiled a one-to-two relationship between P-waves and ventricular activalions as a consequence of simultaneous anterograde fast and slow conduction leading to double ventricular responses lo single P-waves.