Permanent form of junctional reciprocating tachycardia with only even-numbered beats

An analysis of the electrocardiogram of a patient with the permanent form of junctional reciprocating tachycardia is presented. The patient demonstrated near-incessant tachycardia, with a 1:1 atrioventricular relationship and a retrograde P wave (P') occurring closer to the succeeding QRS complexes (ie, with a P'R interval that is shorter than the RP' interval). Each tachycardia episode was characterized by alternating short and long cardiac cycles due to alternation of retrograde conduction time (RP' interval), retrograde Wenckebach periodicity, and an even number of ectopic P' waves. The authors propose that there is an accessory AV connection with decremental functional properties that arborizes into two atrial branches with different conduction times. The fast branch initially exhibits a 3:2 retrograde conduction block followed by a cycle length-dependent 2:1 retrograde conduction block, thereby permitting alternate use of the slow branch, which is the weakest component of the reciprocating process.