Junctional reciprocating tachycardias. The permanent and paroxysmal forms of A-V nodal reciprocating tachycardias.

Stimulation technics have demonstrated a reciprocating mechanism in many supraventricular tachycardias previously classified from a purely morphologic point of view. Three conditions are required for the creation of a circus movement: a potential circuit pathway, undirectional block in this curcuit, and slowed conduction. While all three conditions are readily apparent in reciprocating tachycardias of the WPW syndrome, two or even all of these factors may be concealed in the others forms. Paroxysmal reciprocating tachycardias are characterized by prolongation of the P-R interval in the beat immediately preceding the tachycardia, and are generally accepted as being related to longitudinal dissociation of the A-V node, though the possibility of unidirectional (anterograde) block of an extra-nodal accessory pathway should be appreciated. Permanent reciprocating tachycardias start after a normal P-R interval when the sinus cycle reaches a critical value. Both paroxysmal and permanent forms of reciprocating tachycardia must be differentiated from tachycardias located in the atria: one of the most reliable features of reciprocating tachycardia is the existence of a 1:1 A-V relationship which cannot be altered without interrupting the tachycardia. Study of capture phenomena during the tachycardia, and the modes of termination not only permit the demonstration of the reentry mechanism but may also determine more precisely the actual location of the circus movement.