Observations contributing to certain aspects of the mechanism of spontaneous reciprocating tachycardia.

In 7 patients, none of whom had the WPW syndrome, analysis of recurrent paroxysms of supraventricular tachycardia strongly favoured the existence of re-entry in the AV junction as the underlying mechanisms. Paroxysms were abortive in 4 patients and sustained in the remaining 3. The initiating factors were partial AV block and/or premature beats or premature depolarization from re-entrant impulses. The following phenomena were documented pertaining to reciprocating tachycardias. (I) Variation in configuration and in polarity in the same lead of retrograde P wavesfrom junctional impulses; (2) substantial variation in atrial echo intervals during initial and terminal cycles as well as in sustained reciprocation; (3) regular alternation in atrial echo intervals; (4) irregular ventricular response due to blockage and/or concealment of junctional impulses when sustained reciprocation was associated with partial AV block; (S) continuous recurrence of tachycardia as a result ofunidirectional block below the sitefor thejuncture ofdualAVpathways; (6) persistence for many days of circus re-entry in the AVjunction. It is pointed out that, in the diagnosis of reciprocating tachycardia, the criterion of a difference between the configuration of the P wave which initiates the tachycardia and the P waves recorded during the tachycardia is not obligatory.