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 supra- ventricular 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 initiatingfactors were partial A V block and/or premature beats or premature depolariza- tion from re-entrant impulses.The following phenomena were documented pertaining to reciprocating tachycardias.(I) Variation in configuration and in polarity in the same lead ofretrograde 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 ofjunctional impulses when sustained reciprocation was associated with partial AV block; (S) continuous recurrence of tachycardia as a result ofunidirectional block below the sitefor thejuncture ofdual A Vpathways; (6) persistence for many days of circus re-entry in the A Vjunction.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.Reciprocal beating, the most likely example of re-entry within the AV junction, is a process due to temporal dispersion of refractoriness in the AV junction, in conjunction with areas of unidirectional block in some fibres.When the re-entry in the AV junction is bidirectional, the process may become self-perpetuating and be the source of paroxysms of supra- ventricular tachycardia (Pick and Langendorf, I968) which is designated as reciprocating tachycardia (Kistin, I965).Reciprocating tachycardias may be initiated by atrial or ventricular premature systoles (Kistin, I959, I965; Gettes and Yoshonis, I970) or they may be induced simply by par- tial AV block (Pick and Langendorf, 1950; Codina-Altes and de Beristain, 1950; Bix, I951; Schamroth, I960; Burchell, I964; Massumi, Kistin, and Tawakkol, I967).Vari- ous aspects concerning the mechanism of re- entrant supraventricular tachycardias have been recently studied and analysed by use of