Junctional Tachycardia

IN THE ABSENCE of functioning anomalous connection between atria and ventricles, the A-V junction serves as the only link between these structures, allowing the atrial impulse to be transmitted to the ventricles.The A-V junction may be defined as consisting of the atrial approaches to the A-V node, the A-V node itself, and the penetrating portion of the His bundle.Total transection of the A-V junction at any point should produce A-V block, and stimulation in the presence of a normal His-Purkinje system should produce a narrow QRS.The electrophysiologic function of the A-V junction is primarily that of conduction.However, in addition, the A-V junction possesses intrinsic automaticity, allowing it to serve as a subsidiary pacemaker in the event of failure of proximal impulse formation or propagation.Under pathologic conditions, the A-V junction can serve as an ectopic pacemaker, or as a site of reentry. Electrophysiologic ConsiderationsExperimental studies demonstrating automaticity in the A-V junction were extensively reviewed by Scherf and Cohen.1 Earlier workers produced A-V junctional escape rhythms after depressing sinus node function by various surgical, chemical, mechanical, and thermal maneuvers.In addition, experimental A-V junctional rhythms were pro- duced by increasing sympathetic tone or by warming specific areas in the A-V junction.The site of origin in the A-V junction was determined by noting the timing of atrial and ventricular contrac- tion, and also by noting the initial sites of negativity with direct recording.Zahn suggested classification into upper, middle, and lower A-V nodal rhythms.2In upper nodal rhythms, atrial contraction preceded