Electrophysiological Studies on Preexcitation in the Dog Using an Electronically Simulated Atrioventricular Bypass Pathway

Experiments were performed on open-chest dogs. In the in situ heart, electronic circuits were used to simulate atrioventricular (AV) bypass pathways as they are found in the preexcitation syndrome. Bipolar electrograms were recorded from the atria, His bundle, Purkinje system, and ventricles. A Systron-Donner SD-80 computer was used to simulate conduction over the Kent or the James bundle and over Mahaim fibers. The electronic bypass circuits had the properties of an excitation threshold, conduction delay, and refractoriness and the ability to conduct in the antegrade or retrograde directions. Simulated lateral accessory AV pathways at various regions of the base of the left and right AV margin produced electrocardiogram (ECG) patterns corresponding to type A, type B, and intermediate types of the Wolff-Parkinson-White syndrome. Stimulation of the ventricular septum along that region of the His bundle corresponding to Mahaim fibers did not produce prominent delta waves or obvious fusion of ECG complexes when the timing of ventricular septal stimulation was coincident with His bundle activation or preceded it by 18 msec. The characteristics of the fusion beats during introduction of premature atrial beats and during phasic changes in vagal tone were also investigated. Retrograde conduction through a simulated Kent bundle caused the development of a self-sustained supraventricular tachycardia. Spontaneously occurring echo beats during simulated preexcitation exhibited distorted ECG complexes and verified the possibility of a nodal reentrant atrial tachycardia with rapid antegrade conduction over the Kent bundle. The danger of atrial tachyarrhythmias in individuals with preexcitation was demonstrated by showing that, with the proper conduction parameters across a bypass tract, atrial fibrillation could lead to ventricular fibrillation.