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Accessory pathway tachycardia: techniques of electrophysiologic study and mechanisms.

THE PREEXCITATION SYNDROMES are a group of disorders in which tachycardia is related to the presence of fibers that bypass all or part of the normal conduction system, either anterogradely or retrogradely. Recognized substrates of preexcitation include atrio-Hisian fibers, nodoventricular (-fascicular) fibers, fasciculoventricular fibers, and atrioventricular pathways. Arrhythmias occur either because the parallel circuits of conduction afforded by these various pathways create conditions for reentry, or because they permit rapid ventricular responses during atrial flutterfibrillation due to bypass of areas of normal physiologic delay. ' 2 Nodoventricular (-fascicular) and atrioventricular accessory pathways have been demonstrated to participate in tachyarrhythmias, but fasciculoventricular pathways have not been clearly implicated. Although atrio-Hisian fibers may facilitate rapid atrioventricular conduction, their participation in reentrant rhythms has not been established. A variety of mechanisms of tachycardia may be encountered some related to accessory pathways, others coincidental. The mechanism of reciprocating tachycardia (RT) in patients with preexcitation can often be deduced from careful analysis of the morphology and temporal relationship of the P wave and the QRS complex during tachycardia.3 This obviously requires capture of the event on ambulatory recordings. With the development of sophisticated recording and stimulating techniques, much additional information has become available. The mechanism of most types ofRT can now be clarified by analysis of specific phenomena during spontaneous and induced tachycardia.4 Knowledge of the substrate responsible for RT and the therapeutic alternatives available may provide the basis for a "clinical threshold" to opt for nonpharmacologic forms of therapy. The proper study of a patient with RT due to known or suspected preexcitation must be tailored to the clinical situation. Thus, a study aimed at diagnosis might be limited to establishing the presence of an accessory pathway, whereas a study intended as a guide to nonpharmacologic therapy must establish the number,