SLOW ABNORMAL CONDUCTION IN THE LOW RIGHT ATRIUM - ITS ANATOMIC BASISAND RELEVANCE TO ATRIAL REENTRY

To characterize slow abnormal conduction in the low right atrium, whic h is known to be responsible for atrial flutter, electrophysiologic fi ndings were correlated with anatomic features in a canine model of atr ial flutter with ligation of the crista terminalis in the midright atr ium. Activation in the low right atrium was mapped with a patch electr ode containing 52 bipolar electrodes and a multiplexing system. A part icular region in the low right atrium showed atrioventricular node-lik e electrophysiologic properties, a rate-dependent conduction delay, an d Wenckebach periodicity. This area coincided with an area responsible for slow conduction during atrial flutter and unidirectional block at its initiation. Both pilsicainide and E-4031 preferentially blocked c onduction in the specific area, leading to the termination of atrial f lutter. Although refractoriness could not explain the abnormal conduct ion, anatomic studies consistently found the specific region to be in or around a thick muscle bundle, that is, the crista terminalis, or a thick pectinate muscle branching from the crista, located perpendicula r to the wavefront of the pacing impulse and atrial flutter. These ele ctrophysiologic and anatomic findings suggest that slow abnormal and a trioventricular node-like conduction over a thick muscle bundle, which is a normal anatomic feature of the low right atrium, plays a role in the initiation, maintenance, and termination of atrial reentry.