ATRIOVENTRICULAR NODAL CONDUCTION GAP AND DUAL PATHWAY ELECTROPHYSIOLOGY

Background The gap phenomenon in atrioventricular (AV) conduction is d escribed as a block that occurs within a range of atrial coupling inte rvals. This block is assumed to occur between two adjacent parts of th e conduction system having different refractory properties; thus, a ga p would develop if the functional refractory period of the proximal un it was shorter than the effective refractory period of the distal unit . We describe a new electrophysiological mechanism mechanism based on dual pathways electrophysiology of the AV node. Methods and Results In vitro experiments were performed on isolated superfused rabbit hearts . Standard electrophysiological pacing arid recording techniques were used to generate conduction curves. The gap phenomenon was documented in 9 of 14 nodal preparations. With shortening of the atrial coupling interval, antegrade conduction block of the ''fast'' pathway wave fron t occurred while this impulse was still retrogradely interfering with slow pathway conduction. That is, the fast pathway wave front prevente d propagation of the anterograde ''slow'' pathway wave front by collis ion or by creating a refractory barrier. This mechanism produced a gap and the block persisted until, at even shorter coupling intervals, th e fast wave front penetration became insufficient and conduction was r estored through the released slow pathway wave front. This mechanism w as Verified in AV nodal preparations with separated inputs, in which i ndependent fast and slow wave fronts could be induced and programmed t o collide. Conclusions Our results established the functional interact ion of fast and slow pathway wave fronts as an important electrophysio logical mechanism underlying the AV conduction gap. This mechanism may be responsible for a variety of clinically observed conduction discon tinuities.