Effects of the site and timing of atrioventricular nodal input on atrioventricular conduction in the isolated perfused rabbit heart.

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Abstract
Programmed stimulation was used to study patterns of atrioventricular nodal propagation in an isolated rabbit heart preparation. Stimulation was done at the two major atrioventricular nodal input regions, the crista terminalis and interatrial septum, by use of various sequencing protocols. The influence of stimulation input interactions on atrioventricular nodal propagation was then evaluated with the use of simultaneous extracellular and intracellular recordings from the various regions of the atrioventricular node and His bundle. Activation of the atrioventricular node occurred predominantly via the stimulated input site, although perinodal conduction to the opposite input also occurred and could modify the atrioventricular conduction pattern. Engagement patterns of both AN and N fibers were dependent on the sequence of activation at the major input sites, and similar conduction times to the His bundle were often associated with different local activation times, depending on the site of premature stimulation. Conversely, if premature stimulation was timed to produce constant local activation times but the input site varied, then the time to His bundle activation could also vary. Atrioventricular nodal functional and effective refractory periods and conduction patterns were dependent on both the timing and pattern of engagement at the principal inputs to the atrioventricular node as well as the subsequent organization of activation of the various regions of the atrioventricular node. Furthermore, depending on the sequence of stimulation at the crista terminalis and interatrial septum, activation patterns were either organized or inhomogeneous in the AN and N regions of the atrioventricular node, consequently influencing the H-H interval, and in some instances resulting in conduction block to the His bundle. It is concluded that the relative timing of activation at the principal input regions of the atrioventricular node is critical to patterns of atrioventricular nodal propagation and subsequent conduction to the His bundle. Our results emphasize the complexity of atrioventricular nodal propagation during premature stimulation.