Influence of the site of stimulation on atrioventricular nodal refractory periods and the effect of verapamil

The refractory periods of the atrioventricular (AV) node appear dependent on the pattern of AV nodal input. In 21 superfused AV rabbit heart preparations stimulated from each of the 2 principal input regions, crista terminalis or atrial septum, the effect of changing the site of stimulation of the AV nodal refractoriness and the relative effect of verapamil on AV nodal refractoriness was determined. In 6 of 21 preparations the functional AV refractory curve became discontinuous only when stimulation was applied at the atrial septum and suggested dual AV nodal pathways (dual pathways group). In the 15 other preparations no interruption of the curve occurred with either crista terminalis or atrial septal stimulation (normal conduction group). In the normal conduction group, the difference in the effective refractory period of the AV node obtained by crista terminalis vs atrial septal stimulation was not significant (154 ± 25 vs 150 ± 28 ms). However, the functional refractory period was significantly longer with crista terminalis vs atrial septal stimulation (232 ± 19 vs 239 ± 19 ms, p < 0.001). After verapamil administration, the effective and functional refractory periods during crista terminalis vs atrial septal stimulation were prolonged to 270 ± 49 vs 285 ± 55 ms (p <0.01) and 335 ± 43 vs 351 ± 41 ms (p < 0.001), respectively. Thus, the difference in refractory periods associated with changing the stimulation site was exaggerated with verapamil. In the dual pathways group, dual atrionodal pathways and echo beats were observed in all 6 cases only when the atrial septum was stimulated. The ΔH1–H2 max (the maximal difference in H1–H2 interval resulting from crista terminalis vs atrial septal stimulation with the same coupling interval) was 57 ± 20 ms in this group. Thus, these studies demonstrate that elucidation of dual pathways, determination of AV nodal refractory periods and evaluation of interventions such as verapamil can be influenced by the site of input stimulation to the AV node.