Concealed conduction in nodal dual pathways: Depressed conduction, prolonged refractoriness, or reset excitability cycle?

BackgroundConcealed conduction is recognized as a major determinant of atrioventricular (AV) nodal filtering properties, but little is known about the underlying mechanisms.ObjectivesThe purpose of this study was to consistently elicit concealed conduction through the AV node and to determine the involvement of slow and fast pathways in resultant changes in nodal function.MethodsThe concealment zone (nodal effective refractory period minus nodal functional refractory period of atrium) was determined in six rabbit heart preparations with and without a conditioning cycle (10 ms longer than nodal effective refractory period). Nodal function curves were constructed for concealed cycle lengths selected within the concealment zone. Experiments were repeated after slow pathway ablation.ResultsWhen assessed with a blocked beat alone, a narrow concealment zone (22 ± 12 ms, n = 3) was observed in 50% of the preparations. In contrast, when assessed with a blocked beat preceded by a conducted conditioning beat, a wider concealment zone (77 ± 47 ms, n = 6, P <.03) was observed in all preparations. Increases in the concealed cycle length resulted in graded increases in the nodal effective refractory period and nodal functional refractory period and graded rightward shifts of the recovery curve as a whole, consistent with resetting of the excitability cycle in the slow and fast pathways. These effects were analogous to those expected from a conducted beat. Slow pathway ablation widened the concealment zone but failed to alter fast pathway resetting.ConclusionOur approach reveals a wide concealment zone consistently displayed in all preparations. Concealed conduction acts as a resetting mechanism of the excitability cycle in the slow and fast pathways similar to that expected from a conducted beat.