Beat-to-beat changes in atrioventricular nodal excitability and its modulation by concealed conduction during functional 2:1 block in man.

An atrial pacing model of functional 2:1 block was used in 10 patients to investigate for the first time the electrophysiologic properties of the human atrioventricular node during intermittent conduction. By varying the terminal portion of the 2:1 atrial train and using the extrastimulus technique, we characterized atrioventricular nodal (AVN) conduction and refractoriness after five different methods of AVN activation: a conducted beat (method I), a conducted beat with omission of the prior blocked beat (method II), a blocked beat (method III), a blocked beat converted to one that conducts by omission of the prior conducted beat (method IV), and finally, 1:1 conduction at twice the cycle length of the 2:1 train (control method V). Observed AVN conduction times obeyed the following relationship: method I greater than method II greater than method V, indicating a cumulative effect of concealed penetration by the blocked beats. During 2:1 block, the AVN effective refractory period (ERP) alternated with a...