Quantification of human atrioventricular nodal concealed conduction utilizing S₁S₂S₃ stimulation

We studied antegrade concealed conduction of atrial extrastimuli that blocked in the atrioventricular (AV) node in eight subjects, using a third extrastimulus (A₃), coupled at decreasing coupling intervals to A₂. Three A₁-A₂ intervals were tested in each subject: late (just shorter than AV nodal effective refractory period), intermediate, and early (just longer than atrial functional refractory period). The curves relating the following variables were constructed for each A₂: A₁-A₃, H₁-H₃ and A₂-A₃, A₃-H₃. The former was compared to the control A₁-A₂, H₁-H₂ curve. Concealment of A₂ was demonstrated in all eight subjects at the three tested values of A₁-A₂. The A₂-A₃, A₃-H₃ curve allowed analysis of AV nodal conduction time (A₃-H₃) and AV nodal recovery time (defined as the shortest A₂-A₃ at which the impulse conducted to the His bundle) at identical values of A₂-A₃. In all subjects the timing of blocked A₂ had minimal effect on both AV nodal conduction time and recovery time. In five of the eight subjects a late A₂ sporadically conducted to the His bundle. Conduction of A₂ to the His bundle resulted in marked lengthening of both AV nodal conduction and recovery times. Concealed conduction of A₂ was always demonstrated, but the degree of concealment was relatively fixed, whether A₂ was an early, intermediate, or late blocked premature beat. Slow conduction of A₂ had a much greater effect than concealment of A₂ on subsequent impulse conduction.