Effects of blocked atrial beats on the atrioventricular nodal recovery property: Facilitation or depression?

Introduction: Blocked atrial beats (A(B)) usually have concealed AV nodal penetration, which can change the nodal conduction time (AH) of a subsequent beat. However, without an output marker it is difficult to assess their effect on the node. In this report we used all possible parameters as nodal resting time after A(B) and plotted them against the AH of testing beats to study their effects on the node. Methods and Results: Atrial extrastimulation studies were done in 21 patients in whom one blocked atrial beat (A(2A)) was observed. Nodal recovery curves were obtained for basic pacing (A₁), after a conducted premature beat (A₂), and after A(2B). In six patients there were 2 to 3 consecutively blocked beats (A(12b)) and recovery curves were constructed after each A(2B). Nodal recovery curves were plotted with AH of the testing beat against different nodal resting parameters and fitted to a single exponential equation. We found contradicting phenomena when using different formats. (1) For recovery curves of A(2B), there was a rightward shift from that of the basic curve when using H₁A₃ or A₁A₃ as the gauge (depression phenomenon). On the contrary, there was a leftward shift of the curves when using A(2B)A₃ (facilitating phenomenon). (2) For recovery curves after multiple blocked beats there was a marked rightward shift of all curves except A((n-1)(B))A(n)-curves, which were all leftward shifted. Conclusion: Because these contradicting phenomena were dictated by the presenting formats, the terms 'depression' and 'facilitation' cannot be considered intrinsic AV nodal properties outside of the strict context of the pacing protocol and the format of data presentation.