Atrioventricular nodal conduction during atrial fibrillation in rabbit heart

Atrial fibrillation was induced in 15 superfused rabbit atrial-atrioventricular nodal preparations in which surface bipolar electrograms were recorded simultaneously from the crista terminalis, interatrial septum, and His bundle along with microelectrode action potentials from cells in the atrionodal (AN), nodal (N), and nodal-His (NH) regions of the atrioventricular node. Effective engagement of the atrioventricular node with propagation to the His bundle was critically dependent on the relative timing of activation at the crista terminalis and interatrial septal input regions of the atrioventricular node. Conduction through the AN and N regions appeared dependent on the relative timing of activation wave fronts emerging from the two input regions. Asynchronous engagement of AN and N regions resulted in both distortion of action potentials and concealed conduction, with delayed conduction and block to the NH region and His bundle. Successful engagement of the NH region always produced a 1:1 NH-to-His bundle relationship. It is concluded that during atrial fibrillation 1) activation of the AN region occurs as a result of the variable interaction of inputs from the crista terminalis and interatrial septum; 2) predictably, effective synchronous engagement of the AN and consequently the N region is responsible for conduction to the NH and His bundle regions; 3) conversely, asynchronous activation inputs from the crista terminalis and interatrial septum result in fragmented, asynchronous as well as concealed conduction within the AN and N regions with block in the atrioventricular node and variable conduction to the His bundle.