Filtering role of the atrioventricular node in atrial fibrillations

Experiments on rabbit heart preparations were carried out with the aim of studying the causal relation between the signals at the input and at the output of the atrioventricular node (AVN) during atrial fibrillations (AF). Electrograms were recorded simultaneously from the posterior and anterior inputs and from the output of AVN, together with cellular action potentials from one or two structures of the node. AVN is shown to play the role of a filtering unit for the excitatory waves passing from the atria to the ventricles during AF. The random and fragmentary character of the atrial excitatory front during AF causes summation of excitatory waves, local and manifested re-entry, and conduction along competitive pathways. These processes accompany both the successful conduction through AVN and the blocking of the excitation at different levels in the node. The filtration process takes place mainly in the AN- and N-zones of the node. In the NH-zone conduction does not differ compared to that in the case of spontaneous rhythm. The results obtained show that conduction through AVN during AF does not take place through triggering of specific mechanisms, but through a complex combination of the mechanisms characterizing the deteriorated conduction under extreme conditions.