Cycle-length-dependent properties of AV nodal activation in rabbit hearts

Cycle-length-dependent changes in AV nodal cell activation were studied in isolated preparations from rabbit hearts. Transmembrane action potentials were recorded from the node while it was propagating impulses initiated in the atrium with an accelerating train of stimuli. This train consisting of five successive stimuli separated by progressively shorter intervals was uniformly repeated at every 10th basic beat, each time reproducing a sequence of five different increasing AV nodal delays. The AN and NH cells were found to contribute only slightly to the cycle-length-dependent AV nodal delay which developed mainly in the small N zone, located centrally in the AV node. With the increasing delay, the action potentials from this N zone typically dissociated into two components synchronous with late AN and early NH activity, respectively. The amplitude of the first component decreased, wheareas that of the second increased progressively in N cells activated progressively later. No cells were activated at an intermediate time between the two components. This dissociation was not accompanied by changes in the activation pattern of the AV node. The different nodal cells classified according to their response to the accelerating train delineated functional zones corresponding to different anatomic structures. The possible mechanisms which would explain the cycle-length-dependent AV nodal delay are discussed.