Characterization of refractoriness in the sinus node of the rabbit.

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Abstract
Recovery cycles following premature atrial stimulation introduced during atrial pacing may fall into 4 categories: reset, incomplete interpolation, complete interpolation and echo responses. The transition from reset to incomplete interpolation may represent the point at which premature beats can no longer enter the sinus node and reset it and thus reflect the refractory period of the sinus node. The electrophysiological basis underlying the transition from reset to incomplete interpolation and the spatial orientation of refractoriness in the sinus node was studied. In 15 rabbit sinus node preparations, premature atrial stimuli were introduced at varying degrees of prematurity. Intracellular potentials were recorded with a microelectrode in the sinus node. In 12 of 15 experiments, transition from reset to incomplete interpolation immediately followed a sudden reduction in action potential amplitude, rendering the action potential incapable of resetting the node. This point was interpreted as the effective refractory period of the sinus node. During the zone of incomplete interpolation, low voltage depolarizations were seen in the node, interfering with diastolic depolarization and delaying the recovery beat. These small depolarizations were absent during the zone of complete interpolation. In 6 experiments the microelectrode was moved toward the crista terminalis in steps of 50-100 .mu.m. The stimulation sequence at each site was repeated. By examining relative action potential amplitude at various sites at the same premature interval, curves showing the pattern of block of premature impulses were constructed. Progressively earlier premature beats are blocked at progressively greater distances from the node. Tissue between the crista terminalis and the sinus node provides a progressive gradation of refractoriness, rather than a discrete barrier.