The nature of atriosinus conduction during rapid atrial pacing in the rabbit heart.

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Abstract
During clinical electrophysiologic investigations the sinus node recovery time (SNRT) lengthens as the atrial pacing rate increases; after reaching a maximum value SNRT shortens with further increases in pacing rate. This phenomenon was ascribed to the onset of atriosinus entrance block. The nature of atriosinus conduction during rapid atrial pacing and related changes in SNRT to changes in conduction were studied. In 8 rabbit sinus node preparations, the crista terminalis was paced for 1 min at cycle lengths from 400-100 ms, while crista terminalis electrograms were recorded by surface electrode and sinus node transmembrane action potentials were recorded by microelectrode. In all experiments SNRT prolonged progressively as the cycle length shortened until 2:1 atriosinus block occurred; at that point SNRT shortened. After correction for spontaneous cycle length SNRT was inversely related to the cycle length of action potentials recorded from the pacemaker site in the sinus node (r = -0.84), indicating that SNRT depends on the number of impulses reaching the pacemaker site. In 4 experiments the microelectrode was moved toward the crista terminalis in intervals of 50-100 .mu., repeating the pacing sequence at each site. The site of atriosinus block could be identified as the point at which action potential amplitude fell rapidly. With progressively shorter cycle lengths, the site of block moved progressively farther from the pacemaker site in the sinus node. The shortening of SNRT with rapid pacing may be explained by the presence of atriosinus block and by a reduction in the number of impulses that reach the pacemaker site in the sinus node. This information may prove useful in the study of sinus node dysfunction and pacing rate in patients.

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