Acceleration of Sinus Rhythm During Slow-rate

We evaluated the effects of slow-rate atrial pacing on impulse formation of the sinus node in 13 isolated rabbit right atria using the microelectrode technique. After the spread of activation was mapped to determine true sinoatrial conduction time (SACT) (38 ± 16 msec [± SD]), the atrium was paced for eight beats at constant intervals slightly shorter than spontaneous cycle length in steps of 10 msec. In all preparations, slow-rate atrial pacing failed to capture the pacemaker center, but shortened action potential duration because of electrotonic interaction between atrium and sinus node. The resulting acceleration of impulse formation of dominant fibers ceased instantaneously when atrial pacing was terminated. Estimation of SACT by constant pacing 5 beats/min faster than sinus rhythm seriously underestimated the true values (p < 0.05), because sinus node acceleration prevented capture of dominant pacemaker fibers in 10 of 13 preparations. At pacing rates 10 beats/min faster than sinus rhythm, capture did not occur in eight of 13 experiments; at pacing rates 20 beats/min faster, the pacemaker center was captured in all preparations. This study describes sinus node acceleration as a consistent and hitherto unknown finding during slowrate stimulation of the atrium. This phenomenon accounts for the failure of the constant atrial pacing technique to determine SACT accurately in man.