Atrial excitability and conduction during rapid atrial pacing.

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Abstract
Using temporary atrial wire electrodes placed at selected atrial sites, rapid atrial pacing at rates of up to 368 beats/min was used to study atrial excitability and conduction in seven patients who underwent open heart surgery. The threshold for atrial pacing was found to be an exponential function of pacing rate (r = 0.55, p less than 0.01), increasing threefold when the fastest pacing rates were compared with the slowest pacing rates (p less than 0.005). Atrial conduction times (measured from pacing to recording sites), prolonged during rapid atrial pacing both for studies conducted before institution of cardiopulmonary bypass (p less than 0.005) and for those done 7 days postoperatively (p less than 0.05). However, prolongation of conduction times always depended on achievement of a critically rapid pacing rate. During rapid atrial pacing, we observed a high incidence of alternans of the atrial electrogram (17 of 42 studies). Thus, human atrial excitability, conduction and electrogram morphology are not constant during pacing at rapid rates. Rather, at rapid pacing rates, there is depression of atrial excitability, prolongation of atrial conduction times and alternation in electrogram morphology. These findings have clinical relevance and theoretical implications for the understanding and treatment of rapid atrial rhythms.