Demonstration of an area of slow conduction in human atrial flutter. Ten patients with chronic atrial flutter were studied prospectively using electrophysiologic mapping and pacing techniques to assess the mechanism of atrial flutter and the presence of an area of slow conduction in the atria. Electrograms recorded from greater than or equal to 30 right atrial sites for each patient during atrial flutter demonstrated that right atrial free wall activation was craniocaudal and that the interatrial septum activation was caudocranial, consistent with a reentrant circuit involving the right atrium. In six patients, slow conduction occurred during atrial flutter in the inferior right atrium and was spatially associated with fractionated electrographic recordings. In the other four patients, a "missing" interval of electrical activity occurred in the inferior right atrium for an average of 40% of the atrial flutter cycle. Transient entrainment criteria were demonstrated in each patient during rapid high right atrial pacing. The mean activation time from the high right atrial pacing site to the coronary sinus (inferior left atrial) recording site was long (228 ms) and consistent with activation through an area of slow conduction. During rapid pacing of atrial flutter from the coronary sinus site, no transient entrainment criteria could be demonstrated. The mean activation time from the coronary sinus pacing site to the high right atrial recording site was relatively short (134 ms) and consistent with orthodromic activation of the high right atrium not through an area of slow conduction. High right atrial pacing during sinus rhythm at rates similar to atrial flutter demonstrated a short activation time to the coronary sinus and low right atrial sites (mean 169 and 88 ms, respectively), indicating activation that did not traverse an area of slow conduction. Coronary sinus pacing during sinus rhythm demonstrated the same phenomena. Low right atrial electrograms recorded during sinus rhythm and during rapid pacing of sinus rhythm were not fractionated, although they were during atrial flutter. Thus, atrial mapping and pacing data were complementary, indicating that human atrial flutter in the patients studied was generated by a reentrant circuit in the right atrium, with an area of slow conduction in the low right atrium present only during atrial flutter. 