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Purpose. To evaluate and compare the effects of heart rate on conduction velocity in the cavotricus-pid isthmus (CTI) and septum in patients with and without typical atrial flutter (AF) using electro-anatomicmapping (EAM) of the right atrium (RA).Methods. Ten patients(age 53±10 yrs,7M/3F)with AF and 13 patients (age 51±11 yrs, 5M/8F) withatrio-ventricular nodal reentrant tachycardia (AVNRT) underwent conventional electrophysionogical study,electro-anatomic mapping and radiofrequency ablation. Using EAMs obtained during coronary sinus pacing at pac-ing cycle length (PCL) 600 ms, 400 ms, and 300 ms, we evaluated conduction velocities in the CTIand septum of RA in 10 patients with AF and compared EAMs to 13 patients with AVNRT to determinewhether the conduction slowing required to maintain AFL was related to changes in volume alone or al-tered RA electrophysiology.Results. Conduction velocities in CTI and septum were significantly slower at all PCL when AF wascompared to AVNRT(*P＜0.05). Additionally, in the AF group, septal conduction velocities were slowerat PCL 600 ms and 400 ms, but not at 300 ms compared to CTI (#P＜0.05). In AF, during PCL 300.conduction in CTI slowed significantly compared to PCL 600 and 400 ms such that there was no differ-ence between CTI and septum at PCL 300.Conclusionss. There is slower conduction in the septum compared to the CTI in all patients. However,in patients with AF, there is significant slowing of conduction in the CTI and septum as well as decremen-tal rate-dependent slowing of conduction in the CTI. These findings indicate that in addition to RA en-largement, changes in atrial electrophysiology distinguish AF patients from patients with AVNRT.