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-anatomic mapping （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） with atrio-ventricular nodal reentrant tachycardia （AVNRT） underwent conventional electrophysionogical study

electro -anatomic mapping and radiofrequency ablation. Using EAMs obtained during coronary sinus pacing at pacing cycle length （PCL） 600 ms

400 ms

and 300 ms

we evaluated conduction velocities in the CTI and septum of RA in 10 patients with AF and compared EAMs to 13 patients with AVNRT to determine whether the conduction slowing required to maintain AFL was related to changes in volume alone or altered RA electrophysiology.Results. Conduction velocities in CTI and septum were significantly slower at all PCL when AF was compared to AVNRT（P<0.05）. Additionally

in the AF group

septal conduction velocities were slower at 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 difference between CTI and septum at PCL 300.Conclusions. 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 enlargement

changes in atrial electrophysiology distinguish AF patients from patients with AVNRT.