Atrial activation sequence during atrial flutter in the canine pericarditis model and its effects on the polarity of the flutter wave in the electrocardiogram. Stable atrial flutter induced in both conscious and open chest states was studied in 30 mongrel dogs after production of sterile pericarditis. During the conscious state studies, induced atrial flutter (mean cycle length 128 +/- 15 ms) was always sustained greater than 15 min and was stable. Three types of flutter wave polarity were noted in electrocardiogram (ECG) lead II: positive in 16 dogs, negative in 3 and flat or slightly positive in 11. Sequential site atrial mapping during atrial flutter (mean cycle length 133 +/- 18 ms) in the open chest state showed either clockwise (18 dogs) or counterclockwise (12 dogs) circus movement in the right atrium. In 19 of 30 dogs, the circus movement clearly did not require any naturally existing anatomic obstacle; in 11, the orifice of the superior vena cava probably was also involved. Double potentials were recorded from the center of the reentrant circuit during atrial flutter, and fractionated electrograms were recorded from a pivot point of the reentrant wave front. A positive flutter wave in ECG lead II (12 dogs with counterclockwise circus movement) was associated with early activation of the Bachmann's bundle region compared with the posteroinferior left atrium and activation of the left atrium mainly in a superoinferior direction. A negative flutter was associated with the early activation of the posteroinferior left atrium compared with Bachmann's bundle and activation of a considerable portion of the left atrium in an inferosuperior direction. A flat or slightly positive flutter wave (14 of 18 with clockwise circus movement) was associated with activation of the left atrium almost simultaneously by two wave fronts coming from both these sites. In conclusion, atrial flutter in this dog model is due to circus movement in the right atrium, the center of which does not necessarily require an anatomic obstacle. Although atrial flutter is generated by circus movement in the right atrium, the flutter wave polarity in the ECG is determined primarily by the activation sequence of the left atrium. 