MAPPING THE CONVERSION OF ATRIAL-FLUTTER TO ATRIAL-FIBRILLATION AND ATRIAL-FIBRILLATION TO ATRIAL-FLUTTER - INSIGHTS INTO MECHANISMS

It is not generally believed that there is a relation between atrial flutter, thought to be due to a single reentrant circuit, and atrial fibrillation, thought to be due to simultaneously circulating multiple-reentrant wave fronts. However, there are many reasons to suggest that these rhythms are more closely related than previously thought. To test the hypothesis that the length of an area of functional block in the right atrial free wall is critical to the conversion of atrial flutter to atrial fibrillation and of atrial fibrillation to atrial flutter, we studied spontaneous and ATP-induced conversion of stable atrial flutter to sustained atrial fibrillation and spontaneous conversion of sustained atrial fibrillation to stable atrial flutter. We studied 13 episodes of the conversion of stable atrial flutter to sustained atrial fibrillation and sustained atrial fibrillation to stable atrial flutter in seven dogs with sterile pericarditis. Six episodes were spontaneous and seven were ATP related. All episodes were studied by using a multisite mapping system to record 190 unipolar electrograms (converted in the software to 95 bipolar electrograms) from the right atrial free wall along with ECG lead II. Atrial flutter induction was attempted by atrial stimulation (S1S2 or S1S2S3) or by rapid atrial pacing for greater than or equal to 20 beats from selected sites at selected rates. For both the spontaneous and the ATP-related episodes, stable atrial flutter was defined as any episode of greater than or equal to 5 minutes, and sustained atrial fibrillation was any episode of greater than or equal to 1 minute. During all the episodes of stable atrial flutter, a line of functional block with a mean length of 24+/-4 mm was localized on the right atrial free wall. When the previously stable line of functional block decreased to a mean of 16+/-3 mm (P<.05), either spontaneously or after ATP administration (40 mg IV), the new line of functional block was not long enough to maintain stable atrial flutter, and conversion to atrial fibrillation resulted. This shortened line of functional block continued to change and migrate over the right atrial free wall throughout sustained atrial fibrillation. These observations were similar for both spontaneous and ATP-induced conversions. When sustained atrial fibrillation evolved to stable atrial flutter, there was reformation of a long line of functional block, long enough (greater than or equal to prior length) to create a stable reentrant circuit, which then captured the right atrial free wall and subsequently both atria. This increase in the length of the line of functional block always occurred over several beats. In the sterile pericarditis model, conversion of atrial flutter to atrial fibrillation and conversion of atrial fibrillation to atrial flutter are closely related phenomena. Changes in the length of the line of functional block in the right atrial free wall are critical for these conversions.