MECHANISMS OF ENTRAINMENT OF HUMAN COMMON FLUTTER STUDIED WITH MULTIPLE ENDOCARDIAL RECORDINGS

Background The mechanisms of common atrial flutter entrainment have no t been directly studied in humans. Methods and Results Endocardial map ping in six cases of common flutter showed large right atrial (RA) ree ntry circuits. Activation was craniocaudal in the anterolateral right atrium and caudocranial in the septum. The inferior vena cavatricuspid isthmus (IVC-TV) closed the circuit. The high right atrium was paced at progressively shorter cycle lengths (CLs) in all, and the IVC-TV wa s paced in three cases. We recorded six to eight simultaneous RA elect rograms from septum and anterior wall. Transient entrainment was recog nized from an sites by capture of all electrograms at two or more pace d CLs, with total or partial preservation of baseline flutter sequence and return to baseline after pacing. Antidromic circuit penetration w as documented in five cases during high RA pacing and in one with IVC- TV pacing. Short CLs induced orthodromic conduction delays that result ed in a postpacing pause longer than basal flutter CL. ECG fusion with high RA pacing correlated poorly with antidromic septal penetration. This was related to overlap of orthodromic septal activation with ante rior wall activation of the following cycle. Pacing disorganized flutt er into a brief irregular rapid rhythm in two cases and atrial fibrill ation in one case. In two cases, complete antidromic septal penetratio n led to sudden flutter interruption, and in another case it led to ci rcuit inversion. Conclusions Direct recordings confirm orthodromic and antidromic penetration of flutter circuits by high and low RA pacing. Short CLs modify the circuit. Disorganization is the most common mode of flutter interruption.