Right atrial flutter due to lower loop reentry - Mechanism and anatomic substrates

Background-The mechanisms of an atrial flutter (AFL) that is more rapid and at times more irregular than typical AFL are unknown. Methods and Results-Twenty-nine patients with AFL were studied. Atrial elec trograms were recorded from a 20-pole catheter placed against the tricuspid annulus (TA), with its distal electrodes lateral to the isthmus between th e TA and the eustachian ridge (ER), and from the His bundle and coronary si nus catheters. Atrial extrastimuli were delivered in the TA-ER isthmus duri ng typical AFL. Episodes of a right atrial flutter rhythm that was differen t from typical AFL were induced in 3 patients and occurred spontaneously in 3 patients. This sustained AFL, designated as lower-loop reentry (I;LR), i nvolved the lower right atrium (RA), as manifested by early breakthrough in the lower RA, wave-front collision in the high lateral RA or septum, and c onduction through the TA-ER isthmus. Linear ablation resulting in bidirecti onal conduction block in the TA-ER isthmus terminated spontaneous LLR in 3 patients and rendered LLR noninducible in all patients. The cycle length of LLR was shorter than that of typical AFL (217+/-32 versus 272+/-40 ms, P<0 .01). Alternating LLR and typical AFL in 1 patient resulted in cycle Length oscillation. Conclusions-LLR is a subtype of right atrial flutter and depends on conduct ion through the TA-ER isthmus.