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In typical atrial flutter, circular activation around the tricuspid ring is possible
because the terminal crest prevents short-circuiting on the posterior wall, and the
myocardium between the inferior vena cava (IVC) and the lower rim of the tricuspid
ring is the obligatory pathway to close the circuit in the low right atrium (RA) (Figure 1). This IVC-tricuspid ring isthmus (cavotricuspid isthmus) has become the preferred
target for ablation because it is the narrowest point of the circuit, it is easily
accessible, and it is located far from the AV junction. Variations of the circuit
all share the cavotricuspid isthmus as the obligatory path in the low RA (Figure 1).
Figure 1Schematic representation of the right atrium in the left anterior oblique view. The
tricuspid orifice is artificially increased to show the endocardium with the openings
of the superior vena cava, inferior vena cava, and coronary sinus. A:Arrows show direction of activation during typical flutter (TFL) and lower loop (LL) macroreentry
closing the circuit through the terminal crest. B: Activation in reverse typical flutter (RTFL). C: Activation sequence pacing the anterior end of the cavotricuspid isthmus before ablation
showing collision of ascending and descending fronts on the septal wall. D: Activation sequence pacing the septal end of the cavotricuspid isthmus before ablation
showing collision of ascending and descending fronts on the anterior wall. E: Activation sequence pacing the anterior end of the cavotricuspid isthmus after isthmus
block showing fully descending activation on the septal wall. F: Activation sequence pacing the septal end of the cavotricuspid isthmus after isthmus
block showing fully descending activation on the anterior wall. G: Activation sequence pacing the septal end of the cavotricuspid isthmus with partial
block (slow conduction) showing fully descending activation on the anterior wall.
H: Block at the eustachian ridge causing delay in arrival of activation to the cavotricuspid
isthmus that can mimic cavotricuspid isthmus block.
Role of right atrial structures as barriers to conduction during human type I atrial flutter. Activation and entrainment mapping guided by intracardiac echocardiography.
