The posterior right atrial transverse conduction capability during typical atrial flutter (AFL) is well known, but its relationship to the anatomical characteristics remains controversial. Methods and Results Thirty-four AFL and 16 controls underwent intracardiac echocardiography after placement of a 20-polar catheter at the posterior block site during AFL or pacing. In 31 patients, the effective refractory period (ERP) at the block site was determined as the longest coupling interval that

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... ted in double potentials during extrastimuli from the mid-septal (SW) and free (FW) walls. The block site was located 3.0-29.0 mm posterior to the crista terminalis (CT) in each AFL and control patient. The CT area indexed to the body surface area was larger in AFL patients than in control patients (16.4±6.5 mm 2 /m 2 vs 11.3±6.4 mm 2 /m 2 , p=0.01), and was positively correlated to age (r=0.34, p=0.02). The ERP was longer in the AFL patients than in controls (SW: median value 600 ms vs 220 [200-253] ms; FW: 280 [230-675] ms vs 215 [188-260] ms, p<0.05 for each). Conclusions A functional block line was located on the septal side of the CT in all patients. A limited conduction capability and age-related CT enlargement might have important implications for the pathogenesis in AFL. (Circ J 2007; 71: 636 -642)