Conduction block across the left mitral isthmus (LMI) seems more challenging to achieve and validate compared with the cavotricuspid isthmus (CTI).

This study sought to investigate the relationship between peritricuspid and perimitral circuit times in the same patient and to compare the difficulty in achieving the CTI and LMI linear lesions.

We retrospectively studied 122 consecutive patients (46 paroxysmal and 76 persistent) admitted for atrial fibrillation ablation or subsequent atrial macroreentry who underwent both CTI and LMI ablation. The peritricuspid and perimitral conduction times were measured after validation of bidirectional block across their respective line by pacing from the septal side of the CTI or LMI and recording of the second late potential on the line of block. Atrial dimensions were measured by standard transthoracic echocardiographic techniques.

The mean peritricuspid and perimitral times were 180 ± 35 ms (range 120 to 300) and 189 ± 42 ms (range 120 to 322), respectively, with a mean difference of 7 ± 32 ms (−70 to 95). The correlation between both circuit times was highly significant (r = 0.621, P < .001). In 84 patients (68%), the perimitral time was within 30 ms of the peritricuspid time. In the remaining patients, only 12 (10% of the total patients) had a shorter perimitral time compared with peritricuspid time. Radiofrequency energy delivered was significantly longer for LMI (15 ± 7 min [range 7 to 33]) compared with CTI (7 ± 4 min [range 3 to 17]) (P = .005).

The peritricuspid and perimitral circuit times are strongly correlated. In 90% of patients, the perimitral conduction time is within 30 ms or longer than the peritricuspid time. In addition, both circuit times are always ≥ than 120 ms. Compared with the left mitral isthmus line, the CTI line is significantly easier to perform.