Assessment of Clockwise Cavotricuspid Isthmus Block Based on Conduction Times during Transient Entrainment: A Prospective Study

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In typical counterclockwise atrial flutter (AFL), the route of impulse propagation to anteroinferior right atrium (AIRA) during transient entrainment (TE) from the coronary sinus (CS) is expected to be similar to that during pacing from the same CS site during sinus rhythm (SR) when cavotricuspid isthmus (CTI) block has occurred. This could be used to identify CTI block during ablation procedures.


Thirty-six patients with AFL (cycle length [CL], 240 ± 25 ms) underwent CTI ablation during AFL. CS pacing was performed at a CL of 20 ms less than AFL CL before ablation (n = 36), and at several CL during SR with conduction through the CTI (n = 21) and after CTI block (n = 36).


TE with orthodromic activation of AIRA occurred in all 36 patients. Conduction time from CS to AIRA during TE (T-entr, 199 ± 29 ms) was significantly longer than during pacing in SR (T-CTI) at the same rate not only with CTI conduction (T-CTI-C, 135 ± 24 ms, P < 0.001), but also with CTI block (T-CTI-B, 186 ± 24 ms, P < 0.01). T-entr did not correlate with T-CTI-C, but there was an excellent correlation between T-entr and T-CTI-B (r = 0.874, P < 0.001). A “TE index” that corrected T-CTI for individual T-entr identified CTI block with 97% sensitivity and 91% specificity. T-CTI at low rates differed from T-CTI at high rates but correlated significantly with them.


Comparison of conduction times during TE from the CS and during pacing from the same site and rate in SR can help to establish whether clockwise CTI block has been achieved in patients with typical AFL.

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