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Figure 3 Entrainment from the isthmus entrance. (A) Electrocardiogram recorded during entrainment from the isthmus entrance at FCL–10 and FCL–40 ms compared to the non-paced flutter wave. At FCL–10 the paced flutter wave appears identical to the non-paced flutter wave and was recorded as concealed by three blinded observers. At FCL–40 the paced flutter wave is different from the non-paced flutter wave (note aVL and V1 in particular) and was recorded as manifest entrainment by three observers. (B) Intracardiac electrogram recording during entrainment at FCL–10 ms from the isthmus entrance (TA 1/2). Consistent capture could not be achieved from TA 5/6 on this occasion and the catheter was repositioned so that TA1/2 was now at the isthmus entrance. The PPI minus FCL is 10 ms confirming that this site is within the circuit. The upward arrows indicate the orthodromic activation times from TA 19/20 to TA 7/8 (110 ms), TA 5/6 (120 ms) and TA 3/4 (130 ms). During entrainment there is fusion between the antidromic (downward arrows) and orthodromic wavefronts at TA 3/4, which is now activated slightly earlier (125 ms).
(C) Entrainment at FCL–40 ms from the isthmus entrance. The PPI minus FCL is 20 ms, confirming the site is within the circuit. TA 3/4 and 5/6 are now captured by the antidromic wave front (downward arrows) with associated electrogram morphology change and activation ahead of TA 7/8, which is still orthodromically activated (TA 19/20 to 7/8 activation time 110 ms), though with a slight electrogram morphology change suggesting possible fusion between the orthodromic and antidromic wave fronts at this site. Abbreviations as in Figure 2.
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