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Figure 2 Entrainment from the high lateral right atrium (HLRA). (A)Electrocardiogram (ECG) recorded during entrainment from the HLRA at flutter cycle length (FCL)-10 and FCL–40 compared to the non-paced flutter wave. At FCL–10 the paced flutter wave appears identical to the non-paced flutter wave in the limb leads, with minor differences in amplitude and morphology observed in the anterior leads (V1 to V6). Three blinded observers recorded this as concealed entrainment. At FCL–40 the paced flutter wave is clearly different and was recorded as manifest entrainment by all three observers. (B)Intracardiac electrogram recording during entrainment at FCL–10 ms from the HLRA (TA 13/14). The post-pacing interval (PPI) minus FCL is 15 ms, confirming that the HLRA is within the circuit. The upward arrows indicate the orthodromic activation times from TA 19/20 to 17/18 (10 ms) and TA 17/18 to 15/16 (10 ms) are unchanged during entrainment consistent with orthodromic capture of TA 17/18 and 15/16. However, the entrained TA 15/16 electrogram is different in morphology from the non-entrained electrogram, suggesting fusion between the antidromic (downward arrows) and orthodromic(upward arrows) wave fronts at this point. Also shown: surface ECG leads II and III, and distal and proximal coronary sinus (DCS/PCS) recordings. (C) Entrainment at FCL–40 ms from the HLRA. The PPI minus FCL is 20 ms, confirming that the HLRA is within the circuit. TA 15/16 and 17/18 are now captured by the antidromic wave front (solid downward arrow), with associated electrogram morphology change and activation timing ahead of TA 19/20, which is still orthodromically activated with activation timing and morphology not significantly different from spontaneous flutter (dashed downward line from TA 1/2 to TA 19/20). The phenomenon described by Cosio et al. (17) of orthodromic overlap is also evident with orthodromic activation of TA11/12 and 9/10 (by the n wave front) occurring simultaneously with orthodromic activation (by the previous n–1 wave front) of TA 19/20. Surface ECG fusion may be a "fusion" of these two processes (antidromic penetration and orthodromic overlap), though in the present analysis the observation of manifest entrainment was statistically related to the extent of antidromic penetration.
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