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Figure 4 Entrainment from the isthmus exit. (A) Electrocardiogram recorded during entrainment from the isthmus exit at 10 ms and 40 ms below the flutter cycle length (FCL–10 and FCL–40) compared to the non-paced flutter wave. At both FCL–10 and FCL–40 the paced flutter wave appears identical to the non-paced flutter wave and was recorded as concealed by three blinded observers on both occasions. (B) Intracardiac electrogram recording during entrainment at FCL–40 ms from the isthmus exit (TA 1/2). The PPI minus FCL is 5 ms, confirming that this site is within the circuit. The upward arrows indicate the orthodromic activation time from TA 19/20 to TA 3/4 (80 ms), which is unchanged during entrainment consistent with orthodromic capture of TA 3/4. Allowing for the pacing spike artifact, there is no electrogram morphology change in TA 3/4 either.