We present an example of sudden modification in QRS morphology during ventricular pacing inside a scar-related isthmus. This is explained by a "concealed" sudden block in both the orthodromic and antidromic wavefront directions, allowing then the activation to proceed through a now overt antidromic conduction.
Keywords: Entrainment; Isthmus; Pace-mapping; Reentry circuit; Ventricular tachycardia.
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