Sudden QRS morphology change during ventricular pacing in a scar-related isthmus What is the mechanism?

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.