In VT mapping, what does the isthmus refer to?

• A. Slow conduction through an isthmus, a corridor between scar and normal tissue, forming part of the reentrant circuit.
• B. A rapid conduction pathway near the AV node.
• C. A completely scarred nonconductive area.
• D. A normal conduction pathway in the right ventricle.

Answer: (D)

In VT mapping, the isthmus is a narrow corridor of surviving myocardial tissue that sits between scar and normal tissue and exhibits slow conduction. This slow, channel-like pathway forms a critical part of the reentrant circuit, allowing impulses to travel around the scar and perpetuate VT; interrupting or ablating this isthmus can terminate the arrhythmia. It’s not a rapid conduction pathway near the AV node, which would be a different conduction route; it’s not a completely scarred nonconductive area, which would block conduction entirely; and it isn’t a normal conduction pathway in the right ventricle, since VT circuits in scar-related disease depend on these slow, surviving channels within/around scar tissue.