What is the difference between activation mapping and substrate mapping in VT ablation?

• A. Activation mapping identifies sites of earliest activation during VT; substrate mapping identifies scar tissue and slow conduction zones; both complement.
• B. Activation mapping targets scar tissue; substrate mapping targets earliest activation.
• C. Both mappings are identical and interchangeable.
• D. Activation mapping is used only in AF ablation.

Answer: (A)

Activation mapping and substrate mapping are two complementary ways to guide VT ablation. Activation mapping tracks the real-time electrical activation during sustained VT, marking where activation begins earliest relative to the reference timing. That earliest site typically points to the critical isthmus or entrance of the reentrant circuit, so ablating there can disrupt the VT circuit.

Substrate mapping, by contrast, looks for the structural substrate that enables VT. It identifies scar tissue and regions of slow conduction by mapping during sinus rhythm or pacing, highlighting low-voltage areas, fragmented or late potentials, and channels within scar that can carry reentry. Ablation targets these conductive channels or the scar borders to prevent future VT, even when sustained VT isn’t inducible.

These approaches work together: activation mapping provides functional insight into the actual circuit during VT, while substrate mapping reveals the underlying anatomical substrate that makes VT possible. The other choices mix up targets, claim they’re identical, or state activation mapping is used only for AF—each of which isn’t accurate for VT ablation.