During entrainment of ischemic VT with a long stim to QRS and concealed entrainment (PPI-TCL = 0), the catheter is in a region of what?

• A. Fast conduction near the exit
• B. Nonconductive scar
• C. Slow conduction within the isthmus of the VT circuit
• D. Proximal His bundle

Answer: (C)

Entrainment of a reentrant VT behaves differently depending on where you pace. When pacing inside the tachycardia circuit, you often see concealed entrainment—the QRS during pacing looks the same as the VT (no fusion)—and the post-pacing interval approximates the tachycardia cycle length. A long stimulus-to-QRS interval indicates you’re traversing a slow-conduction path. In ischemic VT, the critical slow-conduction area that completes the circuit is the isthmus, the narrow channel of surviving myocardium within scar. Pacing inside this slow isthmus will entrain the VT with concealed fusion and a PPI equal to the TCL, while the stimulus-to-QRS is prolonged due to the slow conduction. Thus the catheter is located in the slow-conduction isthmus of the VT circuit.