When pacing near the reentrant circuit for entrainment mapping, a PPI equal to TCL is an observation of what?

• A. Pacing outside the reentrant circuit.
• B. Catheter is within the reentrant circuit.
• C. PPI is unrelated to TCL.
• D. The tachycardia is terminated.

Answer: (B)

Entrainment mapping hinges on where your pacing site sits relative to the reentrant circuit. The post-pacing interval (PPI) is the interval from the last paced stimulus to the next intrinsic tachycardia activation, while the tachycardia cycle length (TCL) is the intrinsic interval of the tachycardia. If you pace from a site inside the reentrant circuit, the paced impulse becomes part of the loop and the next tachycardia activation after you stop pacing returns after exactly one cycle length. In this case the PPI essentially matches the TCL. If you pace from outside the circuit, extra conduction time to reach the circuit and come back lengthens the PPI beyond the TCL. So a PPI equal to TCL indicates the catheter is within the reentrant circuit. This does not by itself indicate tachycardia termination.