What is the significance of a PPI equal to TCL during entrainment mapping?

• A. The tachycardia is non-reentrant.
• B. The pacing is outside the circuit.
• C. The pacing catheter lies within the reentrant circuit.
• D. The mapping is unsuccessful.

Answer: (C)

Entrainment mapping uses how the post-pacing interval (PPI) compares with the tachycardia cycle length (TCL) to locate the circuit. If the pacing site lies inside the reentrant circuit, the paced impulse travels around the loop and reenters the site after one full circuit, so the time from the last paced beat to the next tachycardia beat—the PPI—matches the intrinsic tachycardia cycle length. Therefore, a PPI equal to TCL indicates the catheter is within the reentrant circuit, pinpointing the critical area for mapping and potential ablation. If pacing were outside the circuit, the impulse would incur extra conduction time before reentry, making the PPI longer than the TCL.