When pacing near a reentrant circuit for entrainment mapping, PPI = TCL indicates?

• A. Concealed entrainment
• B. Activation of the circuit
• C. Pacing capture
• D. Exit block

Answer: (A)

Pacing during a tachycardia to map a reentrant circuit hinges on where your stimulus lands relative to the loop. The post-pacing interval (PPI) compares how long it takes for the pacing stimulus to return to re-enter the circuit with the intrinsic tachycardia cycle length (TCL). When you pace from inside the circuit, the stimulus already sits in the loop and re-enters after one full circuit, so the time to the next beat at the pacing site matches the intrinsic tachycardia cycle length. That yields PPI ≈ TCL and the rhythm looks exactly like the tachycardia on the ECG—no visible fusion. This situation is known as concealed entrainment because the entrainment occurs without changing the surface activation pattern.

If you were pacing from outside the circuit, or at an exit, extra conduction time to reach the loop would make the PPI longer than the TCL, and you’d see different activation patterns or fusion, rather than the seamless, unchanged tachycardia morphology of concealed entrainment.