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

• A. Concealed entrainment
• B. Normal conduction
• C. Extrastimulus
• D. Pacing failure

Answer: (A)

In entrainment mapping of a reentrant tachycardia, the post-pacing interval (PPI) relative to the tachycardia cycle length (TCL) reveals where your stimulus lands. When you pace from a site inside or very close to the reentrant circuit and successfully capture, the circuit reenters with essentially no extra delay, so the interval from the last paced beat to the first return at the pacing site equals the tachycardia cycle length. Because the impulse travels around the circuit without altering the sequence, the QRS during pacing looks the same as the ongoing tachycardia. This is concealed entrainment. Therefore, observing a PPI that equals the TCL indicates concealed entrainment, meaning the pacing site is within the reentrant circuit. If the PPI were longer than the TCL, that would suggest pacing from outside the circuit with added travel time.