Unusual ventricular entrainment response: What is the mechanism?

Entrainment is an important tool for the diagnosis and location of reentry. The usefulness of this maneuver requires that, prior to cessation of ventricular pacing, the atrial rate accelerates to the ventricular pacing rate. Moreover, it is important to verify the continuation of the tachycardia following cessation of entrainment. The recognizing the last entrained atrial beat is utmost important to avoid erroneous A-A-V labeling.