Entrainment without fusion is known as what?

• A. Concealed
• B. Manifest
• C. Partial
• D. None

Answer: (A)

Concealed entrainment happens when you overdrive pace during a reentrant tachycardia and reset it without producing any fusion on the ECG. The pacing impulse enters the reentrant circuit and drives ventricular activation in the same pattern as the tachycardia, so the QRS morphology looks identical to the tachycardia and no fusion beat is seen. This indicates the pacing site is within or very close to the circuit, and you can still gather diagnostic clues (like the post-pacing interval) to map the pathway. If a fusion beat were visible, that would be manifest entrainment, showing a different QRS morphology due to simultaneous activation by both the pacing impulse and the reentrant wavefront.