If the post-pacing interval equals the tachycardia cycle length, what does this indicate about the pacing site?

If the post-pacing interval equals the tachycardia cycle length, what does this indicate about the pacing site?

• A. It is within the reentrant circuit
• B. It is far from the circuit
• C. It is outside the circuit
• D. It cannot sustain arrhythmia

Answer: (A)

The key idea is entrainment during a reentrant tachycardia and what the post-pacing interval tells you about the pacing site. When you stimulate during VT from a site inside the reentrant circuit, the impulse rides around the loop and returns to the pacing site after one tachycardia cycle. The next tachycardia beat you record at that site then occurs at about the same interval as the intrinsic tachycardia cycle length, so the post-pacing interval closely equals the tachycardia cycle length. This equality indicates the pacing site is within the reentrant circuit, often on the critical isthmus. If the pacing site were outside the circuit, the impulse would have to travel to the circuit and back, adding extra conduction time, so the post-pacing interval would be longer than the tachycardia cycle length.

The key idea is entrainment during a reentrant tachycardia and what the post-pacing interval tells you about the pacing site. When you stimulate during VT from a site inside the reentrant circuit, the impulse rides around the loop and returns to the pacing site after one tachycardia cycle. The next tachycardia beat you record at that site then occurs at about the same interval as the intrinsic tachycardia cycle length, so the post-pacing interval closely equals the tachycardia cycle length. This equality indicates the pacing site is within the reentrant circuit, often on the critical isthmus.

If the pacing site were outside the circuit, the impulse would have to travel to the circuit and back, adding extra conduction time, so the post-pacing interval would be longer than the tachycardia cycle length.