Which of the following is an EGM characteristic of AVNRT?

• A. CS appears 'stacked'
• B. Long VA interval
• C. Short VA interval
• D. QRS prolongation

Answer: (C)

In AVNRT, the reentrant circuit sits around the AV node, so the atria are activated retrogradely through the fast pathway very soon after the ventricles. This makes the ventriculoatrial (VA) interval during tachycardia very short, often under about 70 ms. That near-synchronous activation between the ventricle and atrium is the hallmark you see on the EGM. So the best choice is a short VA interval.

Long VA intervals would suggest other mechanisms, like orthodromic AVRT with more delayed retrograde conduction or other atrial tachycardias. QRS prolongation isn’t a defining feature of AVNRT unless there’s aberrancy; AVNRT typically has a narrow QRS. The appearance of the coronary sinus signals being closely spaced isn’t the defining diagnostic feature here. The key takeaway is the very short VA interval characteristic.