How can typical AVNRT be distinguished from AVRT with WPW during EP?

• A. AVNRT typically lacks pre-excitation on baseline ECG and shows dual AV nodal physiology; AVRT shows pre-excitation (WPW) on baseline rhythm and uses an accessory pathway for retrograde conduction
• B. AVNRT shows pre-excitation on baseline ECG; AVRT lacks pre-excitation
• C. AVNRT uses accessory pathway for retrograde conduction
• D. AVRT cannot conduct retrogradely

Answer: (A)

The difference rests on the substrates: AVNRT is a reentrant circuit inside the AV node that relies on dual AV nodal pathways (fast and slow) and does not require an accessory tract to reach the ventricle. Because of that, you typically do not see pre-excitation on the baseline ECG, and during EP you look for signs of dual AV nodal physiology to confirm AVNRT.

AVRT with WPW, by contrast, uses an accessory pathway between atria and ventricles. If this pathway conducts antegradely, the baseline rhythm shows pre-excitation (delta wave) and the tachycardia is sustained by retrograde conduction through the normal AV node while the ventricle is activated via the accessory pathway. This combination—pre-excitation on baseline rhythm and retrograde conduction via the accessory pathway during tachycardia—distinguishes AVRT with WPW from AVNRT.

So, the best way to tell them apart during EP is to note baseline pre-excitation and the presence or absence of dual AV nodal physiology: no pre-excitation with dual AV nodal pathways points to AVNRT, whereas pre-excitation with an accessory pathway points to AVRT with WPW.