Which ERP defines the longest S1 S2 interval that fails to conduct to the atrium?

• A. VAERP
• B. AERP
• C. VERP
• D. AVERP

Answer: (A)

The key idea is measuring a refractory period in the direction of retrograde conduction. An ERP is the longest S1–S2 interval that fails to produce a response. To see if the stimulus can conduct back to the atrium, you pace the ventricle with S1 and deliver an extrastimulus S2 to the atrium. If the atrium does not respond, the interval you found is the ventriculo-atrial ERP (VAERP). This specifically defines the longest S1–S2 interval that fails to conduct retrogradely into the atrium, reflecting the refractoriness of the retrograde pathway (AV node and related tissue).

In contrast, atrial ERP (AERP) measures forward conduction from atrium to ventricle, ventricular ERP (VERP) measures forward conduction within the ventricle, and AVERP (when used) refers to aspects of AV conduction but not retrograde atrial conduction. Thus the longest S1–S2 interval that fails to conduct to the atrium is VAERP.