Which ERP is defined as the longest S1-S2 that fails to conduct to the atrium?

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

Answer: (A)

The concept here is the retrograde refractory period from the ventricles to the atria. When you pace the ventricle with a train of stimuli (S1) and then deliver a premature stimulus (S2), you’re testing whether the impulse can travel backward through the AV conduction pathway to activate the atrium. The longest S1–S2 interval that still fails to produce atrial activation defines the ventriculoatrial effective refractory period. This is why the term used is VAERP (ventriculoatrial ERP). In some texts this is also called VERP, as it measures the retrograde (ventricle to atrium) refractory property. The other terms refer to foreward-direction refractory periods or different pacing sites (for example, an atrial ERP is measured with pacing at the atrium and looking at retrograde conduction in the opposite direction), so they describe different conduction directions rather than the retrograde ventricle-to-atrium situation described here.