In a person with normal AVN, decrementing S2 by 10 ms during extrastim pacing will cause AH interval to

• A. progressively increase
• B. decrease
• C. remain the same
• D. vary unpredictably

Answer: (A)

The main concept is decremental conduction in the AV node. As the extrastimulus coupling interval (S2) becomes shorter, the AV node conducts more slowly because its conduction velocity decreases with closer-to-refractory intervals. In a normal AV node, this shows up as the AH interval—the time from atrial activation to His bundle activation—lengthening with shorter S2. So decrementing S2 by 10 ms makes the AV nodal conduction time longer, i.e., the AH interval increases progressively. This reflects the predictable, decremental nature of AV nodal conduction. The AH interval would not shorten, remain flat, or vary unpredictably in this normal scenario.