What describes a typical 'jump' in AVNRT when testing conduction?

• A. On decrementing S2 by 10 ms, the fast pathway becomes refractory and conduction shifts to the slow pathway, with AH increasing by more than 50 ms
• B. The AH interval shortens
• C. There is no change in AH interval
• D. The ventricles capture with a delta wave

Answer: (A)

The idea being tested is dual AV nodal physiology and the abrupt switching that can occur between the fast and slow pathways within the AV node during pacing. In AVNRT, two pathways exist: a fast one with rapid conduction but a longer refractory period, and a slow one with slower conduction but a shorter refractory period. When you deliver a premature atrial impulse (S2) and shorten the interval, the fast pathway can become refractory. The impulse then travels down the slow pathway, which has a longer conduction time, causing a sudden, large increase in the AH interval. This abrupt change, typically an AH jump of more than about 50 ms, is the classic sign of a shift from fast to slow pathway conduction—the “jump.”

So, the best description is: as the premature stimulus is delivered earlier, conduction shifts from the fast to the slow pathway, and the AH interval increases abruptly (often >50 ms). This contrasts with a simple shortening of AH, no change, or findings like a delta wave, which point to other mechanisms.