In typical AVNRT, what happens during the jump when conduction shifts from the fast pathway to the slow pathway?

• A. AH Interval Suddenly Increases
• B. AH Interval Shortens
• C. PR Interval Prolongs Without AH Change
• D. QRS Widening

Answer: (A)

The main idea is that AV nodal reentry often involves two pathways inside the AV node: a fast pathway and a slow pathway, each with different conduction times. During the jump, the impulse shifts from the fast pathway to the slow pathway. The slow pathway takes longer to conduct from the atria to the His bundle, so the AH interval—the time from atrial activation to His activation—increases abruptly. On the surface ECG this can translate to a sudden lengthening of the PR interval, while the QRS timing stays the same since ventricular activation isn’t altered. That abrupt AH prolongation is the telltale sign of the jump from fast to slow pathway.