The nurse must differentiate Wenckebach (Mobitz Type I) from Mobitz Type II and understand the electrophysiologic basis for their differing clinical significance. In Wenckebach block, decremental conduction occurs at the level of the AV node (compact node, transitional cells). Each successive impulse encounters AV nodal cells in a progressively less recovered state (relative refractory period), resulting in slower conduction (longer PR interval) until one impulse arrives during the effective refractory period and is completely blocked. The AV node has rich autonomic innervation: parasympathetic (vagal) fibers release acetylcholine which hyperpolarizes AV nodal cells via IKACh current, slowing conduction; sympathetic fibers release norepinephrine which enhances conduction via ICaL current augmentation. This is why atropine (anticholinergic) and isoproterenol (beta-agonist) improve AV nodal conduction in Wenckebach. ECG features distinguishing Wenckebach from Mobitz II: Wenckebach shows grouped beating, progressive PR prolongation, the LONGEST PR interval precedes the dropped beat, the SHORTEST PR interval follows the dropped beat, and R-R intervals progressively SHORTEN before the dropped beat (because the increment of PR prolongation decreases with each beat). The QRS is typically narrow (less than 120 ms)...