Progressive lengthening of the PR interval followed by a dropped QRS complex characterizes which AV block?

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Progressive lengthening of the PR interval followed by a dropped QRS beat is the hallmark of Wenckebach (Mobitz I) AV block. Here, conduction through the AV node becomes increasingly sluggish from one atrial impulse to the next (decremental conduction), so the PR interval lengthens until one atrial impulse fails to conduct to the ventricles, producing a nonconducted P wave and a missing QRS. After that dropped beat, the cycle starts again with a shorter PR interval.

This pattern points to an AV nodal level of block and is often benign, sometimes related to increased vagal tone or medications. Other blocks differ: a constant, prolonged PR with every beat means first-degree AV block; a sudden dropped QRS without prior PR lengthening points to Mobitz II (usually below the AV node and more concerning); complete dissociation with no fixed relationship between P waves and QRS indicates third-degree AV block.