Which finding defines Second Degree AV Block Type I?

• A. Dropped QRS with no PRI prolongation
• B. PRI lengthens until a QRS is dropped
• C. Complete dissociation of P waves and QRS
• D. Normal conduction with occasional blocked beats

Answer: (B)

Progressive slowing of AV nodal conduction with a subsequent nonconducted P wave is the hallmark of Second Degree AV Block Type I. On the ECG you’ll see the PR interval lengthen step by step from beat to beat until a P wave is not followed by a QRS complex, and then the cycle starts over with a shorter PR interval. This pattern shows the block is at the AV node and is usually more benign, often related to vagal tone or certain drugs. The other patterns don’t fit: a dropped QRS without PR prolongation points to Mobitz II, complete AV dissociation indicates third-degree block, and normal conduction with occasional blocked beats lacks the characteristic progressive PR lengthening.