Second-degree AV block

Definition

Second-degree AV block is intermittent failure of atrial impulses to conduct to the ventricles: some P waves are not followed by a QRS, so there are more P waves than QRS complexes. Mobitz I (Wenckebach) shows progressive PR prolongation until a single P wave is blocked and the cycle repeats; with a narrow QRS it most likely reflects block in the AV node, and it is typically benign, in that it does not progress suddenly to complete heart block. Mobitz II shows a suddenly blocked P wave with a constant (non-lengthening) PR before and after it; it usually reflects disease in the His-Purkinje system, is often accompanied by bundle branch block (a wide QRS), and often progresses to complete heart block. 2:1 AV block (every other P wave blocked) cannot be labeled type I or type II from that ratio alone, and high-grade (advanced) block denotes two or more consecutive non-conducted P waves at a constant physiologic atrial rate with some conduction preserved.

Clinical noteIn 2:1 block the ratio alone cannot separate type I from type II, so the level of block is inferred from other clues. QRS width helps but is not decisive: 2:1 block with bundle branch block is often taken to mean block below the AV node, yet 15% to 20% of such patients have block within the AV node. A stretch of the same recording that shows a typical Mobitz II sequence identifies type II.