Mobitz I (Wenckebach): AV node block, progressive PR prolongation
Mobitz I (Wenckebach) is second-degree AV block at the level of the AV node itself. The ECG hallmark is progressive PR interval prolongation with each beat until a P wave is not followed by a QRS complex (the dropped beat), after which the cycle resets and PR shortens back to baseline.
The PR interval before the dropped beat is the longest; the PR interval after the dropped beat is the shortest. The RR interval progressively shortens (somewhat counterintuitive — even though PR is getting longer, the RR interval shortens because the increment of PR prolongation with each beat decreases). A group-beating pattern results from the repeating cycles of PR lengthening and dropping.
Clinical significance: Mobitz I is usually benign. It commonly occurs with inferior MI (RCA territory ischemia of the AV node), during sleep (increased vagal tone), in well-trained athletes, and with inferior pericarditis. It rarely requires pacing, generally does not progress to complete heart block, and typically resolves when the underlying cause is addressed. Monitor closely and reassess.
Mobitz II: infranodal block, sudden QRS dropping, pacemaker urgency
Mobitz II is second-degree AV block at an infranodal level — in the bundle of His or the bundle branches. The ECG hallmark is a constant PR interval (no progressive prolongation) followed by a sudden, unexpected dropped QRS complex. The PR interval does not change before the dropped beat — this is the critical distinguishing feature.
Clinical significance: Mobitz II is dangerous. The infranodal location means the block is in the distal conduction system, which is less reliable than the AV node. It carries a high risk of sudden progression to complete (third-degree) heart block, even when the patient appears asymptomatic. This progression can be abrupt and hemodynamically catastrophic — the escape rhythm in complete heart block at the infranodal level is typically a slow ventricular escape at 20–40 bpm.
Urgency: Mobitz II requires urgent cardiology consultation and pacemaker evaluation even when asymptomatic. Symptomatic Mobitz II (syncope, pre-syncope, hemodynamic compromise) requires immediate temporary transcutaneous pacing while waiting for transvenous pacing. Atropine has limited and unpredictable effect on Mobitz II and should not be relied upon.
2:1 AV block: distinguishing Mobitz I from Mobitz II
2:1 AV block presents every other P wave failing to conduct — exactly half the P waves produce QRS complexes. This pattern makes the PR prolongation of Mobitz I impossible to observe (there are no two consecutively conducted beats to compare). Without consecutive conducted beats, the classic Mobitz I criterion (progressive PR prolongation) cannot be assessed.
Approach to 2:1 block: Look at the QRS width. A narrow QRS suggests block at the AV node level (Mobitz I more likely). A wide QRS suggests block at the infranodal level (Mobitz II more likely). Look for Wenckebach sequences elsewhere in the tracing — periods of 3:2 or 4:3 block with typical PR prolongation. Electrophysiology study is definitive if the site of block cannot be determined from the surface ECG and the clinical decision is high stakes.