Mobitz I (Wenckebach) vs Mobitz II
How to tell Mobitz I (Wenckebach) and Mobitz II apart when a board question makes them look alike.
Step 2 CK pairs are in open beta while clinical review finishes. Spot an issue? Tell us.
Dropped beats with unchanged PR interval identify Mobitz II.
Side by side
Progressive PR prolongation followed by dropped QRS indicates Mobitz I block at the AV node.
Fixed PR intervals with sudden dropped QRS complexes indicate Mobitz II infranodal block.
Fatigable AV nodal conduction intermittently fails after incremental delay.
His-Purkinje conduction disease causes abrupt nonconducted beats and progression risk to complete block.
ECG shows grouped beating with gradually lengthening PR interval before a dropped beat, usually with narrow QRS complexes.
ECG shows constant PR interval with intermittent nonconducted P waves, often with wide QRS or bundle-branch disease.
The trap
All second-degree AV blocks are treated as benign Wenckebach, delaying pacing in Mobitz II. Fixed PR with dropped beats should be treated as unstable conduction disease even if symptoms are initially mild.
The repair move
Next best step: start transcutaneous pacing for symptomatic Mobitz II or high-grade block, arrange urgent transvenous pacing if needed, and plan permanent pacemaker placement. Mobitz I without symptoms usually needs observation and correction of reversible triggers such as AV nodal drugs.
Try a free question
CoreA patient with dizziness has ECG showing constant PR intervals and intermittent dropped QRS complexes. Blood pressure is 86/50 mm Hg. What is the next best step in management?
Show answer and explanation
Answer: A. Urgent pacing for symptomatic Mobitz II block. Symptomatic Mobitz II requires immediate pacing because of progression risk and hemodynamic compromise.
Why the other choices are tempting
- B. Wrong. Observation is unsafe in unstable Mobitz II.
- C. Wrong. AV nodal blockers can worsen conduction block.
- D. Wrong. Outpatient care is inappropriate with hypotension.
- E. Wrong. Thrombolysis does not treat AV conduction disease.
Flashcards
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Which PR pattern differentiates Mobitz II from Wenckebach?
Mobitz II has fixed PR intervals before dropped beats.
Why is Mobitz II considered higher risk than Mobitz I?
It is infranodal and can abruptly progress to complete heart block.
Quick answers
What is the fastest way to tell Mobitz I (Wenckebach) from Mobitz II?
Dropped beats with unchanged PR interval identify Mobitz II.
What is the key clue for Mobitz I (Wenckebach)?
Progressive PR prolongation followed by dropped QRS indicates Mobitz I block at the AV node.
What is the key clue for Mobitz II?
Fixed PR intervals with sudden dropped QRS complexes indicate Mobitz II infranodal block.
What is the common trap with Mobitz I (Wenckebach) vs Mobitz II?
All second-degree AV blocks are treated as benign Wenckebach, delaying pacing in Mobitz II. Fixed PR with dropped beats should be treated as unstable conduction disease even if symptoms are initially mild.