AV Block: 2nd degree, Mobitz I (Wenckebach Phenomenon)

Definition

• Progressive prolongation of the PR interval culminating in a non-conducted P wave
• The PR interval is longest immediately before the dropped beat
• The PR interval is shortest immediately after the dropped beat

Other Features

• The greatest increase in PR interval duration is typically between the first and second beats of the cycle.
• The RR interval progressively shortens with each beat of the cycle.
• The Wenckebach pattern tends to repeat in P:QRS groups with ratios of 3:2, 4:3 or 5:4.

Example of a typical Wenckebach sequence

• Arrows mark the first beat in each Wenckebach cycle
• Non-conducted P waves are hidden in the T wave before each pause

Mechanism

• Mobitz I is usually due to reversible conduction block at the level of the AV node.
• Malfunctioning AV node cells tend to progressively fatigue until they fail to conduct an impulse. This is different to cells of the His-Purkinje system which tend to fail suddenly and unexpectedly (i.e. producing a Mobitz II block).

Causes

• Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone
• Increased vagal tone (e.g. athletes)
• Inferior MI
• Myocarditis
• Following cardiac surgery (mitral valve repair, Tetralogy of Fallot repair)

Clinical significance

• Mobitz I is usually a benign rhythm, causing minimal haemodynamic disturbance and with low risk of progression to third degree heart block.
• Asymptomatic patients do not require treatment.
• Symptomatic patients usually respond to atropine.
• Permanent pacing is rarely required.

An Interesting Case of Wenckebach

Mobitz I in an atrially-paced patient following mitral valve surgery

• Small atrial pacing spikes precede the QRS complexes.
• The interval between the pacing spikes increases progressively until there is a non-conducted pacing spike.
• To find out the story behind this ECG, check out this chapter from the ECG Exigency series:”Post-op Pacing Puzzler“

Related Topics

• AV block: 1st degree
• AV block: 2nd degree, Mobitz II
• AV block: 2nd degree, “fixed ratio blocks” (2:1, 3:1)
• AV block: 2nd degree, “high grade AV block”
• AV block: 3rd degree (complete heart block)

Further Reading

• ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
• ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
• ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
• ECG Exam Template — a framework for the FACEM part 2 exam.
• ECG Reference Sites on the WEB — the best of the rest

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