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Rhythm

Second-degree AV block, Mobitz II

Not medically reviewed · Content last updated · 1 min read ·

This is the date the draft was written, not a medical review date.

In Mobitz II block, P waves are intermittently not conducted without prior PR lengthening. The block usually lies below the AV node, in the His-Purkinje system, and carries a significant risk of progression to complete heart block.

ECG criteria

  • Constant PR interval in conducted beats, then a sudden non-conducted P wave.
  • The PR interval before and after the dropped beat is the same.
  • The pause containing the blocked P wave equals twice the underlying P-P interval (RR interval is constant otherwise).
  • QRS is often wide (bundle branch block pattern), reflecting infranodal disease.
  • Fixed ratios (2:1, 3:1) or variable ratios; advanced (high-grade) block shows two or more consecutive non-conducted P waves.

Mechanism

Intermittent all-or-none failure of conduction in the His bundle or both bundle branches, without the decremental delay seen in the AV node. Because the diseased infranodal tissue can fail suddenly and completely, ventricular escape rhythms may be slow and unreliable.

Common causes

  • Anterior myocardial infarction with septal necrosis.
  • Degenerative fibrosis of the conduction system (Lenègre and Lev disease).
  • Cardiomyopathies, infiltrative disease (sarcoidosis, amyloidosis), myocarditis, Lyme disease.
  • After cardiac surgery or catheter ablation, valvular procedures; rarely drug-induced.

Management principles

Mobitz II is treated as a potentially unstable rhythm because of the risk of sudden progression to complete block. Symptomatic patients follow the local bradycardia and advanced life support protocols, and permanent pacing is usually indicated once it is recognised; follow local guidelines.

High-level principles for study. Treatment decisions follow local guidelines and the clinical situation.

Pitfalls and mimics

  • A 2:1 block with narrow QRS is more likely nodal (Mobitz I physiology) and with wide QRS more likely infranodal; the surface ECG alone cannot always decide.
  • Do not diagnose Mobitz II on a single dropped beat after a long PR; look for PR shortening after the pause, which means Mobitz I.
  • Non-conducted premature atrial beats can mimic dropped beats; check whether the P wave is premature.

Practise in the ECG trainer — Recognise this pattern on generated tracings.

References

  1. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. American College of Cardiology / American Heart Association / Heart Rhythm Society (2018) — Tier 2 (guidelines / primary literature)
  2. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. European Society of Cardiology (2021) — Tier 2 (guidelines / primary literature)
  3. Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. Wolters Kluwer, 12th edition (2013) — Tier 3 (textbook)

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