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Second-degree AV block, Mobitz I (Wenckebach)

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

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

In Mobitz I (Wenckebach) block the PR interval lengthens progressively until a P wave fails to conduct, after which the cycle repeats. The block is usually within the AV node and often benign, particularly with high vagal tone.

ECG criteria

  • Progressive lengthening of the PR interval in consecutive beats, then a P wave not followed by a QRS.
  • The PR interval after the dropped beat is the shortest of the cycle.
  • RR intervals shorten progressively within each group, and the pause is less than twice the preceding RR interval.
  • Grouped beating, for example 4:3 or 3:2 conduction ratios, giving a regularly irregular rhythm.
  • QRS usually narrow.

Mechanism

Decremental conduction in the AV node means each successive impulse arrives at a more refractory node and is delayed more, until one is blocked and the node recovers. The increments in delay get smaller with each beat, which explains the shortening RR intervals.

Common causes

  • High vagal tone: athletes, sleep.
  • Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone.
  • Inferior myocardial infarction (often transient), myocarditis.
  • Degenerative AV nodal disease, increased intracranial pressure.

Management principles

Asymptomatic Mobitz I usually needs observation and review of contributing drugs. Symptomatic or haemodynamically unstable patients are managed by the local bradycardia and advanced life support protocols, and pacing is reserved for symptomatic or infranodal block; follow local guidelines.

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

Pitfalls and mimics

  • A 2:1 block cannot show progressive PR lengthening and cannot be classified as Mobitz I or II from the rhythm strip alone; a wide QRS suggests an infranodal site.
  • Atrial fibrillation or premature atrial beats can produce grouped beating that mimics Wenckebach periodicity.
  • A longer PR after the dropped beat, or a wide QRS, argues against Mobitz I and towards Mobitz II.

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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