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

Mobitz I

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 P-P interval remains relatively constant
  • 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 ECG

  • The first clue to the presence of Wenckebach AV block on this ECG is the way the QRS complexes cluster into groups, separated by short pauses (This phenomenon usually represents 2nd-degree AV block or non-conducted PACs; occasionally SA exit block).
  • At the end of each group is a non-conducted P wave; the PR interval progressively increases from one complex to the next.
  • The Wenckebach pattern here is repeating in cycles of 5 P waves to 4 QRS complexes (5:4 conduction ratio).
  • The increase in PR interval from one complex to the next is subtle. However, the difference is more obvious if you compare the first PR interval in the cycle to the last.
  •  The P-P interval is relatively constant despite the irregularity of the QRS complexes.

Thanks to Dr Harry Patterson, FACEM, for providing this ECG. 

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

wenckebach atrial pacing

  • 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

Diagnosis Wenckebach?

The ECG below was originally featured on this page as an example of Wenckebach AV block. Can you spot the “deliberate” mistake?

Mobitz I

  • Q1. What features of Wenckebach AV block are present on this ECG?
  • Q2. What features of Wenckebach are notably ABSENT?
  • Q3. What possible explanations could exist to explain this tracing?

Now read this blog post for the answers and explanations to these questions.

Related Topics

Further Reading

Author Credits

References

  • Hampton, JR. The ECG in Practice (5th edition), Churchill Livingstone 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
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