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

Definition of Mobitz I block (Wenckebach phenomenon)

Progressive prolongation of the PR interval culminating in a non-conducted P wave:

  • PR interval is longest immediately before dropped beat
  • PR interval is shortest immediately after dropped beat

ECG Wenckebach Phenomenon
AV block: 2nd degree, Mobitz type I
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

Mechanism
  • Mobitz I is usually due to reversible conduction block at the level of the AV node
  • Malfunctioning AV nodal 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 of Wenckebach Phenomenon

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

ECG Examples
Example 1
ECG Wenckebach AV block 1

Mobitz I AV block

  • Progressive prolongation of PR interval, with a subsequent non-conducted P wave
  • Repeating 5:4 conduction ratio of P waves to QRS complexes
  • Relatively constant P-P interval despite irregularity of QRS complexes

The first clue to the presence of Mobitz I 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.

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


Example 2
ECG Wenckebach AV block Mobitz 1

Mobitz I AV block

  • QRS complexes clustered in groups, separated by non-conducted P waves.
  • The P:QRS conduction ratio varies from 5:4 to 6:5.
  • Note the difference in PR interval between the first and last QRS complex of each group.

Example 3
ECG Inferior STEMI RV infarct Wenckebach

Mobitz I AV block associated with inferior STEMI and RV infarction

  • The majority of the rhythm strip shows 2:1 AV conduction, which makes discerning the type of block difficult (i.e. it could represent Mobitz I or Mobitz II)
  • However, there is a single 3:2 Mobitz I cycle visible in the middle of the rhythm strip (QRS complexes 5 + 6). If you look hard, you can see a non-conducted P wave deforming the downslope of the T wave in complex 6
  • Continuous rhythm strip recording revealed that this patient was indeed in Mobitz I AV block

AV block may occur in the context of an inferior STEMI due to ischaemia of the AV node, or due to increased vagal tone (Bezold-Jarisch reflex) 


An Interesting Case of Wenckebach
ECG wenckebach atrial pacing

Mobitz I in a patient with atrial pacing 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

Diagnosis Wenckebach?

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

Wenckebach AV block with Wenckebach SA block
  • 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 ECG exigency 18.2 for answers and explanations to these questions.


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Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

MBBS (UWA) CCPU Emergency Medicine Advanced Trainee based in Melbourne, Australia. Co-author of the LITFL ECG Library. Likes Ultrasound, Echo, ECGs, and anything and everything with caffeine. Part of the 2021 ANZCEN Clinician Educator Incubator Programme | @rob_buttner | ECG Library |

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