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
- 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
- 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
- Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone
- Increased vagal tone (e.g. athletes)
- Inferior MI
- Following cardiac surgery (mitral valve repair, Tetralogy of Fallot repair)
- 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
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.
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.
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
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“
This ECG rhythm strip was originally featured on this page as an example of Wenckebach AV block. Can you spot the “deliberate” mistake?
- 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.
- 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)
- Eponymythology History of Second-degree AV block. LITFL 2018
- Eponym Karel Frederik Wenckebach (1864 -1940). LITFL 2018
- Eponym. Woldemar Mobitz (1889 – 1951). LITFL 2018
- Eponym. John Hay (1873 – 1959). LITFL 2018
- Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
- Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
- Rawshani A. Clinical ECG Interpretation ECG Waves
- Smith SW. Dr Smith’s ECG blog.
- Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
- Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
- Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
- Hampton J. The ECG Made Practical 7e, 2019
- Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
- Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
- Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004
- Smith SW. The ECG in Acute MI. 2002 [PDF]
LITFL Further Reading
- ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis – ECG interpretation in clinical context
- ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
- 100 ECG Quiz – Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS – the best of the rest