• Skip to main content
  • Skip to secondary menu
  • Skip to primary sidebar
  • Skip to footer

Life in the Fast Lane • LITFL • Medical Blog

Emergency medicine and critical care medical education blog

  • MENU
  • ECG
  • CCC
  • Top 100
  • PODCASTS
  • JOBS
  • TOX
  • PART ONE
  • INTENSIVE
Home | ECG Library | AV Block: 2nd degree, Mobitz II (Hay block)

AV Block: 2nd degree, Mobitz II (Hay block)

by Dr Ed Burns, last update May 20, 2019

↪  ECG Library Homepage

AV Block: 2nd degree AV block, Mobitz II (Hay Block)

Intermittent non-conducted P waves without progressive prolongation of the PR interval (compare this to Mobitz I).

  • Intermittent non-conducted P waves without progressive prolongation of the PR interval (compare this to Mobitz I).
  • The PR interval in the conducted beats remains constant.
  • The P waves ‘march through’ at a constant rate.
  • The RR interval surrounding the dropped beat(s) is an exact multiple of the preceding RR interval (e.g. double the preceding RR interval for a single dropped beat, treble for two dropped beats, etc).
ECG Mobitz II Hay AV Block 1
Arrows indicate “dropped” QRS complexes (i.e. non-conducted P waves)

ECG Example of Mobitz II

ECG Mobitz II Hay AV Block 2

Mechanism

  • Mobitz II is usually due to failure of conduction at the level of the His-Purkinje system (i.e. below the AV node).
  • While Mobitz I is usually due to a functional suppression of AV conduction (e.g. due to drugs, reversible ischaemia), Mobitz II is more likely to be due to structural damage to the conducting system (e.g. infarction, fibrosis, necrosis).
  • Patients typically have a pre-existing LBBB or bifascicular block, and the 2nd degree AV block is produced by intermittent failure of the remaining fascicle (“bilateral bundle-branch block”).
  • In around 75% of cases, the conduction block is located distal to the Bundle of His, producing broad QRS complexes.
  • In the remaining 25% of cases, the conduction block is located within the His Bundle itself, producing narrow QRS complexes.
  • Unlike Mobitz I, which is produced by progressive fatigue of the AV nodal cells, Mobitz II is an “all or nothing” phenomenon whereby the His-Purkinje cells suddenly and unexpectedly fail to conduct a supraventricular impulse.
  • There may be no pattern to the conduction blockade, or alternatively there may be a fixed relationship between the P waves and QRS complexes, e.g. 2:1 block, 3:1 block.

Causes of Mobitz II

  • Anterior MI (due to septal infarction with necrosis of the bundle branches).
  • Idiopathic fibrosis of the conducting system (Lenegre’s or Lev’s disease).
  • Cardiac surgery (especially surgery occurring close to the septum, e.g. mitral valve repair)
  • Inflammatory conditions (rheumatic fever, myocarditis, Lyme disease).
  • Autoimmune (SLE, systemic sclerosis).
  • Infiltrative myocardial disease (amyloidosis, haemochromatosis, sarcoidosis).
  • Hyperkalaemia.
  • Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone.

Clinical Significance

  • Mobitz II is much more likely than Mobitz I to be associated with haemodynamic compromise, severe bradycardia and progression to 3rd degree heart block.
  • Onset of haemodynamic instability may be sudden and unexpected, causing syncope (Stokes-Adams attacks) or sudden cardiac death.
  • The risk of asystole is around 35% per year.
  • Mobitz II mandates immediate admission for cardiac monitoring, backup temporary pacing and ultimately insertion of a permanent pacemaker.

Related Topics

  • 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.
  • Eponym: Karel Frederik Wenckebach (1864 -1940). LITFL 2018
  • Eponym: Woldemar Mobitz (1889 – 1951). LITFL 2018
  • Eponym: John Hay (1873 – 1959). LITFL 2018

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

Advanced Reading

  • Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
  • Wagner GS. Marriott’s Practical Electrocardiography 12e
  • Chan TC. ECG in Emergency Medicine and Acute Care
  • Rawshani A. Clinical ECG Interpretation
  • Mattu A. ECG’s for the Emergency Physician
  • Hampton JR. The ECG In Practice, 6e

ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

EKG Library

Share this:

  • Facebook
  • Twitter
  • LinkedIn
  • Print

Related

About Dr Ed Burns

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

Reader Interactions

Leave a Reply Cancel reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Primary Sidebar

  • Twitter
  • Facebook
  • Vimeo
  • LinkedIn
  • Twitter
  • Twitter
  • RSS Feed

Recent Posts

Paediatric Constipation

Pediatric CXR Cases 008

Funtabulously Frivolous Friday Five 301

Creativity in Fiction

Are the Critically Ill Actually Hungry?

Footer

RSS FEED  LITFL posts by EMAIL or RSS

RSS FEED  LITFL Review by EMAIL or RSS

RSS FEED  FFFF by EMAIL or RSS

#FOAMed Medical Education Resources by LITFL is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Based on a work at https://litfl.com

  • Twitter
  • Vimeo
  • Facebook
  • LinkedIn
  • Twitter
  • Twitter

Authors • Blog • Contact • Disclaimer

2018 Launch

  • 22,915,322 visitors

Copyright © 2019 · Powered by vocortex and iSimulate