ECG Conduction Blocks

ECG Library summary of the different types of conduction disturbance, with links to read more about each type of conduction block

Conduction Blocks

First-degree block

• First-degree block PR interval >200 msec (1 large square)

Second-degree block

• Mobitz Type I (Wenckebach Block): progressive prolongation of the PR interval before the missed QRS complex
• Mobitz Type II (Hay Block): absence of progressive prolongation of the PR interval before the missed QRS complex
• Fixed ratio blocks (e.g. 2:1, 3:1): constant relationship between P waves and QRS complexes (e.g. 2:1 = 2 P waves for each QRS complex).
• High grade AV block: 2nd degree AV block with a high P:QRS ratio, producing a very slow ventricular rate

Third-degree block

• Third-degree block: absence of any relationship between P waves of sinus origin and QRS complexes (AV dissociation)

Fascicular Blocks

Left anterior fascicular block (LAFB)

• Left axis deviation
• qR complexes in leads I, aVL
• rS complexes in leads II, III, aVF
• Prolonged R wave peak time in aVL > 45ms

Left posterior fascicular block (LPFB)

• Right axis deviation
• rS complexes in leads I, aVL
• qR complexes in leads II, III, aVF
• Prolonged R wave peak time in aVF

Right bundle branch block

• QRS > 120 ms
• Dominant R wave in V1
• RSR’ pattern (“M”) in V1 with wide, slurred S wave (“W”) in V6 (=MaRRoW)

Left bundle branch block

• QRS > 120 ms
• Dominant S wave in V1
• Deep S wave (“W”) in V1 with broad R wave (“M”) in V6 (=WiLLiaM)

Bifascicular block

• RBBB plus either LAFB or LPFB

Trifascicular block

• Bifascicular block plus 3rd degree AV block

Interventricular conduction disturbance

• QRS > 100 ms, not due to LBBB or RBBB. Most important causes are hyperkalaemia or tricyclic antidepressant poisoning

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Zimmerman FH. ECG Core Curriculum. 2023
• Mattu A, Berberian J, Brady WJ. Emergency ECGs: Case-Based Review and Interpretations, 2022
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Hampton J, Adlam D. The ECG Made Practical 7e, 2019
• Kühn P, Lang C, Wiesbauer F. ECG Mastery: The Simplest Way to Learn the ECG. 2015
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

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