ECG Library summary of the different types of conduction disturbance. Follow the links below to read more about each type of conduction block.
- First-degree block PR interval >200 msec (1 large square)
- 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 absence of any relationship between P waves of sinus origin and QRS complexes (AV dissociation).
- Left axis deviation
- Q waves in leads I and aVL
- Small R in lead III, in the absence of LVH.
- Right axis deviation
- Small R in lead I
- Small Q in lead III, in the absence of RVH
- QRS > 120 ms
- Dominant R wave in V1
- RSR’ pattern (“M”) in V1 with wide S wave (“W”) in V6 (=MaRRoW)
- QRS > 120 ms
- Dominant S wave in V1
- Deep S wave (“W”) in V1 with slurred R wave (“M”) in V6 (=WiLLiaM)
- QRS > 100 ms, not due to LBBB or RBBB. Most important causes are hyperkalaemia or tricyclic antidepressant poisoning
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
- 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