ECG Diagnostic criteria
- QRS duration > 120ms
- Dominant S wave in V1
- Broad monophasic R wave in lateral leads (I, aVL, V5-6)
- Absence of Q waves in lateral leads
- Prolonged R wave peak time > 60ms in leads V5-6
Associated features include:
- Left axis deviation (LAD);
- Poor R wave progression in precordial leads, and
- Appropriate discordance (discussed below)
In normal cardiac conduction, impulses travel equally down the left and right bundles, with the septum activated from left to right and the formation of small Q waves in lateral leads
- In LBBB, conduction delay means that impulses travel first via the right bundle branch to the RV, and then to the LV via the septum
- Septal activation is thus reversed eliminating lateral Q waves
- The overall depolarisation vector from the right to left ventricle produces tall R waves in lateral leads (I, V5-6) and deep S waves in the right precordial leads (V1-3). The delay between activation of the RV and LV produces the characteristic “M-shaped” R wave seen in lateral leads
- Delayed overall conduction time to the LV extends the QRS duration to > 120 ms
ECG QRS Morphology
QRS Morphology in the Lateral Leads
The R wave in the lateral leads may be either “M-shaped”, notched, monophasic, or an RS complex
QRS Morphology in V1
The QRS complex in V1 may be either:
- rS complex (small R wave, deep S wave)
- QS complex (deep Q/S wave with no preceding R wave)
What about the ST elevation?
- Appropriate discordance refers to the fact that abnormal depolarisation should be followed by abnormal repolarisation, which appears discordant to the preceding QRS complex
- Lateral leads with tall, broad R waves will often have associated ST-segment depression and T-wave inversion, and those with deep S waves can have an allowable amount of ST elevation that does not indicate ischaemia (generally viewed as < 25% of the size of the preceding S wave)
- Any concordant ST segment change is concerning for ischaemia. For further reading, see LITFL: Sgarbossa Criteria
Causes of Left Bundle Branch Block
It is unusual for LBBB to exist in the absence of organic disease. Causes are varied and include:
- Aortic stenosis
- Ischaemic heart disease
- Dilated cardiomyopathy
- Anterior MI
- Lenègre-Lev disease: primary degenerative disease (fibrosis) of the conducting system
- Digoxin toxicity
New LBBB in the context of chest pain was once considered a “STEMI-equivalent” and part of the criteria for thrombolysis. However, more up-to-date data suggests that chest pain patients with new LBBB have little increased risk of acute myocardial infarction at the time of presentation.
Practice has now evolved to examining for excessive discordance or concordant ST segment changes indicative of infarction.
ECG Examples of LBBB
Broad notched R waves are best appreciated in leads aVL and I here. There is absence of Q waves in leads V5-6.
LBBB with AF. Note deep S waves in leads V1-3 and tall broad R waves laterally. Appropriate discordance is present.
Incomplete LBBB is diagnosed when typical LBBB morphology is associated with a QRS duration < 120ms.
- Right ventricular paced rhythms will produce a similiar morphology, as impulse conductions originate from the RV and travel across the septum to the LV as is the case in LBBB. Pacing spikes will be present. The same concepts regarding appropriate discordance apply.
- Left ventricular hypertrophy may produce a similar appearance to LBBB, with QRS widening and ST depression / T-wave inversion in the lateral leads.
- Left bundle branch block LBBB
- Right Bundle Branch Block RBBB
- Left anterior fascicular block LAFB
- Left posterior fascicular block LPFB
- Interventricular Conduction Delay IVCD
- Bifascicular block
- Trifascicular block
- Complete Heart block CHB
- Da Costa D, Brady WJ, Edhouse J. Bradycardias and atrioventricular conduction block. BMJ. 2002; 324(7336): 535-538
- Francia P, Balla C, Paneni F, Volpe M. Left bundle-branch block–pathophysiology, prognosis, and clinical management. Clin Cardiol. 2007; 30(3): 110-115
- 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