‍Introduction

Left bundle branch block (LBBB) is a pattern seen on the surface ECG that results from interruption of normal electrical activity of the left ventricle. 

We can divide LBBB by QRS duration to:

• Incomplete LBBB - QRS complex widening to 100 – 120ms and loss of the septal q wave in leads I and V5-6. There is a slowing or delay of conduction through the left bundle, or diffusely through the Purkinje system and the ventricular myocardium

• Complete LBBB – QRS duration > 120 ms. Complete block of stimulus in the left bundle or within the left ventricular Purkinje network. Activation is initiated by conduction through the septum from the right-sided Purkinje system in a right to left direction (opposite to the normal activation – left to right)
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• This delay in left ventricular activation causes interventricular dyssynchrony, with the right ventricle contracting before the left ventricle, which has a deleterious effect on left ventricular function

‍ECG features

• QRS duration > 120ms, slurring in the mid-portion of the QRS
• QS or rS pattern in V1 – usually deep and abnormal waves
• Single broad R wave in leads I, aVL and V5,V6 
• aVR – a QS pattern with a positive T wave
• ST and T wave abnormalities - T wave inversions and ST segment depression, in the opposite direction from the QRS complex
• Absent Q wave in lead V6 – septum depolarizes from right to left
• Leftward QRS axis

Figure 1 QRS morphology in leads I, aVL, V5 and V6

• monomorphic broad R waves
• Prolonged time to peak > 60 ms in lead V6 (delayed intrinsicoid deflection)

Figure 2 QRS morphology in leads V1-V2

• deep S waves with a small r wave and positive T wave
• The electrocardiographic changes in LBBB can cause diagnostic problems in a variety of clinical conditions such as myocardial ischemia or ventricular hypertrophy.

ECG 1 Left bundle branch block (LBBB)

• sinus rhythm with 1st degree AV block and Left bundle branch block
• QRS duration 160ms
• PR interval 240ms
• broad monophasic R waves in leads I, aVL and V6
• deep broad S waves in leads V1-V4
• Axis deviation to the left – 32°

ECG 2  Left bundle branch block (LBBB)

• sinus rhythm with left bundle branch block, rate 88 bpm
• QRS duration 122 ms
• QS waves in leads V1, V2
• monophasic broad R waves in leads I and aVL
• rSR QRS morphology in lead V6
• Q waves present in the inferior leads II, III, aVF suggest prior inferior wall infarction

Causes

• The prevalence of LBBB increases with age, occurring in less than 1 percent of the general population
• Conduction in the left bundle branch can be compromised by both structural and functional factors:
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  1) Structural heart disease: 
  - usually results from slowly progressive degenerative myocardial fibrosis (hypertension, coronary artery diseases, cardiomyopathies)
  - may result following an acute myocardial insult such as myocardial ischemia or myocardial infarction (approx. 7% of acute MI)
  - may also develop following certain cardiac surgeries or procedures
  
  2) Functional LBBB: 
  - Ashman’s phenomenon – functional RBBB as a result of a long preceding R-R interval following by a short cycle

Management

• The presence of LBBB is almost always an indication of underlying pathology and the patient should be assessed accordingly 
• For asymptomatic patients with an isolated LBBB and no other evidence of cardiac disease, no specific therapy is required
• in patients with LBBB and heart failure, cardiac resynchronization therapy may be indicated

Picture 1 Cardiac resynchronization therapy recommendation as per ESC guidelines

References

1. CAMM, A. J., LÜSCHER, T. F., & SERRUYS, P. W. (2009). The ESC textbook of cardiovascular medicine. Oxford, Oxford University Press
2. Houghton, A. (2019). Making Sense of the ECG: A Hands-On Guide (5th ed.). CRC Press.
3. William H Sauer, MD (2020). Right bundle branch block In I. Gordon M Saperia, MD (Ed.), UpToDate. Retrieved February 7, 2021 from: https://www-uptodate-com.ezproxy.is.cuni.cz/contents/left-bundle-branch-block?search=left%20bundle%20branch%20block&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1
4. Ponikowski P, Voors AA, Anker SD, Bueno H, Cleland JGF, Coats AJS, Falk V, González-Juanatey JR, Harjola VP, Jankowska EA, Jessup M, Linde C, Nihoyannopoulos P, Parissis JT, Pieske B, Riley JP, Rosano GMC, Ruilope LM, Ruschitzka F, Rutten FH, van der Meer P; ESC Scientific Document Group. 2016 ESC Guidelines for the diagnosis and treatment of acute and chronic heart failure: The Task Force for the diagnosis and treatment of acute and chronic heart failure of the European Society of Cardiology (ESC)Developed with the special contribution of the Heart Failure Association (HFA) of the ESC. Eur Heart J. 2016 Jul 14;37(27):2129-2200. doi: 10.1093/eurheartj/ehw128. Epub 2016 May 20. Erratum in: Eur Heart J. 2016 Dec 30;: PMID: 27206819.