Introduction

Have you ever been told about a medical investigation called an electrocardiogram (ECG)? Or been told that you have a condition called ‘left bundle branch block (LBBB)’ and felt unsure about what that meant for you and your body? 

LBBB is a condition caused by blocked or disrupted electrical signals in the left side of your heart.¹ This causes a delay in how fast your heart beats and affects how well it pumps blood to the rest of your body. An electrocardiogram  (ECG) can be used by doctors to identify this change in electrical activity and monitor and manage your condition.²

This article will go through what an ECG is and how doctors and healthcare professionals use it to diagnose a LBBB - i.e the diagnostic criteria - and explore what LBBB really is.

What is an ECG?

An ECG is a simple and painless procedure that is used to measure the heart’s electrical activity.² This includes the rate and rhythm of contractions. Trained medical professionals will stick tabs (electrodes) onto your chest and different parts of your body, connecting the ‘leads’ of the ECG. This records your heart’s electrical activity, and the results are usually printed out onto a graph-like piece of paper that is later read and interpreted by doctors, GPs and specialists to gain insight into the inner workings of the various parts of the heart.² 

So what is an LBBB?

Before we try and understand what LBBB is, we must first understand the anatomy of a normal heart and how the electrical impulse of the cardiac muscles (cardiomyocytes) is normally conducted. The heart is made up of four chambers. The two chambers on top are called atria, and the bottom two are called ventricles. These are then further divided into the left and right sides.³

During each heartbeat, the cardiomyocytes become electrically excitable. An easy way to understand how electrical activity passes through the heart cells is you imagine them as a row of dominoes. When one of these cardiomyocytes becomes electrically charged up and ‘excitable’, it passes this impulse on to the next and so forth as the domino ‘falls’. This is what is meant by a ‘wave of depolarisation’.⁴ This wave begins at the sinoatrial node (SAN) within the atria and spreads to the atrioventricular node (AVN).⁴ It then travels to the bundle of His, where it is split into the left bundle branch (LBB) and the right bundle branch (RBB). The electrical impulses then travel across smaller networks into the ventricular walls and cause ventricular contraction.⁴ This results in blood from the left ventricle (LV) moving to the aorta to be pumped around the body and blood from the right ventricle (RV) flowing through the pulmonary artery to the lungs.³ 

LBBB occurs when there is a block in the conduction of electrical signals along the left side of the heart.^1,5 This prevents the electrical impulse from travelling along the LBB to the LV. In LBBB, the signal is forced to travel to the LV the long way, along the right side instead. This results in a delay in the contraction of the left side of the heart, particularly the LV.^1,5 ECGs in the clinic can identify this change and be further used for diagnosis and disease management. 

What causes LBBB?

Now that you've gone through what LBBB is, you may be wondering what is the cause of it. LBBB can arise as either a primary disease or as a subsequent consequence to another condition. A recently published article identified that LBBB is most often associated with structural heart diseases such as congenital heart diseases (CHD), cardiomyopathies (diseases of the heart muscle, including heart failure) and heart valve disease.⁵

Are there any symptoms? 

In most cases, LBBB typically presents without any obvious symptoms (asymptomatically); however, it can be associated with other cardiac conditions that do. Littmann et al⁶ found that as LBBB causes the ventricles of the heart to beat out of pattern, this asynchronous rhythm leads to stress on the heart and ventricular remodelling (a change in ventricular structure). As the structure of the heart changes to try and compensate for its inability to pump the normal amount of blood out, some added stress is put on other organs. 

• Symptoms may include:⁶ Fatigue
• Dizziness
• Syncope (fainting)
• Chest pain (angina)
• Shortness of breath

Diagnosing LBBB on an ECG

Once a LBBB is suspected, doctors must identify specific features on an ECG to confirm your diagnosis and manage your care moving forward. ECGs record the electrical activity of the heart and help identify if the electrical signals are following their normal pathways.² This is aided through the use of multiple ‘leads’, which provide different angles of view of the heart.

A typical ECG reading showing normal ‘sinus’ rhythm with its segments annotated can be seen below: 

Fig. 1: ‘2022 Electrocardiogram’ taken from Wikimedia Commons 

As discussed, LBBB is an abnormal conduction pathway causing delayed impulses on the left side of your heart.^1,5 The changes in how the electrical impulse is conducted are shown on an ECG through distinct features such as abnormal lengths or prominences of various ECG segments, complexes, or waves. By recognising such features, doctors can investigate the underlying cause and decide on the proper treatment.⁷ These are as follows:  

QRS duration > 120ms

The QRS complex represents ventricular depolarisation, or in other words, the electrical activity of the ventricles and how quickly they are able to contract.⁴ As discussed previously, in LBBB, the electrical impulse travels down to the LV via another longer pathway, as the LBB has been blocked. This means contraction takes longer, resulting in a longer and wider QRS complex.⁷

Dominant S wave in V1 and broad R wave in lateral leads

The S wave within the QRS complex represents the final phase of ventricular depolarisation (the electrical impulse reaches the lower chambers of the heart, making the cardiomyocytes electrically ’excitable’).⁴ As a wave of depolarisation continues along the left side of the heart, it creates a wide and deep S wave seen in the right-sided leads (V1-V2) as well as a broad R wave seen in lateral leads (V5-V6).⁷

Absence of Q waves in lateral leads 

The Q wave signifies the initial phase of ventricular depolarisation and typically a left-to-right depolarisation of the interventricular septum (a wall of partially insulated cardiac tissues that separates the left and right ventricles)^4,8.  However, in cases of LBBB where the electrical impulse along the LBB has been blocked, there is a left-to-right activation instead, suggesting the ventricular septal activation has been reversed, manifesting as inhibited lateral Q waves.⁹

Prolonged R wave > 60ms in leads V5 -V6

The R wave is an upward deflection and shows depolarisation of the main mass of the heart (mostly the LV).⁴ Similar to the reasoning behind the prolonged QRS complex, the delayed pathway in which the electrical impulse travels and reaches the LV causes a prolonged R wave peak time. 

Should I be worried?

Finding out that you have LBBB can be alarming and confusing for many. Its degree of severity largely depends on your overall health, any co-existing medical conditions and if you've been experiencing symptoms such as those listed above. While the prognostic significance of LBBB requires more work, a recent study highlighted the gradual nature of LBBB and how it can remain stable for many years in asymptomatic healthy individuals with no significant cardiac-related condition.⁵

However, whilst some research claims LBBB to be a stable diagnosis, especially when asymptomatic, other studies have found LBBB to be generally associated with an increased risk of major adverse cardiovascular events (MACE,) including related death and the worsening of associated conditions in those with pre-existing disorders such as heart failure.^10,5

What happens next?

If you’ve been diagnosed with LBBB, the next steps usually involve investigating how well your heart functions and whether any other abnormalities may be present. The National Health Service (NHS) have identified alternative scans or tests following an ECG that they can conduct to diagnose as well as monitor the extent of your LBBB. 

These include:

• An echocardiogram (‘echo’) or MRI scan - to assess structural abnormalities
• Exercise/’stress’ tests - to assess how well the heart functions under strain
• Blood tests - to check and monitor underlying health and related biomarkers

By using an echocardiogram to capture a moving picture of your heart, doctors and specialists can find out whether the LBBB may be due to other causes like cardiomyopathies, valvular disease and any structural changes.⁶ If you haven’t been experiencing any of the associated symptoms, there may be no need for further investigations or any invasive treatment. 

What’s important is that you and your doctor work together to monitor your heart and try to prevent further disease progression and worsening of any symptoms.⁶  

FAQs

Can LBBB go away on its own?

No, in most cases, LBBB doesn't ‘go away’ on its own.¹ Whilst it's true the symptoms can be intermittent (they come and go), researchers haven’t yet found a cure to mend the block completely. Nevertheless, LBBB can be managed and remain stable for many years.¹

Can LBBB worsen over time?

Yes, LBBB can worsen over time, especially if caused by other cardiac conditions.⁵ It is important to see your doctor regularly, monitor your conditions and let them know immediately if there’s been any change in symptoms or quality of life. 

Lifestyle changes such as exercise and eating a heart-healthy diet can help further manage underlying or related conditions.

Can I live a normal life with LBBB?

Many people with LBBB live normal lives. If there are no other cardiac-related conditions, the risk of mortality and cardiac-related morbidity is very low.⁵

Summary

LBBB is a heart condition caused by blockage of the electrical impulses in the left side of the heart, causing the left side of the heart, particularly the LV, to contract more slowly than the right.¹ This change in electrical activity is detected on an ECG and recognised by doctors and specialists by its distinct features, such as a widened QRS complex and a deep S wave in the right-sided leads.^7,9 By correctly identifying LBBB, doctors can begin to investigate any underlying heart problems and try to find the cause behind the diagnosis. While some people might not develop any symptoms, others can experience some cardiac-related complications like heart failure or stroke.⁵ Regular monitoring and correct treatment can help manage the condition and reduce any risks. With the proper support and management, you can still live a good and fulfilling life despite an LBBB diagnosis.