Introduction

Our heart pumps with us throughout our lives, beating in perfect unison to deliver vital oxygen and glucose to our cells. In left-bundle branch block (LBBB), this can no longer occur perfectly, thus giving rise to interventricular dyssynchrony (a mismatch in the beating of the left and right ventricles).¹ To further explore these two areas whilst delving into the diagnosis and treatment methods used today, read on to find out more.

A closer look at the heart’s electrical wiring

The heart is a four-chambered vessel with 2 atria that receive blood from outside the heart, and 2 ventricles that pump blood away from the heart. The heartbeat follows this anatomical guidance too, with the electrical innervation/signal beginning in the sinoatrial (SA) node in the right atrium, where all the deoxygenated blood from the body arrives.¹

The electrical wiring of the heart then takes the beat to the atrioventricular node (AV) node, which causes both atria to empty blood into their respective ventricles with some delay to ensure that a maximum amount of blood can reach here. The heartbeat then travels down the ‘interventricular septum’, a wall separating the 2 ventricles, with the heartbeat travelling down the central ‘bundle of His’. Partway through the septum, the fibres of the bundle of His split into 2 parts:¹

•  The left bundle branch - carries electrical signal down the middle wall of the heart to the left ventricle causing it to contract
• A right bundle branch - carries electrical signal down the middle wall of the heart causing the right ventricle causing it to contract

The ventricles both begin contracting at the very bottom of the heart, the ‘apex’, with the electrical signal progressing upwards along the ventricular walls. This allows the heart to pump blood as efficiently as possible in a double-pump fashion, supplying the lungs with deoxygenated blood and the rest of the body with oxygenated blood from the lungs.¹ 

What is left bundle branch block?

The electrical signal moves down the heart from the atria, down the central bundle of His and then to the individual bundle branches that carry the electrical signal to the muscle fibres of the heart. This activation ordinarily activates the left bundle branch fibres just before the right bundle branch, so the heart begins to pump in a left-to-right fashion.

However, if there is an abnormality with the anatomy of the heart, such as issues with the aortic valve, or with cardiac tissue such as in cardiomyopathy or heart failure, left bundle branch block may occur. 10-26% of cardiomyopathy patients display LBBB, with incidence rising to 72% in heart failure patients.^1,2 Occurrence is more common in people assigned male at birth than people assigned female at birth, and white people are generally more susceptible than other races. Incidence increases with age as well, although sometimes asymptomatically (without clinical presentation), with those aged 70 and above experiencing LBBB in 1-5% of cases.¹

There is also a significant correlation between those suffering from LBBB and those with coronary heart disease (CHD) or high blood pressure, indicating that whatever caused these issues may also be responsible for the LBBB. It is important to note that LBBB is not a disease in itself, but an indicator that the heart’s electrical system isn’t conducting correctly.

What is interventricular dyssynchrony?

‘Dyssynchrony’ simply means ‘lack of synchrony’. In the heart, the term interventricular dyssynchrony means that the contraction of the heart’s ventricles is irregular, out of sync with the normal rhythm.¹

In LBBB, the faulty electrical signal means that the left ventricle starts squeezing later than it should, after the right ventricle has already begun.¹ This offers a mechanical disadvantage, meaning that the blood cannot be pumped out of the heart efficiently. Several factors are offset by this change in timing and contribute to this lack of efficiency:e:^1,2

•  Less blood is pumped out of the heart with each heartbeat
• The heart has to work harder and therefore requires more oxygen for fuel 
• Eventually, the increased workload results in the heart changing shape to compensate, with weakening and enlarging (dilation) of the heart muscle over time 

As the left ventricle squeezes later than it should, it is important to note that the dyssynchronous delay doesn’t involve the outer lateral wall of the heart.²

How doctors detect dyssynchrony

Recognising dyssynchrony requires more than spotting LBBB on an ECG. Electrical delay does not always equal mechanical delay, so imaging is key. However, ECGs are often the first step to identifying if something is wrong.

Electrocardiogram (ECG)¹

Things that may appear on an ECG in the case of EBB include:

• An ‘LBBB pattern’: a widened QRS complex and characteristic shape
• ST segment may be elevated, indicating potential of a myocardial infarction 
• R waves greater than 60 milliseconds
• Asymmetrical T wave

This is useful for identifying electrical conduction delay but not actual motion differences.

Echocardiography

Depending on the specific technique used, an ‘echo’ or ultrasound of the heart can measure:^2,3

• Timing of wall motion - how each ventricle contracts in sequence
• Tissue Doppler imaging - detects subtle delays between heart wall segments
• Speckle-tracking strain imaging - measures deformation of the heart muscle for more detailed timing analysis

Cardiac MRI

• Provides high-resolution pictures of the heart’s structure and function
• Can precisely quantify differences in contraction timing between ventricles

Imaging research has identified that not every patient with LBBB shows significant dyssynchrony, and not every patient with dyssynchrony benefits from the same treatments due to the heterogeneity of disease presentation that can occur in different patients.² This makes individual assessment and a patient-specific approach critical.

Treatment: getting the heart back in step

The main intervention for LBBB with significant dyssynchrony is Cardiac Resynchronisation Therapy (CRT).

What is CRT?

CRT revolves around the implantation of a medical device to synchronise the left and right ventricles. This uses a specialised pacemaker with leads placed in both ventricles.⁴ The device sends tiny electrical impulses to both sides of the heart at carefully timed intervals, restoring synchronised contraction.

Benefits of CRT⁴

• Improves amount of blood pushed out of the heart with each beat (ejection fraction)
• Reduces symptoms of heart failure
• Decreases hospitalisations
• Can reverse some structural heart changes (reverse remodeling)

Research shows that CRT works best for patients who:^2,4,7

• Have symptomatic heart failure
• Have a low ejection fraction (≤35%)
• Show wide QRS complexes (especially >150 ms)
• Demonstrate clear mechanical dyssynchrony on imaging

Not all patients are the same

Recent research has changed how we view LBBB and dyssynchrony and its treatment:^2,4

• Some patients with LBBB have minimal mechanical delay and may not need CRT
• Others may have dyssynchronised ventricles without LBBB due to other underlying issues, and may still benefit from pacing

This is why personalised assessment is now emphasised, therapy is guided by imaging and functional testing, not just the ECG

Emerging advances

Medicine is moving toward even more targeted ways to restore synchrony:

His-bundle and left bundle branch pacing

Ordinarily, CRT aims to stimulate ventricular contraction in the correct rhythm. However, newer technology has been stimulating the His bundle and branches directly instead of the ventricles, mimicking the body’s natural electrical conduction pathway and anatomy.⁵

Artificial intelligence in imaging

AI algorithms are being developed which may help to automatically detect subtle dyssynchrony patterns on echocardiograms and MRIs, potentially improving patient selection for CRT and reducing processing times, thus leading to faster treatment.⁶ 

Earlier intervention

Ongoing studies are exploring whether correcting dyssynchrony earlier, before major symptoms develop, might prevent heart failure progression more effectively.

Living with LBBB and dyssynchrony

Being diagnosed with LBBB does not always mean urgent treatment. Many people live symptom-free for years. However, monitoring is important, and you should work closely with your healthcare provider to track any changes. Important steps that you can take include:

• Staying active within safe limits to keep your heart strong
• Eating a heart-healthy diet, low sodium, balanced nutrition, plenty of fruits and vegetables
• Taking medications as prescribed for underlying conditions like high blood pressure or heart failure
• Reporting changes such as new or worsening symptoms like shortness of breath, fatigue, or swelling

Summary

Left bundle branch block is a disruption of the heart’s natural electrical wiring and timing of contraction, thus resulting in the left ventricle squeezing later than the right. This leads to inefficient blood circulation, with downstream changes to the structure of the heart to make up for the irregular rhythm and blood supply. Often, LBBB is a condition associated with pathological changes to the heart, such as cardiomyopathy, high blood pressure, heart failure, or aortic valve stenosis. However, it can also occur without any accompanying symptoms. This irregular contraction can first be identified as changes on an ECT, with further imaging helping to accurately identify the exact parts of the heart affected, as these can differ from case to case. Cardiac resynchronisation therapy can correct the misfiring electrical signals and thus restore the heart’s natural rhythm and help it pump more efficiently. Further treatments are also being explored to help identify interventricular dyssynchrony more quickly and accurately, allowing for more effective treatment. Other strategies also aim to treat LBBB by directly targeting the electrical centres involved in line with the body’s natural pathways.