What Is The Left Bundle Branch Block?

The heart is a tireless marvel, beating over 100,000 times a day and propelling approximately 6,000 litres of blood around the body.1 Have you ever stopped to think about the precise workings behind each beat and the delicate orchestration that ensures its flawless synchronised timings? In this article, we’ll explore what keeps our heart pumping and how disruption of this complex process can lead to the occurrence of heart block – more specifically, left bundle branch block (LBBB).

The heart's metronome-like cadence is carefully coordinated by electrical impulses that follow a specific conduction pathway. As these impulses travel, they stimulate the contraction of the areas of heart muscle in sequence and ensure its synchronised movement. The heart is divided into four chambers, consisting of the atria (the upper chambers) and the ventricles (the larger, lower chambers). At the top of the right atrium, there is a group of specialised cells called the sinoatrial node (SAN), often referred to as the ‘heart's intrinsic pacemaker’. Like a master conductor in an orchestra, the SAN sets the tempo (speed) of the heartbeat and initiates the rhythmic performance of each round of this beautiful symphony. 

The spontaneous electrical signal generation, started by the SAN, spreads throughout the right and left atrium, stimulating both atria to contract. This contraction facilitates the movement of blood received by the top chambers to the bottom ventricles. As the blood flows, the signal conduction pathways reach the second group of specialised pacemaker cells called the atrioventricular node (AVN), positioned just at the top of the right ventricle. The AVN, acting as the second in command, has a slower conducting speed in order to provide enough time for the ventricles to fill with blood.1

After reaching the AVN, the electrical impulse travels down the muscular wall that separates the left and right sides of the heart, in a region known as the ‘bundle of His’. The bundle of His then branches into two main pathways, the left and right bundle branches. These branches are further divided into smaller fibres known as Purkinje fibres, which spread throughout the ventricles. The Purkinje fibres play a crucial role in rapidly transmitting the electrical signal, ensuring that both ventricles contract simultaneously and efficiently, pushing the blood out of the heart and into the lungs and the body’s circulatory system. In this manner, the intricate network of electrical conduction pathways in the heart allows for the harmonious coordination of its contractions and, thereby, the continuous circulation of life-sustaining blood throughout the body.2

Causes of left bundle branch block

LBBB is a descriptive name for the condition where there is either a delay or blockage in the electrical conduction pathway of the heart - specifically at the left bundle branch emerging from the ‘bundle of His’ as described above. Left bundle branch block is usually associated with an organic disease such as heart disease, high blood pressure, or inflammation of the heart tissue – whereas right bundle branch block is not.3

In most people, there are no symptoms of LBBB, which is because if one bundle is damaged, the other bundle compensates. However, this compensation causes a delay in the contraction of the ventricles, which can then be observed only by an Electrocardiogram (ECG). However, unusually, some patients do present with symptoms such as acute chest pain, shortness of breath and dizziness. This is rare, and the development of these symptoms is a medical emergency because it potentially indicates heart failure.3

Management and treatment for left bundle branch block

LBBB has no specific treatment and is a permanent, chronic condition; therefore, treatment usually entails healthy lifestyle modifications and management of the underlying condition. However, if an individual has the following circumstances, then cardiac resynchronisation therapy is recommended by the American Heart Association:

  • Heart failure and a reduced ejection fraction – a reduced amount of blood being pumped with each contraction
  • Wider QRS duration – prolonged contraction periods that are now pathological
  • Moderate to severe heart failure symptoms (NYHA class II-IV) –  patients in this category will have a reduced degree of ability to perform daily activities and will often experience symptoms such as shortness of breath and fatigue

Cardiac resynchronisation therapy is not a cure for LBBB.5 The therapy involves using a special pacemaker called a biventricular pacemaker to simultaneously pace the left and right ventricles of the heart. This bypasses the heart’s natural conduction system entirely, which helps improve coordination between the ventricles and assists in the achievement of a better ejection fraction. This means more complete ventricular filling, and thus, a more normal volume of blood is pumped in each heart contraction and beat.4

Diagnosis of left bundle branch block

Left bundle branch block can be diagnosed using an electrocardiogram (ECG), which provides information about the heart rhythm etc. People often discover that they have left bundle branch block after having an ECG for some other reason.3

Assessment will then include a thorough history and physical examination along with another ECG. It might also include the following tests:

  • Obtaining an echocardiogram to determine the rate and nature of blood flow in the heart and the motion of the heart muscles and valves, etc.
  • Cardiac stress testing to look for coronary artery disease or other abnormalities
  • Blood work to assess cholesterol levels and other factors


In healthy individuals, the left bundle branch block (LBBB) does not pose any significant additional risks to health. The risk of mortality is only slightly raised – about 1.3 times higher than normal. However, if a patient develops LBBB suddenly and experiences symptoms such as chest pain, this should be treated with the same urgency as a heart attack. Prompt medical attention is important in such cases.

For patients who are already experiencing heart failure, the presence of LBBB unfortunately increases the likelihood of cardiovascular problems and mortality. It can have a greater impact on individuals with a history of heart failure, potentially leading to more complications and poorer outcomes. Close monitoring and appropriate management are crucial for these patients to optimise their heart health and overall well-being.3


LBBB itself typically does not lead to significant complications in most individuals. However, in some cases, LBBB can be associated with certain complications. One potential complication is an increased risk of developing heart failure or exacerbating existing heart failure.

LBBB can also disrupt the coordinated contraction of the ventricles, leading to reduced pumping efficiency and potentially causing symptoms such as fatigue, shortness of breath, and fluid retention. 

Additionally, LBBB may increase the risk of arrhythmias (irregular heart rhythms) that can disrupt the heart's normal electrical activity. These arrhythmias can result in palpitations, dizziness, and, in rare instances, more serious complications such as fainting or cardiac arrest. While these complications are relatively rare, individuals with LBBB need to be aware of their condition, undergo regular medical monitoring, and seek appropriate medical care if any concerning symptoms arise.6


How can I prevent left bundle branch block?

LBBB is often caused by underlying heart conditions such as heart disease or high blood pressure. While it may not be entirely preventable, adopting a heart-healthy lifestyle can help you reduce the risk of developing such conditions. This includes maintaining a balanced diet, engaging in regular physical activity, avoiding smoking, managing stress levels, and attending regular medical check-ups to monitor your heart health. It is crucial to manage any underlying conditions effectively to minimise the chances of developing LBBB.

How common is the left bundle branch block?

Left bundle branch block (LBBB) is found in approximately 0.06% to 0.1% of the general population. Among individuals with heart failure, around 33% also have LBBB. The incidence of LBBB tends to rise as the severity of left ventricular failure increases in patients with heart failure.3

What can I expect if I have left bundle branch block?

It is important to understand that most people with LBBB do not experience noticeable symptoms or complications. In such cases, LBBB is often an incidental finding during an electrocardiogram (ECG) or other cardiac tests. However, in some individuals, LBBB may be associated with symptoms such as chest pain, shortness of breath, or dizziness. If you do experience these symptoms, it is important to seek prompt medical attention, as they may indicate an underlying heart condition or potential complications associated with LBBB. Your doctor will assess your specific situation and provide appropriate guidance and treatment if necessary.3

When should I see a doctor?

If you have been diagnosed with a left bundle branch block or suspect its presence based on symptoms or cardiac test results, it is important to consult with a healthcare professional. Additionally, seek medical attention immediately if you experience symptoms such as chest pain, difficulty breathing, light-headedness, or fainting. These symptoms could indicate a more serious underlying heart condition or potential complications associated with LBBB. A healthcare provider can evaluate your specific situation, conduct further tests if necessary, and provide appropriate guidance and treatment based on your individual needs.


 Left bundle branch block (LBBB) is a condition in which the electrical conduction mechanism in the heart is disrupted, affecting the coordinated contraction of the ventricles. While LBBB itself doesn't have a specific treatment, management focuses on addressing underlying heart conditions. Cardiac resynchronisation therapy (CRT) may be recommended for certain patients with heart failure and reduced ejection fraction (a decreased volume of blood being pumped with each contraction). Regular medical care, a healthy lifestyle, and close monitoring can help individuals with LBBB lead fulfilling lives while managing any associated symptoms and risks.


  1. How the Heart Works - The Heart | NHLBI, NIH [Internet]. 2022 [cited 2023 Dec 8]. Available from: https://www.nhlbi.nih.gov/health/heart.
  2. Dun W, Boyden PA. The Purkinje cell; 2008 style. J Mol Cell Cardiol [Internet]. 2008 [cited 2023 Dec 8]; 45(5):617–24. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4332524/.
  3. Scherbak D, Hicks GJ. Left Bundle Branch Block. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Dec 8]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK482167/.
  4. Ahmed I, Kayani WT. Biventricular Devices. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Dec 8]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK567773/.
  5. Rao P, Faddis M. Cardiac resynchronisation therapy: current indications, management and basic troubleshooting. Heart [Internet]. 2017 [cited 2023 Dec 8]; 103(24):2000–7. Available from: https://heart.bmj.com/content/103/24/2000.
  6. Søndergaard MM, Riis J, Bodker KW, Hansen SM, Nielsen J, Graff C, et al. Associations between left bundle branch block with different PR intervals, QRS durations, heart rates and the risk of heart failure: a register-based cohort study using ECG data from the primary care setting. Open Heart [Internet]. 2021 [cited 2023 Dec 8]; 8(1):e001425. Available from: https://openheart.bmj.com/content/8/1/e001425.
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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