LBBB vs RBBB ECG: Bundle Branch Block

An ECG reveals the heart's electrical conduction system. Right and left bundle branch blocks are visible on ECG and occur when an electrical signal is blocked, preventing the heart from beating normally. The right bundle branch block (RBBB) and left bundle branch block (LBBB) occur due to a conduction delay or blockage in the electrical pathway of the heart.

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The blocks cause abnormal heart rhythms due to a conduction delay. Understanding the difference in the ECG features and clinical implications of RBBB and LBBB is necessary for the interpretation, intervention, and evaluation of heart diseases.

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The Cardiac Conduction System

The heart can beat because of a coordinated activity between the atria and the ventricles. The conduction system includes.

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• Sinoatrial (SA) node.

• Atrioventricular (AV) node.

• Bundle of His.

• Bundle branches right and left bundle branch.

• Purkinje fiber.

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The conduction system in the heart helps electrical signals spread to the heart muscles and contract, creating a waveform on the ECG. The heartbeat originates in the sinus node (SA), and then the signals travel through the atria from the SA node to the atrioventricular (AV) node. The AV node slows down the electric signals as the ventricles fill with blood. The impulse then travels through the bundle of His, which splits into the right and left bundle branches.

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The left bundle branch then further divides into anterior and posterior fascicles. These branches then spread the signals across the ventricles. The contraction in the right and left bundle branches makes the right and left ventricles contract simultaneously. The septum between the two ventricles contracts due to the left bundle branch, resulting in a contraction wave traveling from left to right.

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ECG Basics

An ECG shows the heart's activity from different angles using multiple leads. Lead V1 is from the right side, and lead V6 is from the left.

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• An up spike means a signal moving toward that lead.

• A down spike means it is moving away from the lead.

• The larger left side asserts more effect on the ECG.

• The right and left sides contract simultaneously. So, there is a single R wave, and the entire process takes less than 120 milliseconds.

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When the pathway is blocked, it takes longer for the QRS to show, and it becomes wide, indicating an abnormality in the path to the ventricles.

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Causes of bundle branch block

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They are

• Congenital heart disease.

• Dilated cardiomyopathy.

• Heart attack.

• Coronary artery disease.

• Heart failure.

• Heart valve disease.

• High blood pressure.

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Lung diseases cause right bundle branch block 

• Chronic obstructive pulmonary disease.

• Pulmonary embolism.

• Pulmonary hypertension.

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Symptoms caused by bundle branch block

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It causes symptoms

• Dizziness

• Fainting

• A feeling of fainting

• Shortness of breath

• Arrhythmia

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Often, there may be no symptoms.

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Left Bundle Branch Block (LBBB)

The left bundle branch block, LBBB, is characterized by a delay or block in the transmission of electrical signals to the left ventricle on the left side conduction pathway.

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Branches of the left bundle branch

The left bundle divides into anterior and posterior fascicles. A bundle branch block occurs when the entire bundle is blocked, not just the fascicles. Due to the greater muscle mass of the left ventricle, its contraction has a greater effect on the left ventricle and causes cardiac axis changes.

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Each branch of the left bundle branch can be damaged in isolation; anterior fascicle blocks are more common and cause left axis deviation. A posterior fascicle block may cause right axis deviation. It works less than the anterior fascicle. The right ventricular muscle does not have enough mass to deviate from the cardiac axis.

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ECG characteristics of LBBB

• Prolonged QRS complex. Measuring more than 120 milliseconds.

• Wide, slurred or notched waves in lateral leads.

• Deep S waves in right precordial leads.

• Absence of Q wave in leads I, V5 and V6.

• Monomorphic R wave in I, V5 and V6.

• ST and T wave displacement opposite to the major deflection of the QRS complex.

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Management of LBBB

• Managing an underlying heart condition.

• Managing associated symptoms.

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Treatment

• Medications: Beta-blockers, calcium channel blockers, or antiarrhythmic drugs. Control the heart rhythm and manage symptoms.

• Lifestyle modifications: Making lifestyle changes such as maintaining a healthy diet, exercising regularly, and avoiding tobacco and alcohol helps to improve heart health.

• Pacemaker: It may be necessary to regulate the heart's electrical activity.

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Right bundle branch block (RBBB)

The electrical signals are delayed or blocked in the right bundle branch, affecting how the right ventricle contracts.

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In RBBB

• The left ventricle contracts normally.

• The right ventricle gets delayed signals by muscle-to-muscle spread and not through the fast conduction system.

• The delay causes a change in ECG appearance.

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ECG characteristics of RBBB

• QRS duration >120 milliseconds. It has a broad complex.

• RSR^/Pattern in V1-V3. As an M-shaped QRS.

• Wide or slurred S wave. In leads I, aVL, V5 and V6.

• Normal axis.

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Diagnosis: Differences Between LBBB and RBBB

LBBB sometimes occurs after cardiac procedures that damage the left branch of the bundle of His. Medical history evaluation, physical examination and diagnostic test (ECG) help in diagnosing, identifying, and distinguishing LBBB and RBBB. Both the right bundle branch block and the left bundle branch block have unique patterns on the ECG.

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Interpreting bundle branch block ECG

The HEART mnemonic helps us read ECGs and spot differences between left and right bundle branch blocks.

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• H Hypertrophy

• E Extent of intervals

• An Axis

• R qRST analysis

• T T wave

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Hypertrophy: assesss left ventricular hypertrophy in patients above 35 years.

• R Wave In aVL > 12mm.

• A sum of the deep S wave in V1 to V3 and the tall R wave in V4 to V6 > 35mm.

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Extent of intervals. Measure the key intervals.

• PR Interval. Normal ranges 0.12.to 0.20 seconds.

• QRS Duration. Normal is < 0.10 seconds; > 0.12 seconds indicate bundle branch blocks.

• QT interval. Prolongation indicates various diseases.

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Axis: Determines the heart's electrical axis by examining lead I and aVF.

• Positive QRS in both leads suggests normal access.

• Deviation indicates left anterior fascicular block.

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QRS T Evaluation:  Q and R wave progression and changes in the ST segment should be evaluated.

• ST elevations or depression indicate ischemia or infarction.

• Poor R wave progression suggests anterior myocardial infarction.

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T Wave: Analyse T wave morphology.

• Inversions or hyperacute T waves can signal ischemia or other cardiac conditions.

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LBBB Identification

• QRS Duration > 0.12 seconds.

• R waves are broad and notched in lateral leads (I,aVL, V5-V6).

• Absence of Q waves in lateral leads.

• Delayed or wave peak time in V5- V 6.

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RBBB Identification

• QRS Duration > 0.12 seconds.

• RSR^/ pattern M-shaped in V1-V3.

• Wide S waves in lateral leads(I, aVL, V5-V6).

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WiLLiaM MaRRoW mnemonic

The WiLLiaM MaRRoW mnemonic helps to recognize left and right bundle branch blocks quickly by looking at V1 and V6.

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• WiLLiaM: marks the left bundle branch block ECG appearance.

• MaRRoW: is for the right bundle branch block ECG appearance. 

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The middle letters indicate which bundle branch block each name refers to. Two L in WiLLiaM is for the left bundle branch block, and two R in MaRRoW are for the right bundle branch block. The first and last letters help us to recognize the ECG feature of the bundle branch block.

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To recognize the right bundle branch block, we use the name MaRRoW 

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• M pattern in V1 resembles the letter M: There is an initial small up deflection of the (R) wave. A large down deflection (S) wave, then a large one up (second R wave).

• W pattern in V6 resembles the letter W: with a small down deflection (Q), a large up deflection (R) wave, and then a wide down deflection S wave.

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 In the left bundle branch block, we use the name WiLLiaM, where the first and last letters indicate the shape of the waveform.

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• W Complexes in V1 resembles the letter W deep downward deflection, which may be notched.

• M complexes in V6 resemble the letter M with a broad notch or M-shaped R wave in V6.

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Clinical Relevance of Bundle Branch Blocks

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Right bundle branch block

RBBB is caused due to damage to the right bundle branch block.

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The causes include:

• Lung Pathology: COPD, Pulmonary emboli, Cor pulmonale.

• Congenital heart disease. Anterior septal defect (ASD).

• Ischemic heart disease.

• Primary degeneration of the right bundle.

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Left bundle branch block

It is always pathological and is due to conduction system degeneration or pathologies like.

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• Ischemic heart disease.

• Valvular heart disease.

• LBBB occurs after a surgical procedure, which damages the left branch of the bundle of His.

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Conclusion

The right bundle branch block and left bundle branch block are conduction abnormalities that ECG can identify. Both present with wide QRS complexes but have distinctive features. Recognizing ECG patterns of RBBB and LBBB is necessary for diagnosing, identifying, and evaluating underlying cardiac conditions. RBBB is benign, and LBBB is pathologic.