Cardiac Conduction System and ECG

• Electrical impulses travel through the heart via a specific conduction pathway, beginning with the sinoatrial node (SAN).
• The SAN acts as the initial pacemaker, and the impulse spreads throughout the atria and to the atrioventricular node (AVN).
• The depolarization wave travels through the heart's septum via the Bundle of His and Purkinje fibres.
• The Bundle of His and Purkinje fibers are organized into the left and right bundle branches.
• The right bundle branch depolarizes the right ventricle.
• The left bundle branch depolarizes the left ventricle simultaneously.
• The septum is depolarized by the left bundle branch, occurring from left to right.
• An ECG is a graphical representation of the net direction of electrical depolarization in the heart and different leads view the heart from different angles.
• V1 views the heart from the right, and V6 views the heart from the left.
• An upwards spike on an ECG means the net depolarization is heading towards that lead, and a downward spike means it's heading away.
• The left side of the heart has greater muscle mass, so left ventricle depolarization has a greater impact on the ECG.
• The right and left ventricles should depolarize simultaneously, producing one uniform R wave.
• Normal ventricular depolarization is complete within 120ms.
• A broad QRS complex always indicates abnormal ventricular depolarization, taking longer than 120ms.
• Broad QRS complexes are the main feature of bundle branch blocks.
• P waves and PR intervals are normal in bundle branch blocks because the problem is below the atria.

ECG Components

• P wave: Represents atrial depolarization.
• PR interval: Represents conduction through the AVN to the ventricles.
• QRS complex: Represents ventricular depolarization.
• Q wave: The first downward deflection.
• R wave: Any upwards deflection.
• S wave: Any downward deflection after an R wave.
• T wave: Represents ventricular repolarization.

Normal Cardiac Conduction

• The sino-atrial node acts as the initial pacemaker.
• Depolarization reaches the atrioventricular node.
• Impulses travel simultaneously down the bundle of His via the left and right bundle branches.
• The septum is depolarized from the left.
• Both the left and right ventricular walls are depolarized simultaneously.

Right Bundle Branch Block (RBBB) Diagnostic Criteria

• Broad QRS complex: >120 ms (3 small squares).
• RSR’ pattern in V1-V3: an initial small upward deflection (R wave), a larger downward deflection (S wave), then another large upward deflection (R’).
• Wide, slurred S wave in lateral leads: I, aVL, V5-V6.

WiLLiaM MaRRoW Mnemonic

• WiLLiaM and MaRRoW mnemonic quickly recognizes left and right bundle branch blocks by looking at V1 and V6.
• WiLLiaM refers to the ECG appearance of left bundle branch block.
• MaRRoW refers to the ECG appearance of right bundle branch block.
• The middle letters help you remember which bundle branch block each name is referring too.
• Two Ls in WiLLiaM = left bundle branch block
• Two Rs in MaRRoW = right bundle branch block
• Each name’s first and last letter helps you recognize the ECG features of the associated bundle branch block.
• MaRRoW: M complexes in V1 resemble the letter M; initial small upward deflection (r wave), a larger downward deflection (S wave), then another large upward deflection (second R wave).
• MaRRoW: W complexes in V6 resemble a W; initial small downward deflection (Q wave), then a larger upward deflection (R wave), and then a wide downward deflection (S wave).

RBBB Pathophysiology

• The sino-atrial node acts as the initial pacemaker
• Depolarization reaches the atrioventricular node
• Depolarization through the bundle of His occurs only via the left bundle branch. The left branch still depolarizes the septum as normal.
• The left ventricular wall depolarizes as normal.
• The right ventricular walls are eventually depolarized by the left bundle branch, this occurs by a slower, less efficient pathway.

RBBB Clinical Relevance

• RBBB can be either physiological or the result of damage to the right bundle branch
• Causes of damage include underlying lung pathology (COPD, pulmonary emboli, cor pulmonale), primary heart muscle disease (ARVC), congenital heart disease (e.g. ASD), ischaemic heart disease and primary degeneration of the right bundle.

Left Bundle Branch Block (LBBB) Diagnostic Criteria

• Broad QRS complex: >120 ms (3 small squares)
• Dominant S wave in V1
• Broad, monophasic R wave in lateral leads: I, aVL, V5-V6
• Absence of Q waves in lateral leads
• Prolonged R wave >60ms in leads V5-V6

WiLLiaM MaRRoW Mnemonic for LBBB

• W complexes in V1 resemble the letter W: deep downward deflection (dominant S wave), which may be notched.
• M complexes in V6 resemble the letter M: broad, notched or ‘M’ shaped R wave in V6.

LBBB Pathophysiology

• When viewed from the right-hand side (V1), net depolarisation travels away (towards the left), resulting in negative ECG deflections. The first downward deflection represents the right ventricle, and the slightly delayed 2nd downward deflection corresponds to the depolarisation of the left ventricle.
• When viewed from the left-hand side (V6), where the net depolarisation is travelling towards the detector, deflections are positive on the ECG. Again, there will be two peaks (RR) due to the delay in left ventricular depolarisation.
• The sino-atrial node acts as the initial pacemaker.
• Depolarization reaches the atrioventricular node.
• Depolarization down the bundle of His occurs only via the right bundle branch. The septum is abnormally depolarized from right to left.
• The right ventricular wall is depolarized as normal.
• The left ventricular walls are eventually depolarized by the right bundle branch; this occurs by a slower, less efficient pathway.

LBBB Clinical Relevance

• LBBB is always pathological
• May be due to conduction system degeneration or myocardial pathologies such as ischaemic heart disease, cardiomyopathy and valvular heart disease.
• LBBB may also occur after cardiac procedures, which damage the left bundle branch or His bundle.
• A STEMI presenting as chest pain with LBBB is exceedingly rare.

Branches of the Left Bundle Branch

• Disruptions in the depolarization of the left ventricular muscle can cause cardiac axis changes due to the relatively greater mass of the left ventricle.
• The left bundle branch splits into anterior and posterior fascicles.
• LBBB = Left anterior fasicular block (LAFB) + Left posterior fasicular block (LPFB)
• Anterior fascicle block is much more common and causes left axis deviation.
• Posterior fascicle block may cause right axis deviation; less work is done here so blockage may happen without ECG changes.
• The right ventricular muscle does not have enough mass to significantly deviate the cardiac axis.

Other Types of Block

• Bifascicular block: Involves both right bundle branch block and the blockade of one of the fascicles of the left bundle branch.
• Trifascicular block: A 3rd-degree heart block alongside bifascicular block.

Key Points

• Consider bundle branch block in an ECG trace with broad complexes.
• Broad complex tachycardia requires differentiation between ventricular tachycardia or sinus tachycardia with concurrent bundle branch block.
• Left bundle branch block is always pathological and indicates significant damage to the cardiac conduction system.
• The appearances of V1 and V6 are often enough to provide the answer using the WiLliaM and MaRroW technique when assessing whether a broad QRS complex is LBBB or RBBB.