Left Axis Deviation

Objectives

Objective 1: To understand the electrophysiology behind left axis deviation (LAD)

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 Objective 2: To be able to explain the signs one would see on an ECG with LAD

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 Objective 3: To understand the meaning of and physiology behind left anterior fasicular block

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 Objective 4: To be able to compare left anterior fasicular block and left posterior fasicular block

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Right Bundle Branch Block

Objectives

To be able to explain the pathophysiology behind right bundle branch block

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To be able to distinguish the ECG changes present in RBBB and demonstrate this
understanding on example ECGs

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Be able to describe the causes of RBBB

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To apply the skills covered to complete the RBBB quiz

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Objectives

 Objective 1

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Definition

In RBBB, activation of the right ventricle is delayed as depolarisation 
has to spread across the septum from the left ventricle

The left ventricle is activated normally, meaning that the early part of the
QRS complex is unchanged

The delayed right ventricular activation produces a secondary
R wave (R’) in the right precordial leads (V1-3) and a wide, slurred S wave in the
lateral leads

Delayed activation of the right ventricle also gives rise to secondary 
repolarization  abnormalities, with ST depression and T wave inversion 
in the right precordial leads

In isolated RBBB the cardiac axis is unchanged, as left ventricular activation 
proceeds normally via the left bundle branch

Right Bundle Branch Block

Objectives

To be able to explain the pathophysiology behind right bundle branch block

Click to Complete

To be able to distinguish the ECG changes present in RBBB and demonstrate this
understanding on example ECGs

Click to Complete

Be able to describe the causes of RBBB

Click to Complete

To apply the skills covered to complete the RBBB quiz

Click to Complete

Objectives

 Objective 1

 Objective 2

 Objective 3

 Objective 4

Return to Top

Definition

In RBBB, activation of the right ventricle is delayed as depolarisation 
has to spread across the septum from the left ventricle

The left ventricle is activated normally, meaning that the early part of the
QRS complex is unchanged

The delayed right ventricular activation produces a secondary
R wave (R’) in the right precordial leads (V1-3) and a wide, slurred S wave in the
lateral leads

Delayed activation of the right ventricle also gives rise to secondary 
repolarization  abnormalities, with ST depression and T wave inversion 
in the right precordial leads

In isolated RBBB the cardiac axis is unchanged, as left ventricular activation 
proceeds normally via the left bundle branch

Objectives

 Objective 1

 Objective 2

 Objective 3

 Objective 4

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Left Axis Deviation

In left axis deviation (LAD) the direction of
overall electrical activity becomes distorted to the left (between -30° and -90°)

Normal Limb Leads

Left Axis deviation Limb Leads

The positive deflection in lead I to becomes more positive
and the deflection in III becomes be more negative:

- QRS is positive (dominant R wave) in leads I and aVL
- QRS is negative (dominant S wave) in leads II and aVF

Left Axis Deviation ECG Example

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Causes of Left Axis Deviation

- Left anterior fascicular block
- Left bundle branch block
- Left ventricular hypertrophy
- Inferior MI
- Ventricular ectopy
- Paced rhythm
- Wolff-Parkinson White syndrome

Objectives

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Left Anterior Fasicular Block

In LAFB (also known as left anterior hemiblock), impulses
are conducted to the left ventricle via the left posterior fascicle,
which inserts into the infero-septal wall of the left ventricle along its
endocardial surface

Diagnostic Criteria for LPFB

- LAD
- Small,Q waves with tall R waves in leads I and aVL
- Small R waves with deep S waves in leads II, III, and aVF
- QRS duration normal or slightly prolonged
- Prolonged R wave peak time in aVL
- Increased QRS voltage in the limb leads

Objectives

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LAFB VS Left Posterior Fasicular Block

LPFB is much less common than LAFB, as the broad bundle of fibres that comprise the left posterior fascicle are relatively resistant to damage when compared with the slim single tract that makes up the left anterior fascicle.

It is extremely rare to see LPFB in isolation. It usually occurs along with RBBB
in the context of a bifascicular block

Do not be tempted to diagnose LPFB until you have ruled out more significant causes of right axis deviation: i.e. acute PE, tricyclic overdose, lateral MI, right ventricular hypertrophy