Right axis deviation

Right axis deviation
Hexaxial reference system
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The electric axis of the heart is the net direction in which the wave of depolarisation travels. It is measured using an electrocardiogram (ECG). Normally, this begins at the atrioventricular node (AV node); from here the wave of depolarisation travels down to the apex of the heart. The hexaxial reference system can be used to visualise the directions in which the depolarisation wave may travel.

On a hexaxial diagram (see figure 1):

RAD is an ECG finding that arises either as an anatomically normal variant or an indicator of underlying pathology.

There are often no symptoms for RAD and it is usually found by chance during an ECG. Many of the symptoms exhibited by patients with RAD are associated with its different causes. The table below displays the four most common causes and the signs, symptoms and risk factors associated with it.

Chest pain

Fatigue

Shortness of breath

Obesity

Gender

Hypertension

Diabetes

Physical activity

Age

Alcohol

Dizziness

Fainting

Pulmonary hypertension

Mitral stenosis

Pulmonary embolism

Congenital heart disease

Arrythmogenic right ventricle cardiomyopathy

Fainting

Palpitations

Drug toxicity (e.g. tricyclic antidepressants)

Hyperkalaemia

Blockage of the left posterior fascicle would lead to activation of the anterior portion of the left ventricle followed by activation of the rest of the ventricle in a superior to inferior direction and directed towards the right. This would lead to right axis deviation findings on an ECG. Bifascicular block is a combination of right bundle branch block and either left anterior fascicular block or left posterior fascicular block. Conduction to the ventricle would therefore be via the remaining fascicle. The ECG will show typical features of RBBB plus either left or right axis deviation.

The lateral wall of the left ventricle is supplied by branches of the left anterior descending (LAD) and left circumflex (LCx) arteries. Infarction of the lateral wall will thus lead to deviation of the axis away from the site of infarction.

Increased thickness of the right ventricle leads to right axis deviation (see above).

Pre-excitation refers to early activation of the ventricles due to impulses bypassing the AV node via an accessory pathway. Accessory pathways are abnormal conduction pathways formed during cardiac development. An example of pre-excitation syndromes is Wolff Parkinson White syndrome. Here, the presence of a left lateral accessory pathway leads to right-axis deviation.

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