Wikipedia:Imperial College/Project Page/Sandbox 1

Drafting space for Imperial College

Write some things here....

Right Axis Deviation

 * Right axis deviation

(Include a definition and brief summary of what Right Axis Deviation is.)

ECG Presentation and Interpretation
Right axis deviation is a finding on the electrocardiogram (ECG). Electrical heart axis refers to the net electrical activity of the heart. The electrical heart axis is usually determined using the hexaxial reference system (Figure 1). The normal electrical heart axis is situated between -30° and -90°. An electrical heart axis between 90° to 180° is classified as right axis deviation.

In general, a positive (upwards) deflection of an ECG lead demonstrates an electrical activity which moves towards the lead, where as a negative (downwards) deflection of an ECG lead demonstrates an electrical activity which moves away from the lead. The electrical heart axis can be estimated from the ECG by using quadrant method or degree method.

1. Quadrant Method : Leads I and aVF are usually observed. If lead I is positive (translating to 0° on the hexaxial reference system) and lead aVF is positive (translating to 90°), the electrical heart axis is estimated to fall in the left lower quadrant within the normal range. On the other hand, if lead I is negative (translation to 180°) and lead aVF is positive, the electrical heart axis is estimated to fall in the right lower quadrant suggesting a right axis deviation. Similarly, leads I and II can also be used.

2. Degree method : First, identify the lead with the smallest QRS complex or isoelectric (flat) QRS complex (lead a). After locating the axis of the lead on the hexaxial reference system, identify the lead which is perpendicular to it (lead b). If lead b is positive, the electrical heart axis can be estimated to lie within the quadrant between axis of lead a and lead b.

Pathophysiology
The pathophysiology depends on the specific cause of right axis deviation. Most causes can be attributed to one of four main mechanisms. These include right ventricular hypertrophy, reduced muscle mass of left ventricle, altered conduction pathways and change in the position of the heart in the chest.

Hypertrophy of right ventricle

Enlargement of right ventricular myocardial mass can result in right axis deviation. There are 2 main reasons for this mechanism. Firstly, more muscle mass will result in greater amplitude of depolarisation of that side of the heart. Secondly, depolarisation of the heart will be slower through the right ventricle relative to the left, and therefore the effects of the right ventricle on the axis of the heart will be dominant.

Left Ventricular Atrophy

Decrease in myocardial mass of the left ventricle will shift the balance of depolarisation towards the right. For example, scarring and atrophy caused by ischaemia of the left ventricle will cause depolarisation of the left side of the heart to be less forceful. Hence, depolarisation of the right ventricle will be greater in amplitude than left, shifting the axis to the right.

Conduction abnormalities

Changes in the conduction pathways of the heart can result in right axis deviation. For example, an accessory pathway from the left atrium to the left ventricle, as in Wolff-Parkinson-White Syndrome, will result in the left ventricle finishing depolarisation earlier than the right. Hence, the right ventricle will have more of an effect on the axis of the heart.

Position of heart in the chest

The apex of the heart is normally orientated towards the left. A more vertical orientation of the heart, shifts the axis to the right. Physiologically, this can occur in tall and thin individuals. Pathologically, conditions such as a left-sided pneumothorax and lung hyperinflation (e.g. COPD) can cause rightwards displacement of the heart. The congenital condition of dextrocardia results in right axis deviation.

Causes
http://www.sciencedirect.com/science/article/pii/0002870348901082 http://lifeinthefastlane.com/ecg-library/basics/right-axis-deviation/ http://www.sciencedirect.com/science/article/pii/S0099176708000366

Most common causes
 * Lateral myocardial infarction
 * Right ventricular hypertrophy
 * Fascicular block
 * Pre-excitation syndromes

Causes to link
 * Pulmonary hypertension (key cause of right ventricular hypertrophy)
 * Bifascicular block
 * Left posterior fascicular block
 * Right ventricular outflow tachycardia
 * Anterior fascicular tachycardia
 * Ventricular tachycardia
 * Hyperkalaemia - inconclusive
 * Ventricular ectopy
 * WPW - pre-excitation syndromes
 * Drug toxicity (Na blockers)
 * PE
 * COPD - pulmonary hypertension
 * Normal in children or thin adults with a horizontally positioned heart

Signs, Symptoms and Risk Factors
There are often no symptoms for Right Axis Deviation 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.