Saturday, 26 March 2011

Vectors and views

Interpretation of the ECG relies on the idea that different leads (by which we mean the ECG leads I,II,III, aVR, aVL, aVF and the chest leads) "view" the heart from different angles. This has two benefits. Firstly, leads which are showing problems (for example ST segment elevation) can be used to infer which region of the heart is affected. Secondly, the overall direction of travel of the wave of depolarisation can also be inferred which can reveal other problems. This is termed the cardiac axis . Determination of the cardiac axis relies on the concept of a vector which describes the motion of the depolarisation wave. This vector can then be described in terms of its components in relation to the direction of the lead considered. One component will be in the direction of the lead and this will be revealed in the behaviour of the QRS complex and one component will be at 90 degrees to this (which will not). Any net positive deflection of the QRS complex (i.e. height of the R-wave minus depth of the S-wave) suggests that the wave of depolarisation is spreading through the heart in a direction that has some component (of the vector) in the same direction as the lead in question.
Diagram showing how the polarity of the QRS complex in leads I, II, and III can be used to estimate the heart's electrical axis in the frontal plane.

The heart's electrical axis refers to the general direction of the heart's depolarization wavefront (or mean electrical vector) in the frontal plane. With a healthy conducting system the cardiac axis is related to where the major muscle bulk of the heart lies. Normally this is the left ventricle with some contribution from the right ventricle. It is usually oriented in a right shoulder to left leg direction, which corresponds to the left inferior quadrant of the hexaxial reference system, although −30° to +90° is considered to be normal. If the left ventricle increases its activity or bulk then there is said to be "left axis deviation" as the axis swings round to the left beyond -30°, alternatively in conditions where the right ventricle is strained or hypertrophied then the axis swings round beyond +90° and "right axis deviation" is said to exist. Disorders of the conduction system of the heart can disturb the electrical axis without necessarily reflecting changes in muscle bulk.
Normal     −30° to 90°     Normal     Normal
Left axis deviation     −30° to −90°     May indicate left anterior fascicular block or Q waves from inferior MI.     Left axis deviation is considered normal in pregnant women and those with emphysema.
Right axis deviation     +90° to +180°     May indicate left posterior fascicular block, Q waves from high lateral MI, or a right ventricular strain pattern.     Right deviation is considered normal in children and is a standard effect of dextrocardia.
Extreme right axis deviation     +180° to −90°     Is rare, and considered an 'electrical no-man's land'.    
The hexaxial reference system showing the orientation of each lead. For example, if the bulk of heart muscle is oriented at +60 degrees with respect to the SA node, lead II will show the greatest deflection and aVL the least.

In the setting of right bundle branch block, right or left axis deviation may indicate bifascicular block.

Clinical lead groups

There are twelve leads in total, each recording the electrical activity of the heart from a different perspective, which also correlate to different anatomical areas of the heart for the purpose of identifying acute coronary ischemia or injury. Two leads that look at neighbouring anatomical areas of the heart are said to be contiguous (see color coded chart). The relevance of this is in determining whether an abnormality on the ECG is likely to represent true disease or a spurious finding.
Diagram showing the contiguous leads in the same color
Category     Color on chart     Leads     Activity
Inferior leads     Yellow     Leads II, III and aVF     Look at electrical activity from the vantage point of the inferior surface (diaphragmatic surface of heart).
Lateral leads     Green     I, aVL, V5 and V6     Look at the electrical activity from the vantage point of the lateral wall of left ventricle.

    * The positive electrode for leads I and aVL should be located distally on the left arm and because of which, leads I and aVL are sometimes referred to as the high lateral leads.
    * Because the positive electrodes for leads V5 and V6 are on the patient's chest, they are sometimes referred to as the low lateral leads.

Septal leads     Orange     V1 and V2     Look at electrical activity from the vantage point of the septal wall of the ventricles (interventricular septum).
Anterior leads     Blue     V3 and V4     Look at electrical activity from the vantage point of the anterior surface of the heart (sternocostal surface of heart).

In addition, any two precordial leads that are next to one another are considered to be contiguous. For example, even though V4 is an anterior lead and V5 is a lateral lead, they are contiguous because they are next to one another.
Wiggers diagram, showing a normal ECG curve synchronized with other major events during the cardiac cycle.

Lead aVR offers no specific view of the left ventricle. Rather, it views the inside of the endocardial wall to the surface of the right atrium, from its perspective on the right shoulder.

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