ECG: hexaxial reference system

Principle

Fig. 1 Hexaxial reference system. Notice that negative is up. Triangle up denotes that with normal polarity the main peak of the QRS complex is positive. This holds for all 6 leads from 0 ^o to 150^o except AVR.

The hexaxial reference system is a diagram that is used to determine 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) by using the polarity of the QRS complex in leads I, II, and III in the frontal plane. It is usually oriented in a right shoulder to left leg direction, which corresponds to the right inferior quadrant of the hexaxial reference system, although a slightly broader range, -30^o to +90^o is considered to be normal.

The diagram is based on the first six leads (I, II, III, aVR, aVL, and aVF) of the 12-lead ECG (see ECG: 12-lead ECG). To determine the heart's electrical axis, first the most isoelectric (or equiphasic) lead should be located on a diagnostic quality ECG with proper lead placement. Then the corresponding spoke on the hexaxial reference system should be found. The perpendicular spoke will point to the heart's (principal) electrical axis. To determine which of the two opposite located numerical value (in degrees) should be used, use the polarity of the perpendicular lead on the ECG. For example, if the most isoelectric lead is aVL, the perpendicular lead, being lead II has an angle of +60^o.

Application

The reference system is used for diagnosis of cordial disorders.

Normal and deviating directions are classified as:

· Normal axis: -30^o to +90^o

· Left axis deviation: -30^o to -90^o, may indicate left anterior fascicular block or Q waves from inferior myocardial infraction.

· Right axis deviation: +90^o to +180^o may indicate left posterior fascicular block, Q waves from high lateral myocardial infarction, or a right ventricular strain pattern.

· Extreme axis deviation: -90^o to -180^o