Approach

The next step in the orderly approach for 12-lead ECG interpretation is determination of the mean QRS frontal axis. In order to successfully determine this, we will review the standard 12-lead ECG system.

12-lead ECG

The standard ECG displays 12 small groups of ECG complexes on the grid and each group is labelled as an ECG lead. The ECG leads are divided into 6 frontal plane leads (also known as limb or extremity leads) and 6 horizontal plane leads (also known as chest or precordial leads).

Frontal plane

Let us first discuss the frontal plane leads. The 6 frontal plane leads, I, II, III, aVR, aVL, and aVF, using the limb leads, record the electrical forces of depolarization in the frontal plane, that is to the right, left, inferior, and superior directions. Active leads are placed on the right and left arms and the left leg (the right leg lead is a ground lead).

Hexaxial system

The hexaxial diagram shows the degrees assigned to each lead in a 360 degree circle. From Lead I at 0 degree, clockwise is positive, and counterclockwise is negative. For illustrative purposes, the negative poles of the leads are cross-hatched.

Horizontal plane

Let us now learn about the horizontal plane. The horizontal or precordial plane, using chest leads, adds an additional direction to help us evaluate the ECG: besides right and left direction it measures electrical potentials oriented in the anterior and posterior direction. Since both the frontal and horizontal planes include right and left directions, some leads from both planes may appear similar. Leads V1 and V2 are especially useful to look at the electrical activity of the atrial and ventricular septal walls, leads V2-4 the anterior left ventricular wall, and leads V5-V6 the lateral left ventricular wall. Click on the box for information on exact chest lead placement.