Step 3

Next we need to determine the AXIS of the EKG tracing. To do this we need to understand the basic 6 leads and their geometry. The EKG waveform comes from a measurement of surface voltages between 2 leads. A wave that is travelling towards the positive (+) lead will inscribe an upwards deflection of the EKG; conversely a wave traveling away from the positive lead will inscribe a downward deflection. Waves that are traveling at a 90 degree angle to a particular lead will create no deflection and is called an isoelectric lead.

As an example in the pictures below, a wave travelling from the head to the feet would be shown as an upwards deflection in AVF, since it is going towards the AVF+ lead.

The axis is the sum of the vectors, produced by the ekg leads, to produce a single electrical vector. Remember that a positive signal in Lead-I means that the signal is going right to left; this produces a vector, which if we take all the leads, we can sum. This summed vector should in general be pointing the same direction (down-left) for a normal heart; this makes sense if we think of the electrical conduction system of the heart which sends a signal from the SA node (top right) to the purkinje fibers (bottom left). Don't worry if you still don't get it, we'll give you a visual example further down the page.

 
Click here to view the Basic 6 Limb EKG Leads in 3D

You will notice that leads I, II and III form the sides of an equilateral triangle, while AVR, AVL and AVF bisect the vertices of the triangle. The easiest way to figure out the axis is to draw a normal X-Y graph and fill in the quadrants that are represented by each lead with a positive deflection.

There are some tricks to save you time, but first think about a normal EKG plot; in a normal EKG both leads I and AVF will be positive as the signal travels from the SA node (top right of the heart) to the tip of the ventricles (bottom left of the heart). This is a normal axis, and leads us to the rule of thumb, if I and AVF are positive the axis is normal. However, just because this is not the case does not mean that the axis is abnormal! (you need to look at more leads in this case) The normal axis actually allows the signal to travel up to 30° above the X axis and 30° to the left of the Y axis. Let's look at an example below and prove this.

If the axis is not completely in the bottom left (the patient's left) quadrant (i.e. I and AVF are positive), it is simply a matter of using additional leads to determine the axis. Looking at the map of all the leads, we notice that almost 360 degrees of axis are covered. Use the same axis determination method you used with I and AVF.