What is Axis?

You’ll often hear people refer to “axis” when interpreting ECGs.

They are typically talking about the mean QRS axis, which is the direction of net ventricular depolarization represented as an angle in the frontal plane. Similarly, you can have a p wave axis or T wave axis, which are the net directions of atrial depolarization and ventricular repolarization in the frontal plane respectively.

Determining the axis of an EKG can be done with a systematic set of rules. However, in order to understand axis, you must become familiar with the hexaxial reference system.

The Hexaxial Reference System

The hexaxial reference system is a way of representing the directions of limb leads (i.e. the leads measuring currents in the frontal plane of the body) as angles.

We can derive the system by starting with the main limb leads: I, II, and III.

With some mathematics and physics that are out of the scope of this module, scientists have shown that the anatomical lead placement can be mapped to an equilateral triangle in the mathematical space. This is known as Einthoven’s triangle.

The significance of Einthoven’s triangle is we know that the internal angles of an equilateral triangle are all 60°.

We know that Lead I is perfectly horizontal (no vertical angulation) and the other leads form a 60° angle with lead I. The augmented leads form 90° angles with our other leads.

By plotting all these Limb Lead vectors on a circle, joined at the tails at a shared origin point, we get the following:

The angles assigned to the vectors depend on whether they are oriented clockwise (positive) or counter clockwise (negative) to Lead I.

Now, if we plot the negative counterparts of these vectors (oriented 180° away from the original), we get the full hexaxial reference system.

The hexaxial reference system will help us determine the direction of current flow through the heart, along the frontal plane.

Defining QRS Axis

As mentioned before, the mean QRS axis is the direction of net ventricular depolarization in the frontal plane.

To dissect this a little:

• We use the limb leads, NOT the precordial leads, to look at the heart from the frontal plane. Recall that the precordial leads view the heart from the transverse plane.

• Net ventricular depolarization essentially means the net polarity of the QRS complex. 

• Finally, we measure the direction of the QRS axis as an angle on the hexaxial reference system.

To determine the net polarity of the QRS, think about how much of the QRS area is above and how much is below the isoelectric line. 

If more is above, it's net positive; if more is below, it's net negative; if it's equal, it's equiphasic.

Overall:

• A net positive QRS means the axis is in the same direction as the lead (within 90°).

• An equiphasic QRS means the axis is perpendicular to the lead.

• A net negative QRS means the axis is in the opposite direction of our lead (>90° away).

Normal Axis

Normal axis is anywhere between -30°and +90°(some sources go up to +100°).

The wide range of normal for axis is due to normal variability in the position of the heart.

In a taller chest, the heart is more vertical, and the axis points down. In a wider chest, the heart is more horizontal, and the axis points to the side.

Since the bottom surface of the heart is attached to the diaphragm, deep breathing may depress the diaphragm enough to make the heart more vertical, pushing the axis towards +90°.

In conditions like COPD, hyperinflation increases the depth of the chest at baseline, causing a more vertical heart and axis.

Axis Deviation

• An axis between +90° to +180° is classified as a right axis deviation (RAD).

• An axis between -30° and -90° is classified as a left axis deviation (LAD).

• An axis between -90° and -180° is classified as an extreme axis deviation (EAD). 

  • We call an axis between -90° and -180° an “extreme” axis deviation because either an extreme RAD or extreme LAD can land us in this territory. Because we can’t figure this out just from the ECG alone, it is also sometimes referred to as an indeterminate axis.

Methods of Axis Determination

In this section, I will cover two different ways of measuring axis, and we will use them to determine the axis in three example cases in the next section.

Method 1: The Equiphasic QRS Method

In this method, you can determine the QRS axis by following these steps:

1. Find an equiphasic QRS in the limb leads. 

2. Use the hexaxial reference system to determine which 2 leads are perpendicular to this lead. This will be a pair of positive and negative versions of one specific lead.

3. Look at the polarity of the QRS in this lead to determine the axis.

4. Use adjacent leads to help guide you when stuck.

This will make more sense with a worked example.

Method 2: The Quadrant Method

This method is slightly more intuitive. 

Let's say you have an ECG where the QRS is positive in leads I and aVF.

For many cases, by just looking at leads I and aVF, we can determine which quadrant the axis lies in.

There's another really handy shortcut that arises with this method:

• If the QRS is net positive in leads I and II, the overlap area is -30° to +90°. This is exactly our definition of normal axis!

• Therefore, if QRS is positive in I and II, the axis is normal.

So, if you have to quickly determine axis, you can just look at I and II to see if they both have positive QRS complexes. If they do, you can move on. If not, you then have to put some more effort in to determine the axis deviation.

Example Workthroughs

Let's look at some example ECGs, and try to determine the axis by using both our methods!

Let's first use Method 1:

At an initial glance, it may appear that there are two biphasic waves to choose from: lead I and lead aVR. Looking closely, it appears that the S wave in lead I is deeper than the R wave, so it's a net negative QRS, making lead aVR the better choice. However, we will work through both examples and yield equivalent results.

If we choose Lead aVR as our biphasic wave, that means our axis must be either +III or -III. 

Since the QRS in lead III is positive, the axis is +III, and we have a right axis deviation.

If we instead choose Lead I as our biphasic wave, our axis must be either +aVF or -aVF.

The QRS is positive in lead aVF, therefore our axis is +aVF. 

However, since Lead aVF lies on the boundary between normal axis and right axis deviation, we must look at the adjacent leads II and III. Since the QRS is larger in lead III than lead II, the axis is oriented more towards lead III. 

This, once again, suggests a right axis deviation.

Now let's try using Method 2:

If, due to inter-observer variability, someone thought Lead I was equiphasic, we would essentially follow Method 1 to find the axis.