 # ECG Axis Interpretation

Axis interpretation using the schematic illustration demonstrates the relationship between QRS axis and the frontal leads of the ECG.

• Normal Axis = QRS axis between -30° and +90°.
• Left Axis Deviation = QRS axis less than -30°.
• Right Axis Deviation = QRS axis greater than +90°.
• Extreme Axis Deviation = QRS axis between -90° and 180° (AKA “Northwest Axis”).

#### Methods of ECG Axis Interpretation

There are several complementary approaches to estimating QRS axis, which are summarized below:

• Super SAM the Axis Man

#### Method 1 – The Quadrant Method

The most efficient way to estimate axis is to look at LEAD I and LEAD aVF.

Examine the QRS complex in each lead and determine if it is Positive, Isoelectric (Equiphasic) or Negative:

• positive QRS in Lead I puts the axis in roughly the same direction as lead I.
• positive QRS in Lead aVF similarly aligns the axis with lead aVF.
• Combining both coloured areas – the quadrant of overlap determines the axis. So If Lead I and aVF are both positive, the axis is between 0° and +90° (i.e. normal axis).

#### Now estimate the AXIS using the Lead I and aVF – Quadrant Method:

Summary Table:

Note: **Possible LAD can be further evaluated using Lead II as detailed in method 2 below…

• positive QRS in Lead I puts the axis in roughly the same direction as lead I.
• positive QRS in Lead II similarly aligns the axis with lead II.
• We can then combine both coloured areas and the area of overlap determines the axis. So If Lead I and II are both positive, the axis is between -30° and +90° (i.e. normal axis).
• Note: Lead III or aVF can both be used in three lead analysis

Now estimate the AXIS using Three Lead analysis:

Summary Table:

#### Method 3 – The Isoelectric Lead

This method allows a more precise estimation of QRS axis, using the axis diagram below.

#### Key Principles

• If the QRS is POSITIVE in any given lead, the axis points in roughly the same direction as this lead.
• If the QRS is NEGATIVE in any given lead, the axis points in roughly the opposite direction to this lead.
• If the QRS is ISOELECTRIC (equiphasic) in any given lead (positive deflection = negative deflection), the axis is at 90° to this lead.

Step 1: Find the isoelectric lead. The isoelectric (equiphasic) lead is the frontal lead with zero net amplitude. This can be either:

• A biphasic QRS where R wave height = Q or S wave depth.
• A flat-line QRS with no discernible features.

Step 2: Find the positive leads.

• Look for the leads with the tallest R waves (or largest R/S ratios)

Step 3: Calculate the QRS axis.

• The QRS axis is at 90° to the isoelectric lead, pointing in the direction of the positive leads.

This concept can be difficult to understand at first, and is best illustrated by some examples.

Example 1

• aVF = POSITIVE
• This puts the axis in the quadrant between 0° and +90° – i.e. normal axis

Lead aVL is isoelectric, being biphasic with similarly sized positive and negative deflections (no need to precisely measure this).

• From the diagram above, we can see that aVL is located at -30°.
• The QRS axis must be ± 90° from lead aVL, either at +60° or -120°
• With leads I (0), II (+60) and aVF (+90) all being positive, we know that the axis must lie somewhere between 0 and +90°.
• This puts the QRS axis at +60° – i.e. normal axis

Example 2

• This puts the axis in the quadrant, between +90° and +180°, i.e. RAD.
• The QRS axis must be ± 90° from lead II, at either +150° or -30°.
• The more rightward-facing leads III (+120°) and aVF (+90°) are positive, while aVL (-30°) is negative.
• This puts the QRS axis at +150°.

This is an example of right axis deviation secondary to right ventricular hypertrophy.

Example 3

• This puts the axis in the quadrant between 0° and -90°, i.e. normal or LAD.
• Lead II is neither positive nor negative (isoelectric), indicating physiological LAD.
• Lead II (+60°) is isoelectric.
• The QRS axis must be ± 90° from lead II, at either +150° or -30°.
• The more leftward-facing leads I (0°) and aVL (-30°) are positive, while lead III (+120°) is negative.
• This confirms that the axis is at -30°.

This is an example of borderline left axis deviation due to inferior MI.

Example 4

• This puts the axis in the upper right quadrant, between -90° and 180°, i.e. extreme axis deviation.

NB. The presence of a positive QRS in aVR with negative QRS in multiple leads is another clue to the presence of extreme axis deviation.

• The most isoelectric lead is aVL (-30°).
• The QRS axis must be at ± 90° from aVL at either +60° or -120°.
• This puts the axis at -120°.

This is an example of extreme axis deviation due to ventricular tachycardia.

Example 5

• This is the easiest axis you will ever have to calculate. It has to be at right angles to lead I and in the direction of aVF, which makes it exactly +90°!

This is referred to as a “vertical axis” and is seen in patients with emphysema who typically have a vertically orientated heart.

#### Extreme Axis Deviation

For a deeper understanding of axis determination, including a detailed explanation of the hexaxial reference system, check out this excellent series of articles from EMS 12-lead. ## ECG LIBRARY

Electrocardiogram

Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM with a passion for rugby; medical history; medical education; and informatics. Asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | vocortex |

1. Kate Johnson

Hello, can I please clarify.. In the section where you are talking about the Quadrant method of calculation, you have written (just about the circular diagrams) “so if leads I and II are both positive”… should this read “so if leads I and aVF are both positive”?

Thanks for an excellent explanation of axis… such a tricky concept!

• Thank you – corrected

2. Kyra

OMG!! YOUR AMAZING. Im taking a ECG coarse for FNP school and i had no idea and was so lost about this ECG axis. I read your explanations and studied the graphs and ended up getting all the practice questions right! Thank you

3. Sarah

That’s the most wonderful explanation that I have ever encountered. I was really confused about ECG. Thank you very much. Appreciated.

4. Muthumani R

Great explanation…God bless you

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