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”).
Haxaxial ECG AXIS Reference

Methods of ECG Axis Interpretation

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

  • The Quadrant Method – (Lead I and aVF)
  • Three Lead analysis – (Lead I, Lead II and aVF)
  • Isoelectric Lead analysis
  • 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:

Hexaxial ECG AXIS Quadrant Method 1
  • 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).
Hexaxial ECG AXIS Quadrant Method Normal axis

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

AXIS: QRS Positive Lead I – QRS Negative Lead aVF
Hexaxial ECG Lead I positive, aVF negative - LAD
AXIS: QRS Negative Lead I – QRS Positive Lead aVF
Hexaxial ECG Lead I negative, aVF positive - RAD
AXIS: QRS Negative Lead I – QRS Negative Lead aVF
Hexaxial ECG Lead I negative, aVF negative - Extreme axis


Summary Table:

ECG AXIS Lead 1 and aVF Axis measurement QUADRANT

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


Method 2: Three Lead analysis – (Lead I, Lead II and aVF)

Next we add in Lead II to the analysis of Lead I and aVF

  • 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).
Lead I Lead II hexaxial analysis Normal axis
  • The combined evaluation of Lead I, Lead II and aVF – allows rapid and accurate QRS assessment. The addition of Lead II can help determine pathological LAD from normal axis/physiological LAD
  • Note: Lead III or aVF can both be used in three lead analysis
Lead I II aVF Hexaxial evaluation Normal axis

Now estimate the AXIS using Three Lead analysis:

QRS Positive Lead I – QRS Equiphasic Lead II – QRS Negative Lead aVF
Lead I II aVF Hexaxial Evaluation LAD Physiological
QRS Positive Lead I – QRS Negative Lead II – QRS Negative Lead aVF
Lead I II aVF Hexaxial Evaluation LAD Pathological
QRS Negative Lead I – QRS Positive Lead II – QRS Positive Lead aVF
Lead I II aVF Hexaxial Evaluation RAD
QRS Negative Lead I – QRS Negative Lead II – QRS Negative Lead aVF
Lead I II aVF Hexaxial Evaluation Extreme axis deviation


Summary Table:

Lead-1-Lead-II-aVF-Axis-measurement

Method 3 – The Isoelectric Lead

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

Haxaxial ECG AXIS Reference

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

Axis Example 001
Answer – Lead I, II, aVF
  • Lead I = POSITIVE
  • Lead II = POSITIVE
  • aVF = POSITIVE
  • This puts the axis in the quadrant between 0° and +90° – i.e. normal axis
Answer – Isoelectric Lead Method

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

RAD due to RVH Axis Example 002

Answer – Quadrant Method
  • Lead I = NEGATIVE
  • Lead II = Equiphasic
  • Lead aVF = POSITIVE
  • This puts the axis in the quadrant, between +90° and +180°, i.e. RAD.
Answer – Isoelectric Lead Method
  • Lead II (+60°) is the isoelectric lead.
  • 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

Borderline LAD inferior AMI Axis example 003

Answer – Quadrant Method
  • Lead I = POSITIVE
  • Lead II = Equiphasic
  • Lead aVF = NEGATIVE
  • 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.
Answer – Isoelectric Lead Method
  • 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

Northwest axis example 004

Answer – Quadrant Method
  • Lead I = NEGATIVE
  • Lead II = NEGATIVE
  • Lead aVF = NEGATIVE
  • 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. 

Answer – Isoelectric Lead Method
  • The most isoelectric lead is aVL (-30°).
  • The QRS axis must be at ± 90° from aVL at either +60° or -120°.
  • Lead aVR (-150°) is positive, with lead II (+60°) negative.
  • This puts the axis at -120°.

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


Example 5

Vertical AXIS example 005

Reveal answer
  • Lead I = isoelectric.
  • Lead aVF = positive.
  • 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.

CXR COPD Vertical Heart 800


Causes of Axis Deviation

Right Axis Deviation

Left Axis Deviation

Extreme Axis Deviation


Further Reading

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.


LITFL Further Reading


Advanced Reading


ECG LIBRARY 700

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 |

3 Comments

  1. 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!

  2. 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

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