ECG Axis Interpretation

Cardiac axis represents the sum of depolarisation vectors generated by individual cardiac myocytes. Clinically is is reflected by the ventricular axis, and interpretation relies on determining the relationship between the QRS axis and limb leads of the ECG (below diagram)

Since the left ventricle makes up most of the heart muscle under normal circumstances, normal cardiac axis is directed downward and slightly to the left:

  • Normal Axis = QRS axis between -30° and +90°.

Abnormal axis deviation, indicating underlying pathology, is demonstrated by:

  • 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 2021

Note that in paediatric ECG interpretation, the cardiac axis lies between +30 to +190 degrees at birth and moves leftward with age.


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 2021
  • 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 2021

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 2021
AXIS: QRS Negative Lead I – QRS Positive Lead aVF
Hexaxial ECG Lead I negative, aVF positive - RAD 2021
AXIS: QRS Negative Lead I – QRS Negative Lead aVF
Hexaxial ECG Lead I negative, aVF negative - Extreme axis 2021


Summary Table:
ECG AXIS Lead 1 and aVF Axis measurement QUADRANT 2021

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 2021
  • 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 2021

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 2021
QRS Positive Lead I – QRS Negative Lead II – QRS Negative Lead aVF
Lead I II aVF Hexaxial Evaluation LAD Pathological 2021
QRS Negative Lead I – QRS Positive Lead II – QRS Positive Lead aVF
Lead-I-II-aVF-Hexaxial-Evaluation-RAD-2021 2
QRS Negative Lead I – QRS Negative Lead II – QRS Negative Lead aVF
Lead I II aVF Hexaxial Evaluation Extreme axis deviation 2021


Summary Table:
Lead-1-Lead-II-aVF-Axis-measurement 2021

Method 3 – The Isoelectric Lead

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

Haxaxial ECG AXIS Reference 2021

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.


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.


Further reading

Online

Textbooks


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 |

MBBS (UWA) CCPU Emergency Medicine Advanced Trainee based in Melbourne, Australia. Co-author of the LITFL ECG Library. Likes Ultrasound, Echo, ECGs, and anything and everything with caffeine. Part of the 2021 ANZCEN Clinician Educator Incubator Programme | @rob_buttner | ECG Library |

10 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

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

  4. Great explanation…God bless you

  5. Fantastic…nothing else needed

  6. Ashleigh McCormack
    Ashleigh McCormack

    Thank you! Finally able to wrap my head around it.

  7. This is really really a great explanation, thanks so much. Nice to be able to now analyze an ECG with more than a three lead analysis and I see the value of the precise method i.e. example 3. And now I finally understand how to do the more precise method!!!

  8. i am overly excited i was finally able to understand cardiac axis .Thank you so much

  9. Amazing explanation I ever read on ECG!!! Thank you Sir!!!!!

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