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ECG Axis Interpretation

The diagram below illustrates the relationship between QRS axis and the frontal leads of the ECG.
Hexaxial Reference system
Image reproduced from Chung

  • Normal Axis = QRS axis between -30 and +90 degrees.
  • Left Axis Deviation = QRS axis less than -30 degrees.
  • Right Axis Deviation = QRS axis greater than +90 degrees.
  • Extreme Axis Deviation = QRS axis between -90 and 180 degrees (AKA “Northwest Axis”).
There are several complementary approaches to estimating QRS axis, which are summarised below.

Method 1 – The Quadrant Method

The most efficient way to estimate axis is to look at leads I + aVF.
LEAD ILEAD AVFQUADRANTAXIS
PositivePositiveLeft lower quadrantNormal (0 to +90 degrees)
PositiveNegativeLeft upper quadrantPossible LAD (0 to -90 degrees)
NegativePositiveRight lower quadrantRAD (+90 to 180 degrees)
NegativeNegativeRight upper quadrantExtreme Axis Deviation (-90 to 180 degrees)

Method 2 – Leads I + II

Another rapid method is to look at leads I + II.
A positive QRS in lead I puts the axis in roughly the same direction as lead I.
lead I
Image reproduced from Chung

A positive QRS in lead II similarly aligns the axis with lead II.
lead II
Image reproduced from Chung

Therefore, if leads I and II are both positive, the axis is between -30 and +90 degrees (i.e. normal axis).
normal axis I and II
Image reproduced from Chung

Combining Methods 1 and 2

By combining these two methods, you can rapidly and accurately assess axis.
LEAD ILEAD AVFAXIS
PositivePositiveNormal (0 to +90 degrees)
PositiveNegativePossible LAD
Is lead II positive?
Yes -> Normal (0 to -30 degrees)
No -> LAD (-30 to -90 degrees)
NegativePositiveRAD (+90 to 180 degrees)
NegativeNegativeExtreme Axis Deviation (-90 to 180 degrees)

Method 3 – The Isoelectric Lead

This method allows a more precise estimation of QRS axis, using the axis diagram below.
Hexaxial Reference system
Reproduced from Chung

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 in any given lead (positive deflection = negative deflection), the axis is at 90 degrees 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 degrees 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 eg1

Example 2

Ex2

Example 3

Ex3

Example 4

Ex4

Example 5

Ex5

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.

References

  • Chung DC, Nelson HM. ECG – A Pictorial Primer [internet]. Accessed 20/10/2014.
  • Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th Edition. Saunders Elsevier 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
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