ECG Axis Interpretation
ECG 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:
- 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 of ECG Axis Interpretation– 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:
- A positive QRS in Lead I puts the axis in roughly the same direction as lead I.
- A 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:
1.AXIS: QRS Positive Lead I – QRS Negative Lead aVF
2.AXIS: QRS Negative Lead I – QRS Positive Lead aVF
3.AXIS: QRS Negative Lead I – QRS Negative Lead aVF
Here is a video description on Method 1 of ECG Axis Interpretation:
Summary Table:
Note:**Possible LAD can be further evaluated using Lead II as detailed in method 2 below…
Method 2 of ECG Axis Interpretation: Three Lead analysis – (Lead I, Lead II and aVF)
Next we add in Lead II to the analysis of Lead I and aVF
- A positive QRS in Lead I puts the axis in roughly the same direction as lead I.
- A 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).
- 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
Now estimate the AXIS using Three Lead analysis:
1.QRS Positive Lead I – QRS Equiphasic Lead II – QRS Negative Lead aVF
2.QRS Negative Lead I – QRS Positive Lead II – QRS Positive Lead aVF
3.QRS Negative Lead I – QRS Negative Lead II – QRS Negative Lead aVF
Summary Table:
Method 3 of ECG Axis Interpretation– 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.
I hope you have understood different methods of ECG Axis Interpretation in this article. Ihave given a few more links to understand the same topic in better detail.