ECG Axis Interpretation

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:

  • 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:

  • 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

Summary Table:

Note:**Possible LADcan be further evaluated usingLead IIas 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

  • 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

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.

 

Leave a Comment