# Note from LITFL: ECG Axis Interpretation

### The note is so good: We hijack it 100% 😂

there are 3 methods here.

1. The Quadrant Method – (Lead I and aVF)  – will leave you with POSSIBLE LAD in some case
2. Three Lead analysis – (Lead I, Lead II and aVF) – the simplest and faster (RECOMMENDED)
3. Isoelectric Lead analysis – MOST ACCURATE

### The diagram below illustrates 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”). #### 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: • 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 II 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:

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

• 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). • Note: Lead III or aVF can both be used in three lead analysis Now estimate the AXIS using Three Lead analysis: Summary Table: #### 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 directionto 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 • aVF = POSITIVE
• This puts the axis in the left lower quadrant (LLQ) between 0° and +90° –  i.e. normal axis

• 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 • This puts the axis in the left lower quadrant, between +90° and +180°, i.e. RAD.

• 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 • This puts the axis in the left upper quadrant, between 0° and -90°, i.e. normal or LAD.
• Lead II is neither positive nor negative (isoelectric), indicating physiological LAD.

• 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 • 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.

• The most isoelectric lead is aVL (-30°).
• The QRS axis must be at ± 90° from aVL at either +60° or -120°.
• This puts the axis at -120°.

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

#### Example 5  