**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.

**Method 3 – The Isoelectric Lead**

** 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.