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 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:**

## AXIS: QRS Positive Lead I – QRS Negative Lead aVF

## AXIS: QRS Negative Lead I – QRS Positive Lead aVF

## AXIS: QRS Negative Lead I – QRS Negative Lead aVF

**Summary Table:**

*Note: **Possible LAD can be further evaluated using Lead II as 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:**

## QRS Positive Lead I – QRS Equiphasic Lead II – QRS Negative Lead aVF

## QRS Positive Lead I – QRS Negative Lead II – QRS Negative Lead aVF

## QRS Negative Lead I – QRS Positive Lead II – QRS Positive Lead aVF

## QRS Negative Lead I – QRS Negative Lead II – QRS Negative Lead aVF

**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 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.

*This concept can be difficult to understand at first, and is best illustrated by some examples.*

**Example 1**

## Answer – Lead I, II, aVF

- Lead I =
**POSITIVE** - Lead II =
**POSITIVE** - aVF =
**POSITIVE** - This puts the axis in the left lower quadrant (LLQ) between 0° and +90° – i.e.
**normal axis**

## Answer – Isoelectric Lead Method

**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**

## Answer – Quadrant Method

- Lead I =
**NEGATIVE** - Lead II =
**Equiphasic** - Lead aVF =
**POSITIVE** - This puts the axis in the left lower quadrant, between +90° and +180°, i.e.
**RAD**.

## Answer – Isoelectric Lead Method

**Lead II**(+60°) is the*isoelectric lead*.- 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**

## Answer – Quadrant Method

- Lead I =
**POSITIVE** - Lead II =
**Equiphasic** - Lead aVF =
**NEGATIVE** - 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.

## Answer – Isoelectric Lead Method

**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**

## Answer – Quadrant Method

- Lead I =
**NEGATIVE** - Lead II =
**NEGATIVE** - Lead aVF =
**NEGATIVE** - 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. *

## Answer – Isoelectric Lead Method

- The most isoelectric lead is aVL (-30°).
- The QRS axis must be at ± 90° from aVL at either +60° or -120°.
- Lead aVR (-150°) is positive, with lead II (+60°) negative.
- This puts the axis at -120°.

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

**Example 5**

## Reveal answer

- Lead I = isoelectric.
- Lead aVF = positive.
- This is the easiest axis you will ever have to calculate. It has to be at right angles to lead I and in the direction of aVF, which makes it exactly +90°!

*This is referred to as a “vertical axis” and is seen in patients with emphysema who typically have a vertically orientated heart.*

#### Causes of Axis Deviation

**Right Axis Deviation**

- Right ventricular hypertrophy
- Acute right ventricular strain, e.g. due to pulmonary embolism
- Lateral STEMI
- Chronic lung disease, e.g. COPD
- Hyperkalaemia
- Sodium-channel blockade, e.g. TCA poisoning
- Wolff-Parkinson-White syndrome
- Dextrocardia
- Ventricular ectopy
- Secundum ASD – rSR’ pattern
- Normal paediatric ECG
- Left posterior fascicular block – diagnosis of exclusion
- Vertically orientated heart – tall, thin patient

**Left Axis Deviation**

- Left ventricular hypertrophy
- Left bundle branch block
- Inferior MI
- Ventricular pacing /ectopy
- Wolff-Parkinson-White Syndrome
- Primum ASD – rSR’ pattern
- Left anterior fascicular block – diagnosis of exclusion
- Horizontally orientated heart – short, squat patient

**Extreme Axis Deviation**

- Ventricular rhythms – e.g.VT, AIVR, ventricular ectopy
- Hyperkalaemia
- Severe right ventricular hypertrophy

**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.

- EMS 12 Lead Axis Determination Part I
- EMS 12 Lead Axis Determination Part II
- EMS 12 Lead Axis Determination Part III
- EMS 12 Lead Axis Determination Part IV
- EMS 12 Lead Axis Determination Part V
- EMS 12 Lead Axis Determination Part VI
- EMS 12 Lead The 360 degree Heart

**LITFL Further Reading**

- ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis – ECG interpretation in clinical context
- ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
- 100 ECG Quiz – Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS – the best of the rest

**Advanced Reading**

- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
- Wagner GS. Marriott’s Practical Electrocardiography 12e
- Chan TC. ECG in Emergency Medicine and Acute Care
- Rawshani A. Clinical ECG Interpretation
- Mattu A. ECG’s for the Emergency Physician
- Hampton JR. The ECG In Practice, 6e

## ECG LIBRARY

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