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”).

#### Methods of ECG Axis Interpretation

There are several complementary approaches to estimating QRS axis, which are summarised 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:**

**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:**

**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**

- 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**

**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**

- 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**.

**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**

- 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.

**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**

- 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. *

- 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

- 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 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 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

#### Advanced Reading

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

#### LITFL Further Reading

- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis –alphabetical diagnostic approach to the ECG
- ECG CLINICAL CASES — ECG’s placed in clinical context with a challenging Q&A approach
- 100 ECG Quiz — Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS — the best of the rest
- LITFL ECG IMAGE Database — Searchable database of LITFL ECG’s
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for answering ECG exam questions.

**Last update:**

Dr Abdulmalik Mumin says

This is fantastic, I’ve been seeking for this explanation for God knows when. I’m pleased to have come across this.

Thanks alot

Dr Frank Leader says

Hey there,

I’m not sure why a third lead needs to be incorporated if you accept a normal axis of -30 to +90. The only leads that are required are I and II. Both are at 90 degrees to the extremes of axis and so if I and II are positive, the axis has to be between -30 and +90. If I is positive and II negative, we have to be beyond -30 but greater than -90 and so we have LAD. If I is negative and II positive, we have to be beyond +90 but less than +150 and so we have RAD. If both are negative, then we are beyond +150 and in extreme RAD territory (if you really care about just how extreme you can use AVF to determine if you are beyond +180, but that is likely to be unnecessary).

The old approach with I and II works perfectly:

Leaving = LAD

Reaching = RAD

Upright = Usual

Neither = NW (or thereabouts)