ECG Axis Interpretation (2024)

Cardiac axis represents the sum of depolarisation vectors generated by individual cardiac myocytes. Clinically is is reflected by the ventricular axis, and interpretation relies on determining the relationship between the QRS axis and limb leads of the ECG (below diagram)

Since the left ventricle makes up most of the heart muscle under normal circ*mstances, normal cardiac axis is directed downward and slightly to the left:

Normal Axis= QRS axis between -30° and +90°.

Abnormal axis deviation, indicating underlying pathology, is demonstrated by:

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”).

Note that in paediatric ECG interpretation, the cardiac axis lies between +30 to +190 degrees at birth and moves leftward with age.

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:

Apositive QRSinLead Iputs the axis in roughly the same direction as lead I.
Apositive QRSinLead aVFsimilarly aligns the axis with lead aVF.
Combining both coloured areas – the quadrant of overlap determines the axis. So If Lead I and aVF are bothpositive, 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

Apositive QRSinLead Iputs the axis in roughly the same direction as lead I.
Apositive QRSinLead IIsimilarly 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 arebothpositive, the axis is between -30° and +90° (i.e. normal axis).

The combined evaluation of LeadI,LeadIIandaVF– allows rapid and accurate QRS assessment. The addition of LeadIIcan help determine pathological LAD from normal axis/physiological LAD
Note:Lead IIIoraVFcanbothbe used in 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 isPOSITIVEin any given lead, the axis points inroughlythesame directionas this lead.
If the QRS isNEGATIVEin any given lead, the axis points inroughlytheopposite directionto this lead.
If the QRS isISOELECTRIC(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 at90°to the isoelectric lead, pointingin the directionof the positive leads.

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

Examples

Example 1

Answer – Lead I, II, aVF

Lead I =POSITIVE
Lead II =POSITIVE
aVF =POSITIVE
This puts the axis in the quadrant 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 thataVL is located at -30°.
The QRS axis must be ± 90° from lead aVL, either at +60° or -120°
With leadsI(0),II(+60) andaVF(+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 quadrant, between +90° and +180°, i.e.RAD.

Answer – Isoelectric Lead Method

Lead II(+60°) is theisoelectric 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 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°) isisoelectric.
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
Acuteright ventricular strain, e.g. due topulmonary 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
Severeright ventricular hypertrophy

Mike Cadogan

BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Emergency physician, Sir Charles Gairdner Hospital. Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

Robert Buttner

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

FAQs

What is the normal ECG reading axis? ›

As evident from the figure, the normal heart axis is between –30° and 90°. If the axis is more positive than 90° it is referred to as right axis deviation. If the axis is more negative than –30° it is referred to as left axis deviation.

Learn More ›

How accurate is the ECG interpretation? ›

In evaluating the ECG-C interpretation of the cardiac rhythm, the ECG-C demonstrated an overall accuracy of 88.0%. Sinus rhythm was correctly interpreted in 95.0% of the ECGs with this rhythm. However, nonsinus rhythms were correctly interpreted with an accuracy of only 54%.

Find Out More ›

When interpreting a 12 lead ECG what findings will indicate a left axis deviation? ›

The axis will be pointed at right angles (perpendicular) to any lead that shows a biphasic complex and toward other leads that show relatively tall R waves. The normal mean QRS axis in adults lies between about −30° and +100°. An axis more negative than −30° is defined as left axis deviation (LAD).

Read On ›

What is normal axis range? ›

Each of the 4 quadrants represents 90° and an axis type (eg, 0° to +90° is a normal axis; +90° to 180° is RAD; 0° to -90° is LAD; and -90° degrees to 180 degrees is an extreme axis). Therefore, if leads I and aVF are positive, the axis falls within the normal range.

Get More Info Here ›

What is an abnormal cardiac axis? ›

An abnormal fetal cardiac axis, either larger or smaller than normal, is suggestive of a cardiac or intrathoracic anomaly and requires further in- vestigation, such as fetal echocardiography. The cardiac axis should be considered with the four-chamber view in fetal ultrasound evaluation.

Discover More ›

What is a good reading on an ECG? ›

If the test is normal, it should show that your heart is beating at an even rate of 60 to 100 beats per minute. Many different heart conditions can show up on an ECG, including a fast, slow, or abnormal heart rhythm, a heart defect, coronary artery disease, heart valve disease, or an enlarged heart.

Read On ›

Does a normal ECG rule out heart problems? ›

Abstract. Background It is commonly held that a normal ECG rules out the diagnosis of heart failure1; however this has only been demonstrated in the elderly.

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What should a healthy ECG graph look like? ›

Normal ECG obtained from ECG heart monitor looks like a smooth curve. The distance between each spike is almost constant. Each spike represents one whole heartbeat, the distance between spikes represents your heart rate.

Show Me More ›

What does AFIB look like on ECG? ›

This means an ECG showing atrial fibrillation will have no visible P waves and an irregularly irregular QRS complex. The ventricular rate is frequently fast, unless the patient is on AV nodal blocking drugs such as beta-blockers or non-dihydropyridine calcium channel blockers.

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How to read ECG results numbers? ›

ECG paper is marked with a grid of small and large squares. Each small square represents 40 milliseconds (ms) in time along the horizontal axis and each larger square contains 5 small squares, thus representing 200 ms. Standard paper speeds and square markings allow easy measurement of cardiac timing intervals.

Tell Me More ›

What is the normal axis of the ECG? ›

The normal QRS axis should be between -30 and +90 degrees. Left axis deviation is defined as the major QRS vector, falling between -30 and -90 degrees. Right axis deviation occurs with the QRS axis and is between +90 and +180 degrees.

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What does axis deviation tell us about the heart? ›

Right axis deviation (RAD) involves the direction of depolarisation being distorted to the right (between +90º and +180º). The most common cause of RAD is right ventricular hypertrophy. Extra right ventricular tissue results in a stronger electrical signal being generated by the right side of the heart.

MEASURING	MEN	WOMEN
HEART RATE	49 to 100 BPM	55 to 108 BPM
P WAVELENGTH	81 to 130 ms	84 to 130 ms
PR INTERVAL	119 to 210 ms	120 to 202 ms
QRS DURATION	74 to 110 ms	78–88 ms

ECG Axis Interpretation (2024)

Methods of ECG Axis Interpretation

Method 1 – The Quadrant Method

Now estimate the AXIS using the Lead I and aVF – Quadrant Method:

Summary Table:

Method 2: Three Lead analysis – (Lead I, Lead II and aVF)

Summary Table:

Method 3 – The Isoelectric Lead

Key Principles

Examples

Example 1

Example 2

Example 3

Example 4

Example 5

Causes of Axis Deviation

Right Axis Deviation

Left Axis Deviation

Extreme Axis Deviation

Further Reading

Further reading

Mike Cadogan

Robert Buttner

FAQs

What is the normal ECG reading axis? ›

What is the normal axis of the ECG? ›