This guide demonstrates how to read an ECG in a systematic & effective manner

It ensures you don’t miss anything & allows you to present the results in a clear way

By following the steps below you can assess any ECG in a quick & effective manner 

Step 1 – Heart rate

Heart rate can be calculated simply with the following method:

  • Work out the number of small squares in one R-R interval
  • Then divide 300 by this number and you have your answer

E.g. If there are 4 squares in an R-R interval 300/4 = 75 beats per minute

Step 2 – Heart rhythm

Heart rhythm can be either regular or irregular

This can be determined by looking again at the R-R wave interval

If the R-R interval is inconsistent then the rhythm would be classed as irregular

An irregular rhythm with no distinct p waves suggests atrial fibrillation


Step 3 – Cardiac Axis

Cardiac axis describes the overall direction of electrical spread within the heart

In a healthy individual the axis should spread from 11 o clock to 5 o clock

To figure out the cardiac axis you need to look at leads I,II & III

To get a better understanding of Cardiac Axis read this article

Normal cardiac axis

In normal cardiac axis Lead II has the most positive deflection compared to Leads I & III


Right axis deviation

In right axis deviation Lead III has the most positive deflection & Lead I should be negative

This is commonly seen in individuals with Right Ventricular Hypertrophy

Left axis deviation

In left axis deviation Lead I has the most positive deflection & Leads II & III are negative

Left axis deviation is seen in individuals with heart conduction defects

Step 4 – P waves

Next we look at the p waves & comment on a number of things;

  • Are P-waves present?
  • Do they occur regularly?
  • Is there sinus rhythm (does a P-wave precede each QRS complex?)
  • Do the P-waves look normal? (smooth, rounded & upright)

If P-waves are absent & there is an irregular rhythm it may suggest atrial fibrillation


Step 5 – P-R interval 

The P-R interval should be between 0.12-0.2 seconds (3-5 small squares)

Are the P-R intervals consistent or do they change throughout the ECG?


A prolonged P-R interval may suggest the presence of heart block

A shortened P-R interval may suggest the presence of  Wolf Parkinson White Syndrome.

Step 6 – QRS complex

Check the width of the QRS complexes

The QRS complex should be 0.12 seconds (3 small squares)

If longer than 0.12 seconds it suggests the complex originated in the ventricles

If shorter than 0.12 seconds it suggests the complex is supra-ventricular in origin

Step 7 – ST segment

The ST segment is the part of the ECG between the end of the S wave & start of the T wave

In a healthy individual it should be an isoelectric line (neither elevated or depressed)

Abnormalities of the ST segment should be investigated to rule out pathology


ST elevation

ST elevation is significant when it is > 1mm (1 small square) in relation to the baseline

It is most commonly caused by acute myocardial infarction

The morphology of the ST elevation differs depending on how long ago the MI occured



ST depression

ST depression is significant when it is >1mm (1 small square) in relation to the baseline

ST-depression lacks specificity, therefore you shouldn’t jump to any diagnostic conclusions

It can be caused by many different things including;

  • Anxiety
  • Tachycardia
  • Digoxin toxicity
  • Haemorrhage, Hypokalaemia, Myocarditis
  • Coronary artery insufficiency
  • MI

As a result you must take this ECG finding & apply it in the context of your patient


Step 8 – T waves
Are the T waves inverted?

Inverted T waves are one of the most common abnormalities found on ECG

This is because T-waves can be altered by many different processes

As a result they lack specificity & should not used alone to form a diagnosis

Inverted T-waves in V1 & V2 are not significant & seen in healthy individuals


Some of the causes of inverted T-waves are:

  • Smoking
  • Anxiety
  • Tachycardia, Haemorrhage & Shock
  • Hypokalaemia, Pericarditis, MI (new & previous)
  • Bundle branch block
  • WPW syndrome

As a result you must take this ECG finding & apply it in the context of your patient



Are the T-waves Tall?

A T-wave is considered tall when it is greater than;

  • 5mm in the standard leads
  • 10mm in the precordial leads


Tall T-waves can be caused by;

  • Hyperkalaemia
  • Myocardial Ischaemia (usually hyper-acute MI)

In hyperkalaemia the T-waves are described as “Tall Tented T-waves”

This is because alongside been tall they are also very narrow, with a sharp apex

In hyper-acute MI you also get tall T-waves however they are not as narrow as in hyperkalaemia

Peaked T

Recommended reading

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Normal axis – wikipedia commons

Left Axis Deviation ECG –

1st Degree Heart Block –

Wolf Parkinson White Syndrome –

Normal QRS –

ST Elevation ECG –

ST Depression –

T wave inversion –

Tall T waves –