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ECG Basics

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The interpretation of ECG's can be a daunting prospect. However, by adopting a systematic process in your analysis of ECG's, you will be able to describe and identify rhythm abnormalities with confidence. There is a variety of systems which can be used to interpret ECG's, however, it is important to grasp the basics first.

Big and Small Boxes

An ECG is made of BIG and SMALL boxes.

Standard UK and US rhythm strips move at a speed of 25mm/second.

This equates to 300 BIG boxes/minute


A BIG box represents 0.2 seconds
A SMALL box represents 0.04 seconds 

A normal sinus rhythm consists of a P-wave, QRS complex and a T wave as seen below:
ECG sinus rhythm
Normal sinus rhythm
In normal sinus rhythm, the sino-atria node is responsible for atrial depolarisation with the atrioventricular node initiating ventricular depolarisation.

ECG sinus rhythm
P wave
QRS complex
T wave
PR interval
Everyone in practice has their own way to interpret ECG's. However, this 11 step method is the one that I can remember and use daily. It consists of; rhythm, rate, axis, P wave, P-R interval, QRS complex, Q wave, R wave, S wave, ST segment changes and T wave.

11 Step Interpretation

1. Rhythm
To identify whether the rhythm is sinus, regular or irregular we will firstly look at the P wave. A P wave before every QRS complex is a sinus rhythm.

If the interval between the QRS complexes is constant then the rhythm is regular. To check the interval between the R waves; mark the tips of 3 R waves on the edge of a piece of paper. Move the paper along the rhythm strip to align with other R waves. If the rhythm is regular the points on the paper will align with the other R waves on the ECG.

2. Rate
3. Axis
4. P Wave
5. P-R Interval
6. QRS Complex
7. Q Wave
8. R Wave
9. S wave
10. ST Segment Changes
11. T Wave
Now that you have been shown the 11 step approach to ECG interpretation, look at as many ECG traces as you can to improve your confidence. Don't be afraid to take your time. If you think this article is missing something or you want to help make it better, please contact us.

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