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The Complete Basics of ECG Interpretation

This document is for doctors, nurses or medical students who have forgotten (or never really understood) ECGs. I’m not going to go into the ins and outs of ECG – just a guide to help you work out if the person in front of you is having a major cardiac event or not. I guess it will help house officers (FY1’s) who are suddenly faced with an ECG on the ward and also new A&E SHO’s who are continually asked to read ECGs for random people around the department. I have written this assuming some basic understanding of medicine and ECG's - if something really doesn't make sense please contact me.

So to start off with it’s a good idea to understand the layout of the ECG. Most departments use the following format below.

ecg zones

As you can see there are a few heart beats for each of the ‘leads’, such as lead I, II, III, V1, V2 etc. Each of these leads show the electrical activity in the heart viewed from a different angle. Our job is to interpret all these leads as a whole. One lead only tells the story from that angle – to understand what is going on you need to see it from all sides.

Different areas of this ECG describe different areas of the heart. As you can see there is are ‘inferior’, ‘anterior’, ‘septal’, and ‘lateral’ sections that we need to remember. Don’t worry about where these are exactly on the heart – knowing what they are called is enough for you to explain what you are talking about to the medical registrar on the phone.

Before we move on just to remind you about the ECG wave or complex - and what it means.


The p wave which corresponds to the atria contracting and then the QRS complex which is the ventricles contracting. The T wave is the just the ventricles relaxing.

At the bottom of the ECG printout (see first ECG) there is a long section with the heart beat shown all the way across the page. This is mainly used to work out the ‘rhythm’ i.e. if the patient as an irregular heart beat or not. What you do is look for the QRS complexes and then compare the distance between the complexes. If you look below at the first ECG you can see the QRS complexes are pretty much evenly spread apart (there are small changes in this distance with breathing which slows and speeds up the heart rate very slightly)

ecg distance

Now look at the ECG below you see I’ve highlighted the peaks of the complexes and you can see obvious differences in the spaces between the complexes. The ECG below is a good example of an irregular heart rhythm. The most common cause for an irregular heart rhythm is atrial fibrillation or AF.

atrial fibrillation

AF is when the atria have lost their ability to beat and contract in an organised well mannered way. Instead they randomly fire off and apparently it looks like a ‘bag of worms’ rather than a healthy pumping action. This random firing of the atria leads to a random heart beat. So to diagnosis AF you see an irregular heart beat and loss of the ‘P’ waves. If the patient goes into AF you won’t see any p waves at all.

In all the books I’m sure they go on about the calculating the rate and the axis etc. The object of this document is just to help you work out if the patient is in real trouble.

Looking for Ischaemia / Angina on an ECG

Obviously the thing to remember here is that a good history and examination is vital here - the ECG is useful but can be completely normal in someone with terrible real angina. So bear this in mind before you read on...

If possible try and get an ECG done previously when the patient isn’t having chest pain so you can see any differences which can be more useful than one ECG by itself.

The main area to look for is the ‘ST’ segment, which is just the flat area between the QRS and the T wave. The ST segment should be at the same level as the rest of the flat sections of the ECG trace – or the baseline. In ischaemia the ST segment sinks lower than the baseline, otherwise known as ST depression.

ST depression - angina

The other thing you can see on the angina ECG above is that the T wave (the last wave) is upside down. T wave inversion or even flattening is another sign of angina, but it is not a good one. It could also signify an old or current heart attack (myocardial infarction). But clues like this are useful when you become more experienced and you are trying to interpret more difficult ECG’s.

There are some things you should bear in mind before you can say if the patient really has angina or not.

wide QRS bundle branch block


If you are looking for evidence of infarction of the heart then you need to look at the same ST segment again. This time however you are looking for ST ‘elevation’. As you can see below (ECG section taken from Wikipedia) there is an elevation of the ST segment.

ST elevation myocardial infarction - image taken from wikipedia

There are some things you should bear in mind before you can say if the patient really has infarcted some cardiac muscle or not.

There are more scary ECG’s out there like ventricular tachycardia and also ventricular fibrillation.
These are always easy to see and nearly always are in a patient who look like they are about to die. In these situations if the patient looks very unwell - you should call the crash team and get someone more experienced to save the patient. I would advise you to attend a resuscitation course to know what to do in this situation.

Depending on the response I get from this document I will consider doing a more in-depth review of ECG interpretation – this is only meant to be a guide for the completely lost and/or scared. if there are any inaccuracies or would like more information – please email me using the contacts section.

Further Reading

The Alan E. Lindsay ECG Learning Centre

Ken Grauer, M.D. 12 lead ECGs a ‘brain web’

EKG World Encyclopedia


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