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Understanding Your ECG

Nikhil Ahluwalia

• 6 min read
Understanding Your ECG

This summary gives a very simple overview of how to analyse your own ECGs. If you have ever wondered what the red lines on your ECG mean then read on. Note- this is not a substitute for medical interpretation, especially if you experience new or worsening symptoms. However, the automated labels can sometimes be 'inconclusive' or wrong, so it is always good to have a basic understanding.

This is a '12-lead ECG' (a reflection of the number of leads and stickers used to generate it). It is the most commonly performed ECG type within hospitals. The black lines represent the heart rhythm over a 10-second period.

The Fundamentals

An electrocardiogram (ECG) is a recording of your heart’s electrical activity over time. The heart is the predominant site of electrical activity in the chest and as ECG leads measure the difference in voltage between two connected points, they are essentially measuring the voltage across the heart. On a hospital ECG machine, this is done by placing leads and stickers stickers across the body and on a smartwatch this is done by placing your two hands on different parts of the watch.

This is an image of an early ECG machine. It's ability to record the electrical activity of the heart was reported in 1893 by Willem Einthoven. He measured the difference in electrical potential between the left arm and left foot. Image taken from here

A standard hospital ECG typically captures 10 seconds of heart rhythm data data. Consumer devices, such as the Apple Watch, usually record over 30 seconds.

The electrical activity is then plotted as a wave tracing across time:

This is a 'single-lead' ECG recording. It shows three complexes which represent three heart beats that are occurring about once per second, based on the gap between each complex.

Hospital ECGs often include multiple rows of complexes. These correspond to different pairs of “leads” - each demonstrating the electrical activity from different perspectives to show regional differences. This allows for assessment of how electrical signals move through the heart and helps localise issues, such as heart attacks that can effect different sections of the heart.

This ECG shows changes in some of the complexes that are associated with a heart attack. These cannot be seen on a smart watch and require a 12-lead ECG to identify. This must be done by a specialist and all new Chest Pain should be considered a medical emergency, requiring urgent ECG assessment as this pattern can suggest heart damage and lead to death.

The 'Normal' Sinus Rhythm ECG

During sinus rhythm each heartbeat generates a repetitive pattern of electrical signals:

  • QRS complex – the tall, sharp spike that represents contraction of the main pumping chambers (the ventricles)
  • P wave – the small hump before the main spike that signals contraction of the upper chambers (the atria)
  • T wave – hump after the spike that shows recovery of the ventricles

In normal sinus rhythm, these occur in a regular sequence and at generally consistent intervals.

This is a single labelled complex to represent one beat of the heart. It shows atrial contraction (P wave) first → followed by the ventricular contraction (QRS) → and finally ventricular relaxation (T wave)

What Happens in Atrial Fibrillation?

In atrial fibrillation (AF), the normal sequence is disrupted:

  • P waves are absent due to disorganised electrical activity in the atria
  • The QRS complexes occur at irregular intervals
  • The heart rate may be elevated and variable

These changes are hallmarks of AFib and can often be seen on a short ECG strip.

This is a single-lead ECG showing AFib. The spikes are at irregular intervals and there are no clear P-waves before the spikes.
I have shown here a short ECG strip of normal sinus rhythm as a comparison. Note the consistent spaces between the spikes and the single P-wave before each QRS complex.

The AF Exceptions

However, it is not as simple as this distinction as not all irregular ECGs are AF. It’s important to differentiate AF from other conditions or artefacts and I have included some of the common misinterpreted cases below:

Supraventricular tachycardia (SVT) – this is a different arrhythmia, a regular, fast rhythm that also arises from the upper chambers. It can be challenging to differentiate from AFib, especially because it can present with overlapping symptoms such as palpitations. A key difference on ECG is the organisation and regularity of an SVT. However, it is important to differentiate this from AF because it is not associated with the same stroke risk and treatment plans can differ.

This is an ECG of an SVT. The spikes are frequent reflecting a fast rate (189 beats per minute) and there are no obvious P-waves. However the rhythm is very regular- which AF is not.

Premature Atrial Complexes (PACs) - Also, known as ectopics, these can appear like an irregular rhythm on an ECG and can be confused with Atrial Fibrillation. They can also present with palpitation symptoms and can co-exist in people with AF.

Focus on the bottom row which shows the heart rhythm from one view over 10 seconds. This ECG here shows an irregular rhythm (variable spacing between the tall spikes) as would be seen in AF. However, there are P-waves before each spike. The shape of the P-wave differs across the strip suggesting it is coming from an 'ectopic' source during these beats.

Artefact – This is a very common reason for ECG misinterpretation. If you have tried to record an ECG on a smartwatch, you will know that movement, tremor or instability will result in a really messy tracing- this 'noise' is known as artefact because it does not reflect underlying electrical activity. It can mimic irregular rhythms and the simplest solution is to try and acquire a repeat recording that is clearer.

This ECG looks like chaos. There is a lot of waviness in the baseline and it is hard to identify what is a genuine spike and what is a result of movement (known as artefact). If you study it carefully you make be able to detect the underlying true QRS spikes that appear to be occuring at regular intervals. However, it is not possible to make any diagnosis from a tracing like this and a repeat recording would be needed.
  • Tachycardia-bradycardia syndrome (tachy-brady) – the rhythm on an ECG is not always continuous across the tracing. The rate and rhythm can vary across the ECG which is why it is important to look at the whole recording. In some cases of atrial fibrillation, the rhythm can vary between very fast (tightly spaced spikes) and very slow (periods with no spikes)- as we discussed last week.
This tracing shows dramatic reversion pauses the sections of fast rhythm (the tightly clustered spikes) are irregular and have no P-waves. These are followed by long pauses where there are no spikes. Then if you look carefully after the pause, there is a sinus rhythm beat (reflected by the P-wave) demonstrating that these are reversion pauses.

In such cases, a single ECG may not provide the full picture. Longer recordings, such as 24-hour Holter monitors or wearable ECG patches, help capture intermittent abnormalities and correlate them with symptoms.

Symptom Correlation Is Always Key

Alongisde ECG interpretation, understanding whether symptoms like palpitations, dizziness, or breathlessness coincide with the ECG changes is critical. Symptom–rhythm correlation allows clinicians to tailor treatment. This is why recording ECGs during symptoms can be so helpful.

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