Electrocardiogram (ECG or EKG) 101: Purpose, Procedure & Risks

What is an electrocardiogram (ECG or EKG) and when do we need it? Learn all you need to know about ECG here.

by Homage team

What is Electrocardiogram (ECG)?

An ECG is a tracing of the heart’s electrical activity. To understand what an ECG is, it’s important to first understand how our heart works.

How Our Heart Works

The heart is a muscular organ with four chambers – the two atria at the top and two ventricles at the bottom. The heart needs to beat in an organised pattern so that blood is squeezed from the atria into the ventricles, then from the ventricles into the lungs or the rest of the body’s circulation. This all needs to happen in the right order and with the right power for every heartbeat – typically around once every second, but often considerably faster.

The heart beats when a tiny electrical current travels through the muscle. At the top of the heart, a natural pacemaker called the sinus node triggers a current and causes a heartbeat. This impulse travels through the top half of the heart to make the top chambers contract, then is slowed and reorganised before being transmitted to the bottom half of the heart. 

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The electrical impulse of the atria shows up as a small spike on an ECG. It is small because the muscles of the atria only need to pump blood into the ventricles, and so are fairly thin. When the impulse travels through the ventricles, a large spike appears on the ECG. An ECG shows electrical activity as a line on a chart. Any spikes in electrical current appear as spikes on the ECG chart. 

The left ventricle is the chamber which pushes blood all the way through the body and so has to be muscular. This results in a larger amount of electrical activity produced and therefore appears as a large spike on the ECG. The heart muscle cells then reset and the process begins again.

As damaged areas of muscle do not conduct electricity the same way as healthy tissues, they show up as abnormalities on an ECG. This allows us to identify the presence and location of damage, structural and electrical abnormalities, as well as the heart’s rhythm and rate. An abnormal ECG can tell us whether the top and bottom halves of the heart are working together in an organised way and even abnormalities in blood tests, such as potassium levels.

What does an ECG do?

An ECG allows us to measure and examine the electrical activity of the heart muscle. It gives medical teams more information about how a person’s heart is working and can be used to diagnose a wide range of different heart conditions, electrolyte imbalances, and some drug toxicities.

ECGs are often performed when someone displays symptoms of cardiac (heart) problems. They can show the kind of damage caused by a heart attack. ECGs can also detect abnormal heart rhythms (cardiac arrhythmias).

When do we need an ECG?

There are lots of reasons we might need an ECG. As it is a quick, inexpensive and safe procedure, there are no real contraindications to having an ECG.

Your doctor may recommend an ECG for individuals who:

  • Have a pre-existing heart condition
  • Have had symptoms that could indicate a heart condition, such as chest pain, breathlessness, dizziness, fainting, or fast or irregular heartbeats
  • Have a family history of heart diseases
  • Are a smoker
  • Are overweight
  • Have diabetes
  • Have high cholesterol
  • Have high blood pressure

Certain medications can also have an effect on the heart rhythm and function, and therefore individuals taking these medications may have ECGs as part of their regular health check.

Acute heart conditions can be life-threatening medical emergencies, so anyone with symptoms that could indicate an acute heart condition should have an urgent ECG. ECGs can be used to confirm or rule out heart attacks, heart rhythm disturbances such as unusually fast, slow or irregular heartbeats, significant pauses between heartbeats, and many other conditions. An ECG can also show abnormalities indicating some non-cardiac conditions, such as pulmonary embolism (a blood clot in the lung) or certain central nervous system disorders.

An ECG may also be taken routinely as part of a fitness assessment, particularly for elite athletes, and can be a medical screening requirement for people entering certain occupations, such as applying for a pilot’s license.

Types of Electrocardiograms

12-lead ECG

An ECG can take a snapshot and print a page displaying a few seconds of the heart’s electrical activity. This printout is commonly called a 12-lead ECG. There are actually 10 wires attached to the body for a 12 lead ECG, and the ‘leads’ refer to the different ‘views’ of the heart captured by detecting the electrical activity between two different points of attachment.

This can also be referred to as a ‘resting’ ECG, because you lie down and stay still for the procedure. Some heart rhythm monitors are ‘ambulatory’; they can be worn while up and about.


Electrocardiography can also refer to a longer recording of the heart rhythm. If the doctor suspects that you may have or are at a high risk of cardiac arrhythmia, you may be attached to a ‘telemetry’ device. A telemetry is a heart monitor attached to your chest by a wire at several points. It transmits a live recording of the ECG to a manned computer station, which records events and alerts staff to attend a patient if there are any issues.

Short-term home monitoring – Holter monitor or 24, 48, or 72 hour tape

If someone is suspected to have intermittent arrhythmias causing palpitations, blackouts or other symptoms, they may be fitted with a heart monitor which can be taken home. This is used to record a day or more of their heart’s electrical activity while they are going about their normal life. This is useful if someone has occasional symptoms which may be cardiac in origin, but have not occurred during hospital monitoring. The heart monitor is attached by a wire to stickers on the chest and should fit in a pocket or small bag.

Implantable Loop Recorder (ILR)

For people who have occasional symptoms of cardiac arrhythmias, it is sometimes helpful to have an implantable loop recorder. This is a small device implanted just under the skin, usually on the left upper chest, and has a battery which can last for several years. It records heart rhythm events and transmits relevant information for heart rhythm teams to review. Some ILRs automatically detect heart rhythm disturbances, while others record ECG samples when the person develops symptoms and activates the device.

Implanting a loop recorder is a safe procedure which only takes a few minutes. It is done with local anaesthetic and a small incision. The loop recorder can be removed easily once a diagnosis has been made.

Stress test or exercise tolerance test

During a stress test, an ECG is taken while the heart is encouraged to work harder. This usually happens while a person is exercising, such as walking on a treadmill or riding an exercise bike. Some medications can also be used to make the heart work harder for a short time during the test, but this method is usually for people who are unable to exercise due to other health problems or limited mobility.

Preparing for an ECG

An ECG is non-invasive and does not require much preparation. You may be told to avoid caffeine and cigarettes before the test as they can drive the heart to beat faster and make the ECG harder to interpret. The ECG stickers need to attach well so it’s good to be clean and dry and not to use any lotions, ointments or talc. People with hairy chests may need to have small patches shaved where the stickers will be placed.

An ECG can be conducted in a hospital or clinic, and some people find it helpful to have support from home carers or nurses to help them during appointments and travel.

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What happens during an ECG Test?


The test is usually performed by a healthcare assistant or nurse who has been trained to use the equipment. They should explain what they are doing at every step. It’s important that your chest is clean and dry and free from any greasy products or talc. You may be able to wear your own clothes as long as they can be moved to allow the ECG stickers to be attached to your chest and limbs. If you are wearing a bra, you may be asked to remove it. You can bring someone with you as a chaperone or as support for the procedure if you want to.


You will usually be asked to lie on a bed and expose your chest. Six stickers will be placed at specific points on the chest. There will be one sticker on each limb and a wire will be attached to each sticker. The medical professional will then ask you to lie still for a short time while the ECG is printed out. The whole procedure is painless and you will not feel anything from the ECG being taken.


You can get up and dressed immediately after an ECG. There are no after-effects. Removing the stickers straight away helps to prevent skin irritation.

The ECG should be reviewed by a doctor. If the ECG was a routine appointment, this may not happen immediately and you may only get the results after a couple of days. If your ECG is taken because you have ongoing symptoms or are in an urgent situation, the ECG should be reviewed almost immediately.

Risks of an ECG

Taking an ECG is a procedure with minimal risk; it is completely non-invasive and only reads electrical activity – it does not emit a current or give any treatment. There is only a minor risk of slight skin irritation from the small sticky pads attached to your body.

ECG findings can sometimes be subtle and there can be variances in the appearance of the ECG depending on various factors including lead position, obesity, breast tissue, swelling, movement or tremor, and the effects of some drugs. This means that there is a chance that ECG findings alone will not be enough to make an accurate diagnosis or could theoretically contribute to a misdiagnosis.

ECG Reading: Normal ECG vs Abnormal ECG

An ECG can be used to show a wide range of heart conditions – some of the most common serious conditions which can be diagnosed by an ECG include:

Myocardial Infarction (MI) or Heart Attack

A heart attack is a blockage or narrowing in the blood vessel(s) supplying oxygen and nutrients to the heart muscle. When this causes damage to an area of the heart muscle, the way electricity is conducted across those cells changes. This affects the appearance of the ECG.

MIs are categorised according to the severity of the ECG findings: a STEMI (ST-elevation MI) is usually more serious and requires more immediate action than an NSTEMI (NON-ST elevation MI).

Heart Blocks

The electrical conduction pathway from the top half to the bottom half of the heart can be disrupted. There are several forms of heart block, all of which have specific ECG patterns, and may cause the heart to beat unusually slowly or irregularly.

Atrial Fibrillation (AF)

AF is the most common heart rhythm disturbance and occurs when there is little organised electrical activity in the top half of the heart, resulting in an irregular heartbeat.

Inflammation or Infection In or Around the Heart

Pericarditis refers to an inflammation of the pericardium – the sac around the heart. Myocarditis is the inflammation of the heart muscle. Both conditions can cause ECG changes.

Do note that not all heart problems show up on an ECG. An ECG only shows the electrical pattern of the heart function and some conditions do not necessarily affect this. Other heart problems may also only show up on an ECG intermittently or occasionally.

If you have any concerns about heart problems, it’s important to see a doctor urgently. Symptoms like unexplained blackouts, chest pain, or sudden severe shortness of breath should be treated as a medical emergency.

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  1. American Heart Association (2015) Electrocardiogram (ECG or EKG) https://www.heart.org/en/health-topics/heart-attack/diagnosing-a-heart-attack/electrocardiogram-ecg-or-ekg
  2. https://www.healthy-heart.org (2021) Warning Signs and Symptoms https://www.healthy-heart.org/your-heart/warning-signs-symptoms/
  3. NHS (2018) Electrocardiogram (ECG) https://www.nhs.uk/conditions/electrocardiogram/
  4. De Bacquer D, De Backer G, Kornitzer M (2000) Prevalences of ECG findings in large population based samples of men and women. Heart. https://heart.bmj.com/content/84/6/625
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