Electrocardiogram (ECG or EKG)

Each heartbeat is represented by an electrical impulse (or wave) that travels through the heart muscle, enabling the heart to contract and pump blood towards the organs. An electrocardiogram, also called an ECG or EKG, is a clinical test that records and measures the electrical activity of the heart. This test is done to assess if the heart is functioning properly and to check if this electrical activity is regular or not.

How does an ECG work?

Sensors, specifically called electrodes, are placed on the patient’s body (on the chest, arms, and legs). These electrodes detect the electrical signals generated by the heart with each beat. These signals are then recorded by a machine that displays them as a trace (or waves) on a screen and on paper.  

Why have an ECG?

An ECG may be prescribed for patients who are experiencing chest pain, palpitations or arrhythmia. It is used to detect abnormalities in the heart’s electrical activity and to diagnose various heart rhythm disorders. 

An ECG can detect numerous heart abnormalities: 

• heart attack or myocardial infarction
• heart rhythm disorders (arrhythmia)
• insufficient blood and oxygen supply (ischemia)
• thickening of the heart muscle walls (hypertrophy)

How does the exam take place? 

An ECG is a very quick, painless and completely safe test that usually lasts between 5 and 10 minutes. There are no contraindications to this test, which is done without anesthesia. It can be performed in the hospital, in a cardiologist’s office or even sometimes at home. 

The ECG is performed at rest, i.e. in a lying position. This is called a resting ECG. First, the physician will ask the patient to take off their shirt and lie down on an examination table. 

Then, self-adhesive sensors, called electrodes, will be placed directly on the skin at specific locations on the chest, arms (wrists) and legs (ankles). These electrodes are connected to cables and are used to pick up the electrical signals emitted by the patient’s heart. During a standard ECG, a total of 10 electrodes are placed on the patient’s skin. The resulting measurements are called leads.

If it is difficult to attach the electrodes to the skin due to significant hair growth, for example, sometimes certain areas of the torso might need to be shaved. 

How to prepare for it

An ECG does not require any special preparation. For example, it is not necessary to fast before this examination. 

Conversely, the following are recommended right before the test: 

• Get a good night of sleep 
• Avoid stimulants such as coffee or alcohol 
• Avoid smoking 
• Avoid using a moisturizing cream or balm on the chest area so that the electrodes will stick better to the skin

How to read an ECG

An ECG provides physicians with a lot of information about the electrical behavior of the patient’s heart. 

Each heartbeat is represented by an electrical impulse (or wave) that travels through the heart muscle, enabling the heart to contract and pump blood towards the organs. These waves leave the well-known trace that can be seen on the ECG. 

This sequence of signals is interpreted as follows: 

• the P wave corresponds to the moment when the atria contract. 
• the electrical impulse (or current) then travels to the ventricles. The interval between the P-wave and the Q-wave measures the time it takes for the nerve impulse to travel from the atria to the ventricles.
• The QRS complex corresponds to the moment of electrical activation of the ventricles.
• The ST segment corresponds to the end of the depolarization (contraction) of the ventricles.
• The T wave corresponds to the repolarization of the ventricles. The ventricles undergo an electrical change to return to rest and to allow them to prepare for the next heartbeat. 

Reviewed and approved by Marine Arnaud, MD, Cardiologist at Bordeaux University Hospital (CHU)