Electrocardiogram (ECG)
An electrocardiogram (ECG) is a simple test that can be used to check your heart's rhythm and electrical activity.
Sensors attached to the skin are used to detect the electrical signals produced by your heart each time it beats.
These signals are recorded by a machine and are looked at by a doctor to see if they're unusual.
An ECG may be requested by a heart specialist (cardiologist) or any doctor who thinks you might have a problem with your heart, including your GP.
The test can be carried out by a specially trained healthcare professional at a hospital, a clinic or at your GP surgery.
Despite having a similar name, an ECG isn't the same as an echocardiogram, which is a scan of the heart.
When an ECG is used
An ECG is often used alongside other tests to help diagnose and monitor conditions affecting the heart.
It can be used to investigate symptoms of a possible heart problem, such as chest pain, palpitations (suddenly noticeable heartbeats), dizziness and shortness of breath.
An ECG can help detect:
arrhythmias – where the heart beats too slowly, too quickly, or irregularly
coronary heart disease – where the heart's blood supply is blocked or interrupted by a build-up of fatty substances
heart attacks – where the supply of blood to the heart is suddenly blocked
cardiomyopathy – where the heart walls become thickened or enlarged
A series of ECGs can also be taken over time to monitor a person already diagnosed with a heart condition or taking medication known to potentially affect the heart.
How an ECG is carried out
There are several different ways an ECG can be carried out. Generally, the test involves attaching a number of small, sticky sensors called electrodes to your arms, legs and chest. These are connected by wires to an ECG recording machine.
You don't need to do anything special to prepare for the test. You can eat and drink as normal beforehand.
For the purpose of producing an electrocardiogram, the standard practice now is to place 10 electrodes in the following 10 different positions on the body: 4 Limbs ( Left Arm, Right Arm, Left Leg and Right Leg) + 6 on the chest on specific locations near the heart. Potential (Voltage difference) is measured between two of the electrodes or a common virtual electrode, called as the Wilson’s central terminal. Wilson’s central terminal is the average potential measurements between three of the limb electrodes – Right arm, Left arm and Left Leg. These voltage difference measurements are called “Leads”.
In all 12 ‘leads’ are required to complete the ECG graph. These 12 leads include, 3 limb leads, 3 augmented limb leads and 6 chest leads (also called precordial leads). While the three limb leads are measurements of potential / voltage difference between the limb electrodes, the augmented limb leads and chest leads (also called precordial leads) are potential difference between the limb and chest electrodes vs the virtual electrode (Wilson’s central terminal).
Often people use the term ‘lead’ for electrodes also and hence cause confusion as to whether there are 10 leads or 12 leads in an ECG machine.
When a patient cardiac activity needs to be monitored for a longer period of time; ECG variants with fewer number of electrodes are used to help aid mobility – such as with 3-electrodes or 5-electrodes (These may be confusingly called 3-lead / 5-lead ECGs – but the numbers 3 & 5 here refer not to the number of leads obtainable but the number of electrodes used). As against the 12-leads ECGs, the 3 & 5-electrode ECGs capture less information. The 3-elecrode ECGs -capture only the limb-leads but no anterior view, hence useful for rhythm monitoring. In a 5-electrode ECG all the 6 limb leads and one chest view is also captured. If someone says e.g. 5-channel ECG or 5-leads ECG, they essentially mean 5-electrodes ECG. So, what is a channel?
Channel – The other term commonly used in reference to ECG machine is number of ‘channels’. ECG machines may come as single channel, 3-channel, 6-channel or 12-channel configurations. Each lead measurement done aPre-owned GE Mac ECG machines detailed above is then passed through an ‘amplifier’ channel and recorded. In a single-channel system, each lead is recorded and displayed one at a time.
In a multi-channel system, the ECG has a microprocessor which manages this processing of signals based on the number of channels. The signals are amplified, filtered and sent to a multiplexer. The multiplexer receives all the inputs, reconstructs signals, de-multiplexes and sends the output to display screen or chart recorder. If it is a 3-channel ECG, the microprocessor groups the signals received from the 12-leads, in 4 groups of 3 and processes them. Similarly, in 6-channel it is in two groups and 12-channel device the same 12-leads are processes simultaneously. Hence, higher the number of channels, faster the ECG recording would be. Clearly, ‘channels’ is different from ‘electrodes’ or ‘leads’.
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