Resting Electrical Recording
Electrocardiography (ECG)
An electrocardiogram records the electrical activity of the heart through electrodes placed on the skin. It is a quick, painless test and one of the first steps in assessing the heart rhythm.
On this page
Understanding the test
What Is an ECG?
An electrocardiogram, or ECG, picks up the tiny electrical signals the heart produces with each beat. Sticky electrodes on the chest, arms and legs detect these signals, and a machine records them as a trace.
A standard ECG lasts only a short time. It gives a snapshot of the heart rhythm at the moment of the test.
Reading the trace
What Does an ECG Record?
Each heartbeat produces a pattern of electrical activity. The ECG shows the stages of that pattern, which doctors interpret together.
Atrial activation
The upper chambers are activated, which produces the first wave of the beat.
AV conduction
The signal pauses briefly at the AV node before passing on.
Ventricular activation
The lower chambers are activated, which produces the largest part of the trace.
Ventricular recovery
The lower chambers reset, ready for the next beat.
The test
How an ECG Is Performed
You lie down and rest. Small electrodes are placed on the skin, and the skin may be cleaned first. Leads connect them to the ECG machine.
The recording is made while you lie still and takes a short time. The test is non-invasive and does not involve electricity entering the body. The exact routine depends on the clinic.
Electrode placement, conceptual illustration
What the trace shows
Heart Rate and Rhythm
The ECG helps answer several basic questions.
Rate
How fast the heart is beating at the time of the recording.
Regularity
Whether the beats are evenly spaced or irregular.
Origin of the rhythm
Whether the beats start in the normal pacemaker or somewhere else.
Conduction pattern
Whether signals pass normally between the chambers.
The electrical pathway
Electrical Conduction
The ECG reflects the way the electrical signal travels through the heart. Problems at any step can change the trace.
SA node
The natural pacemaker starts the signal.
Atria
The signal spreads through the upper chambers.
AV node
The signal slows briefly before passing on.
His-Purkinje
Fast fibres carry the signal into the lower chambers.
Ventricles
The lower chambers contract.
The Electrophysiology Study records these steps from inside the heart when more detail is needed.
Possible findings
What an ECG May Show
An ECG can suggest or show a range of rhythm and conduction findings if they are present during the recording.
An irregular rhythm without a regular atrial pattern.
A regular atrial pattern that repeats.
A fast, regular rhythm from above the ventricles.
Slow rhythms or delays in conduction.
Early beats from the ventricles.
A fast rhythm from the ventricles.
Signs of an extra pathway, as in WPW.
An ECG shows these only if they happen during the recording. Intermittent rhythms can be missed.
Where to read next
ECG and Arrhythmias
Each rhythm disorder has its own page with more detail on how it is assessed and treated.
Comparison
ECG vs Holter
The two tests record the same kind of signal in different ways. Neither is better. They answer different questions.
Resting ECG
Short snapshot
A brief recording at one moment, usually lying still.
Useful for rhythm and conduction at that time.
Holter monitor
Continuous recording
A portable recorder worn during normal daily life.
Useful when symptoms come and go over hours or days.
Comparison
ECG vs Event Recorder
When symptoms are infrequent, a longer monitoring strategy may be used.
Resting ECG
Short snapshot
A brief resting recording made in clinic.
Shows the rhythm at that moment only.
Event recorder
Intermittent, longer monitoring
Worn or carried for longer and used when symptoms occur or when the device detects a change.
Helps link rare symptoms to the rhythm.
Important to know
Limitations
A normal ECG does not rule out a rhythm disorder that comes and goes. If the heart rhythm is normal during the short recording, the ECG may look normal even when symptoms are caused by an arrhythmia.
For this reason doctors may combine the ECG with longer monitoring, other tests and the history of your symptoms. Where the question concerns rhythm during exertion, an Exercise ECG may also be considered.
After the test
Results
A physician reviews the trace and interprets it together with your symptoms, history and examination. Some results are explained straight away. Others lead to further tests.
An unusual finding on an ECG does not always mean there is a problem, and the significance depends on the situation.
Questions