In people with heart block the electrical pulses that control the heart rate are disrupted, causing the heart to beat more slowly.
It's a type of arrhythmia, which is a medical term used to describe problems with the rate or rhythm of the heartbeat.
There are three levels of heart block (see below), and usually only the most serious type causes symptoms. This type of heart block is known as a complete, or third-degree, heart block.
Symptoms of heart block can include:
When the heart beats normally, the heart muscle contracts (pulls inwards) ready to pump blood around the body. The contractions are triggered by electrical pulses generated by a group of specialised cells in the heart known as the sinoatrial node (SA node).
The SA node, also known as the pacemaker, generates electrical pulses at regular intervals. The pulse is sent to another group of cells, called the atrioventricular node (AV node), which relays the pulse to the two lower chambers of the heart (the ventricles).
A heart block occurs if the transmission of the pulse between the SA node, the AV node and the ventricles is interrupted.
If you have a heart block, your heart won't stop beating altogether – other ‘back-up’ systems will take over. But it can cause an abnormally slow heartbeat (bradycardia), which deprives the body’s organs and tissue of oxygen.
There are three main types of heart block:
In first-degree heart block, there's a split-second delay in the time it takes electrical pulses to move through the AV node. First-degree heart block rarely causes any noticeable symptoms.
In second-degree heart block there's a series of increasing delays in the time it takes the AV node to send the pulse to the ventricle. This will eventually lead to a heartbeat being skipped.
There are two sub-types of second-degree heart block that are known as:
In third-degree or complete heart block there's no transmission of electrical pulses between the atria and the ventricles through the AV node.
A third-degree heart block causes a wide range of symptoms, some of which are life-threatening. This type of heart block can be a medical emergency, although in many cases it's mild and doesn't require treatment.
All types of heart block can increase your risk of developing other types of arrythmia, such as atrial fibrillation (an irregular and abnormally fast heart rate).
Some people are born with heart block, either because of a congenital heart defect or a genetic condition. This is known as a congenital heart block.
More commonly, heart block occurs later in life. It can be caused by:
Heart block that occurs later in life and is caused by factors such as these is known as acquired heart block.
Read more about the causes of heart block.
Unless you're experiencing symptoms, heart block is often diagnosed during routine tests for other conditions.
Babies are sometimes diagnosed with heart block during pregnancy.
An electrocardiogram (ECG) is the main test that's used to diagnose heart block. It measures the electrical activity of your heart.
During an ECG, small electrodes are placed on your chest and connected by wires to a monitor. The electrodes record a trace of your heart's rhythm and measure the strength and frequency of the elctrical signals.
An ECG can be carried out at rest or while you're exercising, and provides a useful overall assessment of how well your heart is working.
The results of an ECG can also sometimes indicate the type of heart block you have.
Read more about diagnosing heart block.
Heart block doesn't always need to be treated, unless the symptoms are serious. However, acquired heart block usually needs treatment.
Transcutaneous pacing (TCP) is a temporary method of pacing the heart. Pads are applied to your chest and electrical pulses are passed through them to stabilise your heartbeat and restore your heart rate to normal.
Once your heartbeat has been stabilised, a permanent pacemaker may be recommended. Pacemakers are small battery-operated devices that are inserted under the skin of your chest. It sends frequent electrical pulses to keep your heart beating regularly.
Treatment for heart block usually works well if it's given when required. Deaths caused by heart block are rare.
Read more about how heart block is treated.