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:
- shortness of breath
- palpitations (irregular heartbeat)
Heart block is an abnormal heart rhythm where the heart beats too slowly ( bradycardia). In this condition, the electrical signals that tell the heart to contract are partially or totally blocked between the upper chambers (atria) and the lower chambers (ventricles).
For this reason, it is also called atrioventricular block (AV block).
How the heart beats
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.
What is Heart Block?
A normal heartbeat is initiated by an electrical signal that comes from the heart’s natural pacemaker, the sinoatrial (SA) node, located at the top of the right atrium.
The electrical signal travels through the atria and reaches the atrioventricular (AV) node. After crossing the AV node, the electrical signal passes through the bundle of His.
This bundle then divides into thin, wire-like structures called bundle branches that extend into the right and left ventricles.
The electrical signal travels down the bundle branches and eventually reach the muscle cells of the ventricles, causing them to contract and pump blood to the body.
Heart block occurs when this passage of electricity from top to bottom of the heart is delayed or interrupted.
Types of Heart Block
First-degree heart block – The electrical impulses are slowed as they pass through the conduction system, but they all successfully reach the ventricles.
First-degree heart block rarely causes symptoms or problems. Well-trained athletes may have first-degree heart block. Medications can also cause this condition. No treatment is generally needed for first-degree heart block.
Second-degree heart block (Type I) – The electrical impulses are delayed further and further with each heartbeat until a beat fails to reach to the ventricles entirely.
It sometimes causes dizziness and/or other symptoms. People with normal conduction systems may sometimes have type 1 second degree heart block when they sleep.
Second-degree heart block (Type II) – With this condition, some of the electrical impulses are unable to reach the ventricles. This condition is less common than Type I, and is more serious.
Usually, your doctor will recommend a pacemaker to treat type II second degree heart block, as it frequently progresses to third degree heart block.
Third-degree heart block – With this condition, also called complete heart block, none of the electrical impulses from the atria reach the ventricles.
When the ventricles (lower chambers) do not receive electrical impulses from the atria (upper chambers), they may generate some impulses on their own, called junctional or ventricular escape beats.
Ventricular escape beats, the heart’s naturally occurring backups, are usually very slow.
Patients frequently feel poorly in complete heart block, with lightheadedness and fatigue.
Bundle Branch Block – With this condition, the electrical impulses are slowed or blocked as they travel through the specialized conducting tissue in one of the two ventricles.
Symptoms of Heart Block
Some people with heart block will not experience any symptoms. Others will have symptoms that may include the following:
- Fainting ( syncope)
- Dizziness Lightheadedness
- Chest pain
- Shortness of breath
Risk factors for Heart Block
Some medical conditions increase the risk for developing heart block. These medical conditions include:
- Heart failure
- Prior heart attack
- Heart valve abnormalities
- Heart valve surgery
- Some medications or exposure to toxic substances
- Lyme disease
What causes heart block?
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:
- other heart conditions, such as a heart attack
- certain infections, such as diptheria\certain medicines
- having open heart surgery
Heart block that occurs later in life and is caused by factors such as these is known as acquired heart block.
Diagnosing 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.j
The results of an ECG can also sometimes indicate the type of heart block you have.
Read more about diagnosing heart block.
Treating 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.
If a pregnant woman has an autoimmune condition, such as lupus, her unborn baby may develop congenital heart block. Congenital heart defects can also cause heart block.
However, congenital third degree heart block is rare, occurring in one in every 20,000 births.
Acquired heart block can affect people of all ages, but older people are more at risk because the risk factors are more common with age.
For example, your risk of developing heart block is increased if you have a history of heart disease, heart attacks or heart failure.
Elite athletes are at increased risk of developing first-degree heart block. This is because their hearts are often enlarged which can cause mild disruption to the heart’s electrical signals.
Taking certain medications, such as beta-blockers for treating high blood pressure, can also sometimes cause heart block.