Heart block can have a number of different causes. It can occur if you have another heart condition or if you take certain medications.

A person can be born with heart block (congenital) or it can develop over time (acquired).

First-degree heart block

First-degree heart block is common among well-trained athletes. Vigorous and prolonged exercise can enlarge the heart muscles, which can cause mild disruption to the heart’s electrical signals.

Other causes of first-degree heart block include:

• myocarditis – inflammation of the heart muscle
• low levels of potassium in the blood (hypokalaemia)
• low levels of magnesium in the blood (hypomagnesemia)

Certain medications can also cause first-degree heart block, including:

• medications for treating abnormal heart rhythms (antiarrhythmics), such as disopyramide
• medications used to treat high blood pressure (hypertension), such as calcium channel blockers
• digoxin – a medication used to treat heart failure

Second-degree heart block

Athletes can develop second-degree heart block for the reasons discussed above.

Some children born with congenital heart disease (heart defects present at birth) can also develop second-degree heart block.

Other causes of second-degree heart block include:

• damage that develops during a heart attack
• Lyme disease – a bacterial infection spread by tics
• certain medications, such as calcium-channel blockers (used to treat high blood pressure), amiodarone (used to treat abnormal heart rhythms) and pentamidine (used to treat some types of pneumonia)

Third-degree congenital heart block

Many cases of third-degree congenital heart block occur in babies whose mothers have an autoimmune condition, such as lupus (a long-term condition that causes inflammation in the body’s tissues).

In autoimmune conditions, the immune system produces antibodies (proteins) that attack and damage tissue and cells.

The immune system is thought to mistake the unborn baby for a foreign object, as it would a virus or bacteria, and sends antibodies to attack it which damage the heart.

Some children with congenital heart disease are also born with third-degree heart block.

Third-degree acquired heart block

Many cases of third-degree heart block are caused by damage to the heart. The heart muscle can become damaged for a number of reasons, including:

• a complication of heart surgery – this is thought to be one of the most common causes
• coronary heart disease – where the heart doesn’t receive enough blood due to acquired narrowing or closing of the coronary arteries
• a complication of radiotherapy – a treatment for conditions such as cancer, thyroid disorders and some blood disorders
• a serious infection, such as diphtheria (a bacterial infection that can cause heart inflammation) or rheumatic fever (a bacterial infection that damages the heart’s valves)
• poorly controlled high blood pressure (hypertension)
• cancer that has spread from another part of the body to the heart
• a penetrating trauma to the chest, such as a stab wound or gunshot wound
• a complication of treatment for heart rhythm problems (arrhythmia), known as radiofrequency ablation

A number of medications can also cause third-degree heart block including:

• digoxin
• calcium-channel blockers
• beta blockers – used to treat high blood pressure
• tricyclic antidepressants – an older type of antidepressant
• clonidine – used to treat a sudden, sharp rise in blood pressure (hypertensive crisis)

Unless you’re experiencing symptoms, heart block is often diagnosed during routine tests for other conditions.

Your GP may suspect heart block based on your medical history, family history and a physical examination.

During a physical examination, your GP may check your pulse and listen to your heart using a stethoscope to check for any abnormal heart rhythms.

They may refer you to a cardiologist (heart specialist) who’ll be able to carry out more specialised tests, such as an electrocardiogram (ECG).

Electrocardiogram (ECG)

An electrocardiogram (ECG) measures your heart’s electrical activity. It records a trace of the rhythm of your heart and the strength and frequency of the elctrical signals.

An ECG can be carried out while you’re exercising (usually on a treadmill or an exercise bike) or while you’re resting.

The results can provide a useful overall assessment of how well your heart is working. In some cases it can also indicate whether you have a Mobitz type 1 or Mobitz type 2 heart block.

An ECG is sometimes recorded over a 24-48 hour period because the heart rate can become much slower at various times of the day or night. This is often known as Holter monitoring.

Other tests

Several other tests can be used to diagnose the type and cause of heart block.

First-degree heart block

First-degree heart block doesn’t usually require a medical diagnosis. But an exception may be made if it’s thought the condition is due to low potassium or magnesium levels. In such circumstances, blood tests may be used to check potassium and magnesium levels.

Second-degree heart block

Blood tests are usually recommended to check whether your heart block is due to an infection, such as diptheria, or high levels of medication, such as a calcium channel blocker. You’ll also be referred for an ECG.

Congenital third-degree heart block

Congenital third-degree heart block may be detected during pregnancy using an ultrasound scan. The scanner can measure how quickly your baby’s heart is beating. An abnormally slow heartbeat may signify a heart block.

Congenital third-degree heart block can usually be confirmed before or after birth using an ECG.

Acquired third-degree heart block

Acquired third-degree heart block is usually diagnosed using a combination of blood tests and an ECG. In some cases, an ECG may also be carried out if it’s thought that inflammation is affecting the heart muscles.

Due to the often critical nature of third degree heart block, treatment may begin before all blood test results are known.

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)
• fainting

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.

Types of heart block

There are three main types of heart block:

• first-degree heart block
• second-degree heart block
• third-degree heart block

First-degree 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.

Second-degree heart block

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:

• Mobitz type 1 – this is the less serious type. It occasionally causes mild dizziness and doesn’t usually require treatment. It’s also known as Wenckeback second-degree AV block.
• Mobitz type 2 – this is the more serious type. It can cause light-headedness, dizziness and fainting and usually requires treatment.

Third-degree heart block

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).

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.

Read more about the causes of 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.

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.

Read more about how heart block is treated.

The symptoms of heart block depend on the type and severity.

First-degree heart block 

First-degree heart block doesn’t cause any noticeable symptoms. Most people are only aware they have the condition when they’re tested for an unrelated medical condition. It’s only recognised by an electrocardiograph (ECG).

Second-degree heart block

Most people with second-degree Mobitz type 1 heart block won’t experience any symptoms. However, some people may have symptoms such as:

• light-headedness
• dizziness
• fainting (temporary loss of consciousness)

People with second-degree Mobitz type 2 heart blocks are much more likely to experience the symptoms listed above. They may also have additional symptoms such as:

• chest pain, which may be worse during physical activity, such as climbing the stairs
• shortness of breath 
• tiring easily when doing physical activity
• feeling very dizzy suddenly when standing up from a lying or sitting position; this is caused by having low blood pressure (hypotension)

Congenital third-degree heart block

Many cases of congenital third-degree heart block are diagnosed during pregnancy because an ultrasound scan can often detect whether the baby has a slow heartbeat (bradycardia).

If the diagnosis is overlooked during pregnancy, the symptoms of congenital third-degree heart block won’t usually become apparent until the child is older and a greater demand is placed on their heart.

Symptoms of congenital third-degree heart block in older infants or young children include:

• unusually pale and blotchy skin
• lethargy (a lack of energy)
• unwillingness to take part in exercise or physical activity
• dizziness
• fainting

Acquired third-degree heart block

Symptoms of acquired third-degree heart block include:

• light-headedness
• dizziness
• fainting
• fatigue (extreme tiredness)
• chest pain
• slow heart beat (bradycardia)

Heart block usually only needs to be treated if it’s causing symptoms (symptomatic).

However, most cases of acquired heart block need to be treated regardless of whether there are symptoms or not. This will usually be in cases of:

• Mobitz type 2 second-degree heart block
• third-degree heart block

First-degree heart block may not need to be treated. You may need to have a pacemaker fitted if you have second degree heart block (see below). If you have thrid degree heart block you’ll need a pacemaker.

Due to the potentially serious nature of symptomatic heart block, if you experience a sudden onset of the symptoms of heart block, you should dial 999 to request an ambulance.

You’ll be taken immediately to the nearest hospital.

Transcutaneous pacing (TCP)

Transcutaneous pacing (TCP) is a temporary method of pacing the heart in a medical emergency. It’s used to restore your heartbeat to normal.

During TCP, pads are applied to the chest and electrical pulses are delivered through them to help restore your heart rate.

The procedure can feel uncomfortable so you may be given medication to sedate you (a sedative). This will make you feel drowsy so you have little or no awareness of what’s going on around you.

Temporary transvenous pacing (TTP) is an alternative to TCP. It involves a tiny wire being fed into your heart and linked to an external pacemaker to restore your heart beat. As with TCP, this will be carried out under sedation.

Once your heartbeat has been stabilised, a permanent pacemaker may be recommended (see below).

Pacemaker

A pacemaker is a small electrical device that produces tiny electrical signals that replace the function of your natural pacemaker.

Pacemakers are run by a small dry cell battery. They vary in size but most are around two-thirds the size of a credit card and about 5mm thick.

The pacemaker is connected to up to four wires that are fed into your heart through one of your veins. It’s placed in the appropriate area of the heart using X-ray guidance. 

Read more about pacemaker implantation for more information about how they work and are fitted.