Heart block

About heart blocks

A heart block is when the electrical impulses that control the beating of the heart muscle are disrupted. The most serious type of heart block known as a complete, or third degree, heart block will have symptoms, but often those with less serious heart block can have symptoms too.

Symptoms can include:

  • shortness of breath
  • palpitations (irregular heartbeat)
  • dizzy spells/fainting
  • nausea

How the heart beats

When the heart beats, the heart muscle contracts (pulls inwards) in order to pump blood around the body. The contractions are triggered by electrical impulses that are generated by a selection of specialised cells that are known as the sinoatrial node (SA node).

The SA node is often referred to as a natural pacemaker because, like artificial pacemakers, it generates a series of electrical impulses at regular intervals.

The impulse is then sent to another selection of cells that are known as the atrioventricular node (AV node). The AV node conducts the impulse into the two lower chambers of the heart (the ventricles).

A heart block occurs if the transmission of the impulse between the AV node and the ventricles is interrupted.

A heart block does not mean that the heart stops beating altogether, because other ‘back-up’ systems can take over. However, it can result in an abnormally slow heartbeat (bradycardia), leading to the body’s organs and tissue becoming deprived 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

These are described in more detail below.

First degree heart block

A first degree heart block is where there is split-second delay in the time that it takes electrical pulses to move through the AV node. First degree heart block does not usually cause any noticeable symptoms and treatment is rarely required.

Second degree heart block 

A second degree heart block is where there is a series of increasing delays in the time that it takes the AV node to send the pulse to the ventricle, until eventually a heartbeat is skipped.

There are two sub-types of second degree heart block:

  • Mobitz type 1 – this is the least serious type of second degree heart block – it may occasionally cause symptoms of mild dizziness and does not usually require treatment
  • Mobitz type 2 – this is a more serious type of second degree heart block – it may cause symptoms of light headedness, dizziness and fainting in some people and usually requires treatment

Third degree heart block

A third degree, or complete, heart block is where there is no transmission of electrical pulses between the AV node and the ventricles. As with second degree heart block, there are two sub-types of third degree heart block:

  • congenital – where the condition is present at birth
  • acquired – where the condition develops as a result of heart damage

A third degree heart block can cause a wide range of symptoms, some of which are life-threatening. This type of heart block is usually regarded as a medical emergency and may require immediate treatment with a pacemaker (an artificial electrical device that is used to regulate heartbeats).

Read more about pacemakers 

How common are heart blocks?

First degree and Mobitz type 1 heart blocks are uncommon but not rare. It is estimated that 0.5-2% of otherwise healthy adults have these types of heart blocks.

Mobitz type 2 heart block is rare in the general population, but it is more common in people with certain heart conditions. For example, it is estimated that 1 in 30 people with heart failure will develop Mobitz type 2 heart block.

Congenital third degree heart blocks are rare, occurring in 1 in every 20,000 births.

Conversely, acquired third degree heart block is a common complication of heart disease, particularly in older people. It is estimated that 5-10% of people who are over 70 years of age, and have a history of heart disease, will develop a third degree heart block.


The outlook for all types of heart block is very good as long as appropriate treatment is given as and when it is required.

The most serious types of heart block respond very well to treatment with a pacemaker, and deaths that are caused by these conditions are very rare.

Symptoms of a heart block

First degree heart block does not always cause any noticeable symptoms, and most people only become aware they have the condition when they are tested for an unrelated medical condition.

Second degree heart block

Most people with second degree Mobitz type 1 heart block will not experience any symptoms. However, some people may experience 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 experience additional symptoms such as:

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

Third degree heart block (congenital)

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 will not 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 (lacking in energy)
  • unwillingness to take part in exercise or physical activity
  • dizziness
  • fainting

Third degree heart block (acquired)

Symptoms of acquired third degree heart block include:

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

Causes of a heart block

It may sound contradictory, but being very fit is one of the most common causes of first degree heart block. For example, the condition is widespread among elite long distance runners.

This is because vigorous and prolonged exercise can enlarge the heart muscles, causing mild disruption to the electrical signals of the heart.

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 that are used to treat abnormal heart rhythms (antiarrhythmics), such as disopyramide
  • medications that are used to treat high blood pressure, such as calcium channel blockers and beta blockers
  • digoxin – a medication that is used to treat heart failure

Second degree heart block

As with first degree heart block, it is possible for athletes to develop second degree heart block for the reasons discussed above.

Some children who are born with congenital heart disease (heart defects that are 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 that is spread by tics
  • certain medications, such as calcium-channel blockers and beta 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 heart block (congenital)

A large proportion of congenital third degree heart block cases develop in mothers who have an autoimmune condition, such as lupus (a long-term condition that causes inflammation in the body’s tissues).

An autoimmune condition is where the immune system mistakenly attacks healthy cells and tissue. It is thought that the immune system mistakes the unborn baby for a foreign object (such as a virus) and sends antibodies to attack it. The antibodies cause damage to the heart.

Some children with congenital heart disease will also be born with third degree heart block.

Third degree heart block (acquired)

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

  • as a complication of heart surgery – which is thought to be one of the most common causes
  • coronary heart disease – a condition where the heart does not receive enough blood
  • as a complication of radiotherapy (a treatment for conditions such as cancer, thyroid disorders and some blood disorders)
  • as a result of a serious infection, such as diphtheria (a bacterial infection that can cause inflammation of the heart) or rheumatic fever (a bacterial infection that damages the valves of the heart)
  • poorly controlled high blood pressure (hypertension)
  • cancer that spread from another part of the body into the heart
  • a penetrating trauma to the chest, such as a stab wound or gunshot wound

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

  • digoxin – used to slow and steady a rapid heartbeat
  • calcium-channel blockers – used to treat high blood pressure
  • 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)

Diagnosing a heart block

First degree heart block does not usually require a medical diagnosis. However, an exception may be made if it is thought that the condition is the result of low potassium or magnesium levels. In such circumstances, blood tests may be used the check your potassium and magnesium levels.

Second degree heart block

Blood tests are usually recommended to check that your heart block is not the result of an infection such as Lyme disease, or high levels of certain medication.

You may also be referred for an electrocardiogram (ECG). An ECG is a test that measures the electrical activity of your heart. It may be carried out while you are exercising (usually on a treadmill or an exercise bike) or while you are resting.

An ECG can provide a useful overall assessment of how well your heart is working. In some cases, it may also be able to determine whether you have a Mobitz type 1, or type 2, heart block.

Third degree heart block (congenital)

Congenital third degree heart block may be detected during pregnancy using an ultrasound scan. The scanner can be used to 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 echocardiogram. An echocardiogram is a type of ultrasound scanner that is specifically designed to assess how well your heart is pumping.

Third degree heart block (acquired)

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

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

Treating a heart block

Treatment is usually only required when a heart block is causing symptoms (symptomatic). This will usually be in cases of:

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

Due to the potentially serious nature of symptomatic heart block, you will usually be taken immediately to the nearest hospital.

Transcutaneous pacing (TCP)

Transcutaneous pacing (TCP) is the treatment of choice for symptomatic heart block. TCP is a way of restoring your normal heartbeat.

During TCP, pads will be applied to your chest. Electrical pulses will then be delivered through the pads and these pulses will be used to correct your heartbeat.

TCP can feel uncomfortable so you may be sedated before the procedure begins. If you are sedated, you will be given medication to make you feel drowsy so that you have little or no awareness of what is going on around you.

Once your heartbeat has been stabilised, a permanent pacemaker may be recommended.


A pacemaker is a small electrical device that is similar to a battery and can be inserted into one of your veins before being moved next to your heart.

The pacemaker has wires that are inserted into your heart and will generate a regular series of pulses to keep your heart beating.

Having a pacemaker fitted

Last updated:
02 July 2024