Heart block

A heart block is when electrical pulses that control the beating of the heart are disrupted.


A heart block is when electrical pulses that control the beating of the heart are disrupted.

There are three levels of heart block and usually only the most serious type of heart block 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

Unless you are experiencing symptoms, heart block is often diagnosed during routine tests for other conditions. Babies may be diagnosed with the condition during pregnancy.

Read more information about diagnosing heart block.

How the heart beats

When the heart beats, the heart muscle contracts (pulls inwards) in preparation for pumping blood around the body. The contractions are triggered by electrical pulses generated by a selection of specialised cells 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 a group of cells known as 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.

A heart block does not mean that the heart stops beating altogether – other ‘back-up’ systems take over. However, it can result in an abnormally slow heartbeat (bradycardia), which deprives the body’s organs and tissue of oxygen.

What causes heart block?

Some people are born with heart block, either because of a heart defect or genetic condition. This is known as a congenital heart block.

Heart block can also be caused by:

  • conditions such as a heart attack
  • certain infections such as diptheria
  • certain medicines
  • having open heart surgery

This is known as acquired heart block.

Read more about the causes 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

In first degree heart block there is a split-second delay in the time that it takes electrical pulses to move through the AV node. First degree heart block rarely causes noticeable symptoms.

Second degree heart block

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

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

  • Mobitz type 1 – this is the less serious type; it may occasionally cause mild dizziness and does not usually require treatment. It is more commonly known as Wenckeback second degree AV block
  • Mobitz type 2 – this is a more serious type; it may cause light headedness, dizziness and fainting, and usually requires treatment

Third degree heart block

In third degree or complete heart block there is 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 may be a medical emergency but in many cases may be benign and does not always require treatment.

How is heart block treated?

Heart block does not always require treatment, unless there are serious symptoms. However, acquired heart block usually needs treatment.

Transcutaneous pacing (TCP) uses electrical pulses to help stabilise your heartbeat and is the main treatment for heart block.

More serious cases will often require a pacemaker. A pacemaker is a small battery-operated device which is inserted under the skin of your chest. It sends regular electrical pulses to keep your heart beating regularly.

Treatment for heart block works well if given when required. Deaths caused by these conditions are rare.

Read more information about how heart block is treated.

Who is affected?

First degree and Mobitz type 1 heart blocks are uncommon but not rare. It is estimated that up to two out of 100 otherwise healthy adults have these types of heart blocks.

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

Congenital third degree heart blocks are rare, occurring in one 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 over 70 years old who have a history of heart disease will develop a third degree heart block.


The symptoms of heart block depend on which type you have and how severe it is.

First degree heart block

First degree heart block does not cause any noticeable symptoms. Most people are only aware they have the condition when they are tested for an unrelated medical condition. It is only recognised by an electrocardiograph (ECG).

Second degree heart block

Most people with second degree Mobitz type 1 heart block will not 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 will usually not 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

Acquired third degree heart block

Symptoms of acquired third degree heart block include:

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


Heart block can be caused by several conditions and certain medications. People can be born with the condition (congenital), or develop it over time (acquired).

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

Second degree heart block

As with first 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](yourmd:/condition/blood-pressure-high), 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 the immune system mistakes the unborn baby for a foreign object (such as a virus) and sends antibodies to attack it. The antibodies damage the heart.

Some children with congenital heart disease are also 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 – this is thought to be one of the most common causes
  • coronary heart disease – a condition where the heart does not receive enough blood due to acquired narrowing or closing of the coronary arteries
  • 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 has spread from another part of the body into the heart
  • a penetrating trauma to the chest, such as a stab wound or gunshot wound
  • as 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 (hypertension)
  • tricyclic antidepressants – an older type of antidepressant
  • clonidine – used to treat a sudden, sharp rise in blood pressure (hypertensive crisis)


The different types of heart block are diagnosed by a heart tracing using an electrocardiograph (ECG).

Electrocardiogram (ECG)

An electrocardiogram (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 can determine whether you have a Mobitz type 1 or type 2 heart block.

Sometimes an ECG may be recorded over a 24-48 hour period as the heart rate can become much slower at various times of the day or night. This is ofter referred to as Holter monitoring.

Other tests

There are several other tests used to diagnose the cause of heart block depending on symptoms. The condition is often diagnosed during tests for other conditions.

First degree heart block

First degree heart block does not usually require a medical diagnosis. However, an exception may be made if it is thought the condition is the result of 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 that your heart block is not the result of an infection, such as diptheria, or high levels of medication, such as a calcium channel blocker.

You will also be referred for an ECG.

Third degree heart block (congenital)

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.

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 ECG may also be performed if it is 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.


Treatment for heart block is usually only required when it is causing symptoms (symptomatic). However, most cases of acquired heart block need treatment whether there are symptoms or not.

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 to the nearest hospital immediately.

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 be delivered through the pads and these pulses will restore your heart rate.

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 you have little or no awareness of what is going on around you.

An alternative to TCP is temporary transvenous pacing (TTP) where a tiny wire is fed into the heart and linked to an external pacemaker to restore your heart beat. This is carried out under sedation.

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


A pacemaker is a small electrical device that generates tiny electrical signals which replace the function of your natural pacemaker. It is run by a small dry cell battery. Pacemaker sizes vary but most are around 2/3rd's the size of a credit card and 5mm thick.

The pacemaker is linked to up to 4 wires that are fed into your heart through one of the veins. It is placed in the appropriate part of the heart using an X-ray for guidance.

Read about pacemaker insertion for detailed information about how pacemakers work and are fitted.

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