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Heart block

Page last reviewed: 13/07/2011

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

  • shortness of breath
  • palpitations (irregular heartbeat)
  • fainting

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 that are generated by a selection of specialised cells 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 pulses at regular intervals.

The pulse is then sent to a group of cells known as the atrioventricular node (AV node). The AV node 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 can take over. However, it can result in 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

These are described in more detail below.

First degree heart block

A first degree heart block is where there is a 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 happens when 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 least serious type of second degree heart block; it may occasionally cause 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 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 regulates heartbeats).

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 more common in people with certain heart conditions. For example, an estimated 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 old 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. Deaths that are caused by these conditions are very rare.

Page last reviewed: 13/07/2011

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.

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)

Page last reviewed: 13/07/2011

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 that are used to treat high blood pressure, such as calcium channel blockers
  • digoxin - a medication that is 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 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's disease - a bacterial infection that is 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 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 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
  • 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
  • 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)

Page last reviewed: 13/07/2011

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 that the condition is the result of low potassium or magnesium levels. In such circumstances, blood tests may be used to 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's disease, or high levels of medication, such as a calcium channel blocker.

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 can 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 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 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.

Page last reviewed: 13/07/2011

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 to the nearest hospital immediately.

Transcutaneous pacing (TCP)

Transcutaneous pacing (TCP) is the treatment of choice for symptomatic heart block is. 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 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 attached to your heart and will generate a regular series of pulses to keep your heart beating.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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