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Heart block

Heart block is a type of arrhythmia (abnormal heartbeat). The electrical signals that control your heartbeat are slowed down or blocked as they travel through your heart. Sometimes heart block doesn’t cause any problems, but if it’s more serious, you might feel dizzy, or you can even blackout.

An image of an older couple looking at a tablet device

What happens in heart block?

Your heartbeat is controlled by electrical signals (impulses) that travel through your heart to make it contract.

If you have heart block, the electrical signals that control your heartbeat are slowed down or blocked as they travel through your heart. This can happen at various points in the electrical pathway.

Watch our animation to see how your heart beats normally and what happens in heart block.

AV heart block

This happens when there’s a delay or block in the electrical impulses travelling between your atria and your ventricles. Your atria are the top chambers of your heart and your ventricles are the bottom chambers. Sometimes AV heart block can be mild and you might not know you have it, but it can be more serious and cause symptoms. It’s categorised into three levels.

  • First-degree – this rarely causes any symptoms.
  • Second-degree – you might feel dizzy and faint or blackout.
  • Third-degree (complete heart block) – this is the most serious type where you’ll usually have a very slow heartbeat and you may even blackout. It can sometimes lead to your heart stopping completely so you’ll need treatment.

Bundle branch blocks

This happens when the electrical impulses travel through your ventricles slower than normal because of a delay or block in the electrical pathway. It can affect the left- or right-hand side of your heart. It doesn’t usually cause any symptoms but it can be a sign that you have another heart condition.

Symptoms of heart block

Whether you have any symptoms from heart block depends on the type of heart block you have, and how serious it is.

You may:

  • feel tired
  • feel very faint
  • feel short of breath
  • get palpitations (a thumping in your chest)

If you have serious heart block and your heart rate becomes so slow that not enough blood reaches your brain, you might blackout. The medical name for this is syncope. Just before you lose consciousness, you might look pale and feel light-headed.

You might find that the symptoms of heart block come and go but if you get any, go and see your GP.

Diagnosis of heart block

If you’re unsure about any symptoms you’re having, it’s best to go and see your GP to talk them through. Our Cardiology Clinical Adviser, Dr Yassir Javaid, explains why in our video: When should you see a doctor about your heart?

Your GP will ask about your symptoms and medical history, and examine you. They’ll check your pulse and blood pressure too. Your GP will ask if you're taking any medicines to make sure they're not causing your heart block. They might do some tests to find out if you have any underlying health condition that could be causing your symptoms too.

Your GP might refer you to go and see a cardiologist for further tests. A cardiologist is a doctor who specialises in identifying and treating heart conditions. 

The main way to test if you have heart block is to have an electrocardiogram (ECG). This measures the electrical activity in your heart to see how well it's working. You might need to wear a 24-hour heart monitor, which is called ambulatory ECG. This records the electrical activity of your heart while you go about your usual activities over 24 hours (or longer if necessary).

Depending on the results of these tests, your cardiologist may need to investigate further and do more tests. They’ll either confirm that you have heart block, or confirm or rule out another health condition.

  • Blood tests to check the levels of electrolytes such as potassium and magnesium in your blood.
  • An implantable loop recorder. This is a small device (about the size of a computer memory stick), which is implanted under the skin. You activate the device when you feel an abnormal rhythm.
  • Chest X-ray. This will enable your doctor to look for any changes in your heart or lungs that might be causing the problem.
  • Tilt table test. In this test, you’ll lie down on a table, which will be tilted upwards. A technician will measure your heart rate, rhythm and blood pressure at different points.
  • Echocardiogram. This test uses ultrasound to provide a clear image of your heart muscles and valves and can show how well your heart is working.
  • Exercise ECG (cardiac stress test). This is when you have an ECG while you exercise to see how your heart works under pressure. It may trigger abnormal heart rhythms, but you’re in the best place for this to happen as you’ll be surrounded by a medical team.
  • Electrophysiology studies. This checks the electrical activity of your heart to see what’s causing an abnormal heart rhythm.

Treatment of heart block

Your treatment for heart block will depend on how serious it is. If you have mild (first-degree) heart block, you probably won't need any treatment. If your heart block is caused by taking medicines, your doctor can review them. If it’s caused by an underlying health condition, your doctor will focus your treatment on that. 

If your heart block is more serious, and isn’t caused by medicines or an underlying health condition, you’ll probably need to have a pacemaker.

Having a pacemaker fitted

A pacemaker is a small device that monitors your heartbeat and sends electrical signals to stimulate your heart to beat at a specific rate. It only kicks in and sends signals if your heartbeat slows below a certain pre-set level.

A cardiologist will put the pacemaker under the skin on your chest, or under the muscle. The pacemaker has two leads that your surgeon will pass through a vein and into your heart. Usually, pacemakers with two leads called dual-chamber pacemakers are used for heart block. The two leads connect to two different points in your heart – usually your right atrium and right ventricle.

You’ll usually have a pacemaker fitted under local anaesthesia and sedation. This will completely block pain from your chest area and you’ll stay awake during the operation. The sedative will help you to relax. Normally, you’ll be able to go home the day after your procedure, once your cardiologist has checked that the pacemaker is working correctly. You might even be able to have the pacemaker fitted and go home the same day.

Some electrical devices, or devices that have strong magnetic fields, can interfere with a pacemaker by disrupting its electrical signalling. These include mobile phones and metal detectors, for example, but ask your doctor or clinic for a full list and what precautions to take. For more information about pacemakers, see our FAQs below.

Causes of heart block

Causes of heart block include:

  • getting older – your heart may get damaged over time, or substances that affect how electrical signals are transmitted in your heart can build up
  • having a heart attack, as this can damage your heart
  • coronary artery disease
  • some types of medicines, such as beta-blockers and calcium-channel blockers
  • heart muscle diseases, such as sarcoidosis
  • infectious diseases, such as Lyme disease
  • inflammatory health conditions, such as systemic lupus erythematosus and rheumatoid arthritis
  • heart conditions you're born with (congenital heart disorders)
  • having surgery on your heart, such as an operation on your heart valves
  • having low levels of electrolytes like potassium or magnesium in your blood

FAQ: Does a slow heart rate mean I have a heart block?

No, there are lots of things that can slow your heart rate. Many people who are physically active have hearts that beat more slowly than usual – this doesn't mean that there's anything wrong.

More information

Athletes or people who are physically very fit often have hearts that beat more slowly than usual. This is because the amount of exercise they do over time physically changes the structure and function of their heart. Some athletes have been found to have a heart rate of below 40 beats per minute when they’re resting. This is perfectly normal for them. But for most of us, a heart rate is considered to be unusually slow if it drops below 50 beats per minute at rest. 

As well as heart block, a variety of other things can cause a slow heartbeat. These include everything from taking certain medicines to other health conditions that affect your heart. Since lots of things can slow your heartrate, it’s best to go and see your doctor to identify why your heart rate is lower than normal.

FAQ: Will I be able to feel the pacemaker?

You might be aware of the pacemaker at first and it may feel uncomfortable to lie in certain positions. But most people quickly get used to it. Once your surgical wound has healed, the pacemaker shouldn’t feel uncomfortable, and you probably won't be aware of its effects on your heart. You can live a normal healthy life with a pacemaker, and can continue to drive and travel by plane, for example.

More information

A pacemaker is small (about the size of a £2 coin) and your cardiologist will put it under the muscle or skin on your chest, usually near your collarbone. You may be able to feel the pacemaker box and connecting leads under your skin as the wound heals. It's really important that you don't try to move them. Let your doctor know if they continue to bother you after the wound has healed. 

Your pacemaker will be programmed while you're in hospital to the best settings for you, so you shouldn't be aware of it working. But if you felt your heartbeat was very slow before it was fitted, then you may notice your heart beating faster.

FAQ: Can I exercise or play sports with a pacemaker?

A cardiologist will put the pacemaker under the skin on your chest, or under the muscle. They’ll usually put it on your left-hand side if you’re right-handed, or right side if you’re left-handed. This should help you continue with sports.

Your doctor might advise you not to do any strenuous activity until after you’ve been to your pacemaker follow-up appointment. This will be about four to six weeks after the pacemaker was fitted. After this appointment, you should be able to increase your level of activity and start to exercise or play sports again. But if you want to play contact sports, you should take extra care. Wear a protective pad over your pacemaker to help protect it if you get a knock.

FAQ: How long will my pacemaker last?

A pacemaker battery usually lasts between seven and 10 years. You’ll have regular check-ups to predict when you’ll need a new one – your hospital won't let it completely run down. 

More information

You’ll usually have check-ups every six months to a year depending on the type of pacemaker you have, and how well it's working. Some types of pacemaker allow your clinic to monitor its effect remotely using software installed in your home. Any problems or changes in your pacemaker’s function will be picked up quickly and the clinic will contact you to come in if necessary. Your clinic will then check the data and reprogram your pacemaker during your check-up.

Most of the pacemaker box is taken up with the battery, so the whole box is replaced when the battery starts to run out. The leads of the pacemaker are usually left in place but sometimes they may need to be replaced eventually. Your healthcare team will assess the leads during your check-ups. Replacing a pacemaker box is a similar procedure to when it was originally fitted, and is usually done under local anaesthesia.


  • Other helpful websites Other helpful websites

    Further information


    • Overview of dysrhythmias (cardiac). BMJ Best Practice., last updated 20 March 2017
    • Atrioventricular block. The MSD Manuals., last full review/revision July 2015
    • ECG identification of conduction disorders. PatientPlus., last checked 26 May 2016
    • Conduction system of the heart. Medscape., updated 28 June 2016
    • Atrioventricular block. BMJ Best Practice., last updated 21 June 2016
    • Arrhythmias. Oxford handbook of cardiology (online). Oxford Medicine Online., published May 2012
    • Bundle branch and fascicular block. The MSD Manuals., last full review/revision July 2015
    • Syncope. Medscape., updated 13 January 2017
    • Dealing with abnormal heart rhythms. British Heart Foundation., published 1 December 2015
    • Bradycardia. BMJ Best Practice., last updated 10 August 2016
    • Electrocardiography. Medscape., updated 17 April 2017
    • Tilt-table testing technique. Medscape., updated 17 March 2016
    • Cardiac tests. Medscape., updated 18 December 2016
    • Dual‑chamber pacemakers for symptomatic bradycardia due to sick sinus syndrome and/or atrioventricular block. National Institute for Health and Care Excellence (NICE)., 1 November 2014
    • Permanent pacemaker insertion. Medscape., updated 21 May 2014
    • Pacemakers. British Heart Foundation., accessed 21 May 2017
    • Pacemakers. British Heart Foundation., published 31 March 2012
    • Pacemakers and pacing. PatientPlus., last checked 19 September 2014
    • Athlete’s heart. The MSD Manuals., last full review/revision February 2017
    • Personal communication, Dr Matthew Wright, Consultant Cardiologist and Electrophysiologist, 28 June 2017
    • Pacemaker. Arrhythmia Alliance., accessed 21 May 2017
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    Reviewed by Rachael Mayfield-Blake, Freelance health Editor, June 2017
    Expert reviewed by Dr Matthew Wright, Consultant Cardiologist and Electrophysiologist
    Next review due June 2020

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