The Martin Pollins Blog

History, economics, business, politics…and Sussex

Cardioversions – what  they are and how they  work

Graphical user interface Description automatically generated
ECG Results on an Apple Watch giving evidence of Atrial Fibrillation

A Cardioversion is a procedure used to return an abnormal heartbeat to a normal rhythm. This procedure is used when the heart is beating very fast or irregular. This is called an arrhythmia. Arrhythmias can cause problems such as fainting, stroke, heart attack, and even sudden cardiac death. With electrical Cardioversion, a high-energy shock is sent to the heart to reset it to a normal rhythm. It differs from chemical Cardioversion, in which medicines are used to try to restore a normal rhythm[1].

Arrhythmia explained[2]
If you have an irregular heartbeat (you might hear it called arrhythmiaatrial fibrillation, AF or AFib), your doctor or specialist will probably suggest a treatment called Cardioversion to get a normal rhythm back. The most commonly treated arrhythmia is atrial fibrillation. Another rhythm commonly treated with electrical Cardioversion is atrial flutter.

If your heart beats too fast or unevenly, it can be dangerous. It may not be pumping enough blood to meet your body’s needs. An irregular heartbeat also can lead to a stroke or a heart attack. There are two kinds of procedures to return to a normal heart rhythm. Your doctor will talk to you about which one is right for you:

  • Chemical Cardioversion: If your arrhythmia is not an emergency, a doctor might use medication to get your heart back to normal. This is called chemical or pharmacologic Cardioversion. You typically get the medicine through an IV while doctors check your heart. But sometimes, people can take it as a pill. The following are some examples of medications that your healthcare provider might use:
  • Amiodarone (Cordarone)
  • Dofetilide (Tikosyn)
  • Flecainide (Tambocor)
  • Ibutilide (Corvert)
  • Propafenone (Rhythmol)

ECG Results on an Apple Watch giving evidence of Atrial Fibrillation
The following are some examples of medications that your health care provider might use:

  • Amiodarone (Cordarone)
  • Dofetilide (Tikosyn)
  • Flecainide (Tambocor)
  • Ibutilide (Corvert)
  • Propafenone (Rhythmol)
  • Electrical Cardioversion: Drugs alone may not correct your heartbeat. Electrical Cardioversion gives shocks through paddles to regulate your heartbeat.

What happens during a Cardioversion procedure?[3]

  • You’ll be given a short-acting general anaesthetic or heavy sedation to be pain-free and asleep throughout.
  • A doctor or nurse will put electrodes stuck to large sticky pads on your chest.
  • The electrodes are connected to a defibrillator machine and will give you one or more controlled electric shocks through to your chest wall to restore a normal heart rhythm.
  • A defibrillator monitors your heart rhythm throughout the procedure so the medical staff can see immediately if the Cardioversion was successful.

Once the setup and sedation are complete, the Cardioversion itself takes only a few seconds. The patient usually awakens five to ten minutes after the Cardioversion when the anaesthetic drug administered for sedation wears off. The whole procedure usually lasts up to 30 minutes. You will usually only need to be in hospital for part of the day, although some people need to stay overnight depending on their medical condition. If you have a cardioversion treatment coming up, you might want to watch the British Heart Foundation video (at: to find out what to expect.

Are there complications of Cardioversion?
Yes. This is what the WebMD website[4] says:

  • Blood clots: Either kind of Cardioversion could knock loose blood clots created from your abnormal heartbeat. Before the procedure, a type of ultrasound may be taken to look for blood clots in your heart. You’ll probably get medicine to take for 3-4 weeks before and after the procedure to help prevent blood clots.
  • StrokeIf a clot travels to your brain, it can cause a stroke.
  • It might not work: Cardioversion doesn’t always fix a fast or irregular heartbeat. You may need medicine or a pacemaker to control things.
  • It might make things worse: It’s unlikely, but there’s a small chance that Cardioversion could damage your heart or lead to more arrhythmias.
  • Irritated skin: This often happens in the area where the paddles are applied. The doctor can give you a cream to treat it.

This is what the British Heart Foundation[5] says:

‘Complications aren’t common, and if you experience any side effects after the Cardioversion, they will usually be temporary.  After your Cardioversion, you may get headaches and dizziness from a drop in your blood pressure. You may also feel a small amount of discomfort in your chest where the shock was given. Feeling sick is also a common side effect of an anaesthetic. Some people may have blood clots in their heart which could cause a stroke if they move during the procedure. If necessary, your doctor may check for clots with an echocardiogram and prescribe some blood thinners before the procedure is undertaken.’

What’s the Success Rate?
Cardioversion doesn’t always restore normal heart rhythm. Sometimes it’s successful to start with, but then your abnormal heart rhythm could come back several days, weeks or even months later. If this happens, your doctor may want to repeat the Cardioversion, or they may consider another treatment for you.

  • Electrical Cardioversionis more than 90% effective, though many have AFib again shortly after having it. Taking an antiarrhythmic drug before the procedure can prevent this. How well it works depends on the size of your left atrium and how long you’ve had AFib. If you have a large left atrium or you’ve been in constant AFib for a year or two, it may not work as well. Taking antiarrhythmic drugs can also prevent AFib after a successful electrical cardioversion.
  • Chemical Cardioversion: You should quickly know if it works. It usually takes effect within hours, but sometimes it takes days. If it doesn’t work for you, the doctor might suggest Electrical Cardioversion.

Ways that a Cardioversion procedure can fail[6]
There are two common ways that a cardioversion procedure for Afib can fail. First, in some cases, the energy required to restore the heart to a normal rhythm is higher than the maximum energy delivered by the Cardioversion equipment. When this happens, newer generation equipment able to provide a more effective type of energy waveform, known as a biphasic waveform, can be used if it were not used initially. Almost all hospitals now have defibrillators that deliver biphasic waveforms. Other potential solutions include applying pressure to the paddles during the shock and administering an intravenous medication called ibutilide. Ibutilide lowers the energy required to restore the heart to a normal rhythm and is highly effective in patients who can safely receive the medication. When this drug is given, patients must be monitored for at least four hours before going home.

A second way a Cardioversion procedure for AFib can fail is when AFib recurs immediately after a normal rhythm has been restored. In some patients, administration of ibutilide and repeating the Cardioversion is effective in restoring sinus rhythm.

Thromboembolism (blood clot travelling through the bloodstream) can occur after Cardioversion if a thrombus (blood clot) becomes dislodged from the atria as the heart begins to beat normally. Blood clots can form within the atria during AF since blood flow is slowed. Most patients are given an anticoagulant (a blood thinner such as warfarin or one of the newer oral anticoagulants) for at least three to four weeks before and after Cardioversion to prevent an embolism unless the AFib has been present for less than a day or two. Many patients will be required to take an anticoagulant indefinitely after Cardioversion. (See “Patient education: Atrial fibrillation (Beyond the Basics)”.) However, anticoagulants may not be suitable for patients with a propensity to gastric bleeding.

Cardioversion for patients with implanted devices[7]
Some patients who require a cardioversion procedure already have a pacemaker or implantable ICD (cardioverter-defibrillator). An ICD is a device that is surgically placed under the skin in the chest and constantly monitors the heart’s rhythm. It delivers a cardioverting or defibrillating shock if a life-threatening arrhythmia develops.

Several precautions are necessary when attempting external Cardioversion in a patient with a permanently implanted device. Cardioversion can change the settings of the ICD or pacemaker, or may damage the ICD or pacemaker, the leads, or the heart’s tissue. To reduce these risks, the patches or paddles are placed at least 12 cm from the pacemaker or ICD and usually in a front-to-back position. After Cardioversion, the pacemaker or ICD is usually evaluated to ensure that it still functions normally.

When a patient with an ICD undergoes Cardioversion, the shock can be delivered using the ICD rather than the standard approach. The advantage of using the ICD is that it avoids the risk of skin irritation from an external shock and the small chance of damage to the ICD system from the shock. The disadvantage of using the ICD is that it consumes some of the battery in the device and does not always work for Cardioversion of atrial arrhythmias.

An ICD does not cure arrhythmia or heart disease. It manages your condition(s) and helps prevent cardiac arrest and death[8].

Recovery from Cardioversion
Once your heart is back in a normal rhythm, your doctor will prescribe medication to ensure it stays that way. You’ll go back to your doctor in a few weeks for an electrocardiogram (you may hear it called an ECG) to make sure your beat is still regular.

Keep up with your doctor visits and follow your treatment plan, which may also include antiarrhythmic drugs to help your heart maintain its normal rhythm.

 Sources and Further Reading

See also my paper on Atrial Fibrillation (Living with Atrial Fibrillation) in the Occasional Papers library.

A picture containing text

Description automatically generated Screenshot from British Heart Foundation YouTube video at:
All rights of BHF duly acknowledged

CAUTION: This paper is not medical advice. No advice is implied or given in articles published by us but is only for general information. You should always seek the advice of your GP or other qualified health provider with any questions you may have regarding a medical condition. This paper is compiled from the sources stated but has not been medically reviewed. It should never be used as a substitute for obtaining advice from a consultant Cardiologist or other qualified clinician/medical practitioner. If you have already been given dietary advice, you should not make changes without first talking to your GP, consultant or dietitian. Any medications mentioned may include names for US drugs which may have a different name to those available in the UK. The facts are believed to be correct as at the date of publication, but there may be certain errors and omissions for which we cannot be responsible. There is no implied endorsement or promotion of any organisation by the writer. The hyperlinks were valid at the date of publication.

  1. Source:
  2. See:
  3. Source:
  4. See:
  5. See:
  6. According to:
  7. Source:
  8. Source:

Leave a Reply

Blog at

%d bloggers like this: