What Is Cardioversion?

Cardioversion is the conversion of a cardiac (heart) arrhythmia into an alternative cardiac rhythm. Cardioversion refers to a variety of medical procedures. The most common involve either medications (pharmacological cardioversion) or electricity (electrical cardioversion or defibrillation). Which method is used depends on the patient's condition and overall stability.

Man's chest with a defibrillator electrode on it
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Electrical cardioversion uses electrodes that are several inches across to conduct electricity through the heart muscle. The electrodes can be placed externally on the chest wall or internally directly on the heart muscle.

There are different types of electrical cardioversion, but they all use the same device known as a defibrillator. Defibrillators come in manual and automated versions. Some of them are capable of being used in either mode. Defibrillators are used when a patient is in certain cardiac rhythms such as ventricular fibrillation or unstable ventricular tachycardia. These rhythms are life-threatening and require a defibrillator to defribrillate the heart, meaning the defibrillator uses electricity to get the heart back into a regular stable rhythm.

Use of the term cardioversion for pharmacological solutions is less common, probably because there is a variety of uses for medications that might cause an immediate change in the heart rhythm—traditionally known as cardioversion—but can also be used chronically to control heart rate or rhythm.

Oftentimes electrical cardioversion is preferred over pharmacologic for many reasons.

Types of Cardioversion

The types of cardioversion that might be performed by either a medical professional or a lay rescuer depend mainly on the medical condition experienced by the patient and on the severity of the patient's condition. Electrical and pharmacological are the two most common types of cardioversion. However, within both categories, there are several different types of cardioversion.

Defibrillation (Electrical Unsynchronized Cardioversion)

Ventricular Fibrillation

Ventricular fibrillation is a condition in which the heart no longer beats effectively. Instead, it quivers uncontrollably in a way that cannot conduct blood flow. This is a primary cause of sudden cardiac arrest. Stopping the fibrillation—called defibrillation—involves using a focused electrical shock that courses through most of the heart muscle cells, causing them to depolarize.

The electrical shock can be monophasic or biphasic and either direct current (DC) or alternating current (AC). Most devices currently manufactured use a biphasic, direct current shock of no more than 360 Joules.

Defibrillation causes most of the heart muscle cells to depolarize (contract) all at the same time. This sudden depolarization provides an opportunity for the natural pacemakers in the heart, located on the right atrium, to regain control of the pace and rate of the heart's rhythm. This is a form of cardioversion because the patient is being cardioverted from ventricular fibrillation to a cardiac rhythm capable of sustaining life.

Pulseless Ventricular Tachycardia

A second, less common, cause of sudden cardiac arrest that can often be cardioverted using unsynchronized electrical shock (defibrillation) is pulseless ventricular tachycardia. In this cardiac arrhythmia, the patient's heart is beating in an organized rhythm, but too fast for the heart to fill with blood between beats and keep blood flowing.

This use of cardioversion is also called defibrillation even though the caregiver or the lay rescuer is not removing fibrillation but a different lethal arrhythmia. This is why defibrillation is not always the correct terminology for certain types of emergency cardioversion.

Synchronized Cardioversion

Some forms of tachycardia are still organized cardiac rhythms but are going at too fast of a rate to adequately allow the heart to effectively pump blood. In these cases, the patients are still able to pump blood and therefore will have a pulse and will most likely be conscious.

During cases of extremely rapid tachycardia, an electrical shock delivered at just the right moment in the cycle of a heartbeat can result in a higher chance of successful cardioversion.

To deliver an electrical shock at that precise moment requires the shock to be synchronized to the heart's rhythm. Synchronization is done using an electrocardiogram (ECG) to monitor the rhythm and time of the delivery of the shock using the same electrodes as those used to provide defibrillation.

Pharmacological Cardioversion

Medications can be used to speed up or slow down the rate of heart rhythm or to completely change an arrhythmia into a different cardiac rhythm. Pharmacological cardioversion does not require sedation. Electrical cardioversion is preferred in patients who are unstable—that is they have a dangerously abnormal blood pressure or other symptoms.

The medications or classes of medications used for pharmacological cardioversion are specific to the condition being treated:


Used for supraventricular tachycardia (SVT) that is not atrial fibrillation, adenosine is the newest of the pharmacological cardioversion agents. Adenosine has a transient (short-lived), noncurative effect on ventricular tachycardia and on atrial fibrillation.


Certain supraventricular tachycardias can be successfully slowed to a rate that properly conducts blood and reduces symptoms with the use of beta-blockers. Beta-blockers are not always seen as cardioversion agents but might be used for long-term control of tachycardia or hypertension (high blood pressure).

Calcium Channel Blockers

Like beta-blockers, calcium channel blockers can either be used for acute cardioversion of supraventricular tachycardia in certain circumstances or be prescribed for chronic control of recurring tachycardia and hypertension.

Both calcium channel blockers and beta blockers can have potentially dangerous side effects if used on a particular type of condition called Wolf-Parkinson-White (WPW) Syndrome.

Atropine, Dopamine, and Epinephrine

Cardioversion of a rhythm that is too slow (bradycardia) into a proper cardiac rhythm can be accomplished through medications such as atropine, dopamine, or epinephrine depending on what is causing the slow heart rate.

An implanted pacemaker is a long term treatment of bradycardia.

Risks and Contraindications

The risks and contraindications of cardioversion depend on the type of cardioversion being used.

One contraindication for defibrillation is the presence of a pulse. Defibrillation should not be done if the victim is in a body of water.

Cardioversion is a two-way street. If a defibrillator is used to shock a patient that is not experiencing fibrillation, the heart could be cardioverted into fibrillation. Proper application of defibrillation is the most important consideration for this type of cardioversion.

If the patient's heart is already in ventricular fibrillation, there is no contraindication for a defibrillation shock.

Atrial Fibrillation

Using electricity for cardioversion of atrial fibrillation can potentially result in stroke, pulmonary embolism, or myocardial infarction from a blood clot embolism. Patients in atrial fibrillation are known to develop blood clots in some areas of the heart that can be susceptible to becoming dislodged during cardioversion. For this reason, anticoagulants are often given before and/or after cardioversion.

Alert Patients

The use of electrical cardioversion in patients who are awake and alert can lead to significant discomfort, even if the cardioversion results in cessation of signs and symptoms of the original dysrhythmia. Healthcare providers control for this with the use of sedation if the patient is stable enough to tolerate waiting a few minutes for sedation to work.

In the event the patient is not stable enough to wait for sedation before cardioversion, sedation is often used after the fact to help the patient manage discomfort post-shock. Patients often report a retrograde amnesia effect from the use of sedation after cardioversion and cannot remember the actual procedure.

Pharmacological Risks and Contraindications

Using medications to achieve cardioversion can have reactions that are more intense than intended. In those cases, it might be necessary to apply corrective measures, either electrically or with other medications. For instance, if a patient reacts too aggressively to the use of atropine and develops ventricular tachycardia, electrical cardioversion might be used to convert the heart back into a proper rhythm.

During Cardioversion

What to expect during cardioversion depends on the type of cardioversion used: electrical or pharmacological.

Defibrillation during sudden cardiac arrest is an emergency procedure performed on a patient who is unconscious and unresponsive. The patient is very unlikely to remember anything about the procedure.

Electrical Cardioversion

Patients who are alert and require electrical cardioversion will likely be experiencing signs and symptoms that include any combination of fatigue, dizziness, weakness, chest pain, confusion, or shortness of breath. The patient will have an ECG attached that allows the healthcare provider to continuously monitor the patient's heart dysrhythmia.

The patient will often receive a sedative prior to the administration of the electrical shock. Once the patient is sufficiently sedated, an electrical shock will be delivered through large electrodes that are attached to the patient's chest and back with adhesives. If the patient has a lot of chest hair, the hair might be shaved off prior to the attachment of the electrodes.

The electrical shock might be delayed a second or two if the patient is receiving synchronized cardioversion. Synchronization requires the ECG monitor to inform the defibrillator of the exact moment to deliver the energy. In most cases, the patient is not likely to notice a slight delay.

Pharmacological Cardioversion

Patients who receive medications to achieve cardioversion can sometimes feel heart palpitations as the medication is working to change the cardiac rhythm. In some cases, the feelings can be prolonged a few seconds. Patients who've experienced both electrical and pharmacological cardioversion usually describe pharmacological cardioversion as much less uncomfortable.

After Cardioversion

Immediately after receiving cardioversion, the patient might experience significant improvement of her signs and symptoms. When it works, cardioversion immediately resolves the patient's condition. The underlying cause of the cardiac dysrhythmia might still be present, which may require additional treatment.

If the initial attempt at cardioversion is unsuccessful, caregivers could try cardioversion again without delay. In most cases, patients can receive multiple electrical shocks or doses of medication without harm.

Managing Side Effects

Some side effects of electrical cardioversion include pain and irritation at the site of the electrodes, soreness in the chest, and anxiety. As noted above, the best management for after electrical cardioversion is to provide the patient with sedation. Pain medication may also be administered if the patient's pain is significant.

Side effects of pharmacological cardioversion are specific to the medication used. Adenosine has an extremely short half-life and the effects of the medication wear off almost immediately. Atropine can make a patient feel very anxious. Beta-blockers and calcium channel blockers have the opposite effect. Most medications for cardioversion can also be proarrhythmic, causing a different arrhythmia.

A Word From Verywell

The use of electrical cardioversion to treat cardiac arrhythmia has been around since the 1950s. These are very safe and effective treatments that will often be administered in an emergency department and in an emergency setting without too much deliberation. If the cardiac arrhythmia is significant enough to warrant electrical cardioversion in an emergency, there probably won't be much communication with the patient first.

If you are awake and need cardioversion, ask your caregiver to walk you through the entire process as it's happening. In many cases, you can receive sedation and won't have any memory of the event.

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7 Sources
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