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Cardioversion vs Defibrillation: What’s the Difference?

Cardioversion vs Defibrillation: What's the Difference?

Photo of Greta

by Greta Kviklyte

Life Saver, AMC
Co-authored by Kim Murray, RN, M.S.

posted on Aug 26, 2021, at 8:30 am


Each year, an estimated 300,000 cardiac arrests happen in U.S. hospitals, of which some 81% are non shockable rhythms (asystole or pulseless electrical activity). In 50-60% of cases, cardiac arrest happened because of primary heart failure, followed by primary respiratory failure at somewhere over 15%. This is according to a JAMA study published at

The only proven rhythm method to improve the survival rate to discharge is timely defibrillation after a cardiac arrest, secondary to ventricular tachyarrhythmia. However, for certain types of cardiac arrhythmias, the most effective treatment is synchronized cardioversion. So it’s important to know the difference between cardioversion and defibrillation, when they’re used, and when one or the other may be dangerous and even cause cardiac arrest.

Synchronized Cardioversion Vs. Defibrillation: Overview

The goal of both defibrillation and cardioversion is to transfer electrical energy to the heart. This stuns the heart instantly in hopes of generating a normal sinus rhythm from the heart’s natural pacemaker, which the heart can then maintain on its own.


While both use the same general action and can use the same equipment, the method is different, as well as when you use them, how you use them, and why you’re shocking the heart.

Primarily, you use defibrillation during an immediate life-threatening situation. You have a cardiac arrest, and if you don’t get this heart restarted, the victim will officially transition.

Conversely, doctors use cardioversion to convert heart rhythm when a patient has an unstable heart but is not immediately in danger of dying. Cardioversion stabilizes the heart rhythm. It is, therefore, usually an elective procedure. With guidance from their doctor, a patient chooses cardioversion to correct their heart rhythm. But it can also be urgent if the patient is experiencing tachycardia or a feeling of breathlessness.

Second, doctors synchronize cardioversion to achieve a specific rhythm outcome. You don’t time defibrillation. You just need a shockable rhythm.

So, as soon as there is a shockable rhythm, an automatic defibrillator tells you to “hands off” or “clear”. Then a shock occurs. But during cardioversion, the machine waits a couple of seconds to sync up with the rhythm to shock at a precise time in that rhythm.

Third, a defibrillator delivers a higher energy dose vs. cardioversion.

Ultimately, cardioversion performed when needed could prevent a subsequent cardiac arrest later that day, week, or year. That’s because it transforms an irregular rhythm into a stable one. But once a cardiac arrest occurs, defibrillation is your only option–that is, if you have a shockable rhythm.

Below you’ll find further information about these procedures to further distinguish between them.

What Is Defibrillation?

Defibrillation is a shock-based treatment for life-threatening arrhythmias when a patient does not have a pulse. These could include ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT). It’s done when someone is in cardiac arrest.

Someone can perform it in a hospital, an ambulance, or an out-of-hospital setting. While medical professionals perform defibrillation in the hospital, the average person with CPR training can also perform defibrillation if they have access to an automatic external defibrillator (AED). You can buy one for your home or office.

A Brief History of the Defibrillator

Two physiologists (Prévost and Batelli) discovered the power of fibrillation around the end of the 18th Century by running dog experiments using shock. They found that when they applied small shocks to the dog’s heart, they could put the heart into V-fib and then shock it again with a larger shock to reverse the change.

But it would be another 14 years before a cardiothoracic surgeon did defibrillation on a 14-year-old boy on whom he was performing cardiothoracic surgery for a congenital heart disorder. He placed electrodes on the open heart for the procedure.

The technology remained mostly unchanged for another 150+ years until the 1950s, when Russian researchers developed a defibrillator that they could use on a closed chest. This would later become the modern-day monophasic defibrillator. In the 1980s, researchers discovered a biphasic waveform, which would replace monophasic shocks in some instances.

Defibrillation and Cardioversion share this history since they both use a defibrillator to work.

Monophasic Vs. Biphasic Shock

A monophasic shock only travels in one direction from one paddle to another, while a biphasic shock travels from one paddle to the other and then back several times. Biphasic shock results in fewer burns and has a higher first shock success rate (90% to 60%), although studies comparing how use may impact discharge success haven’t been thoroughly studied.

Types of Defibrillators

There are four primary types of defibrillators, which are used in different settings:

  • Automated External Defibrillators (AEDs) are often found in CPR kits for the average person with CPR training.
  • Semi-automated AEDs can be overridden and are therefore only used by a trained professional like a paramedic.
  • Standard defibrillators with monitors require professional skills to operate. You’ll find them in hospitals mostly.
  • Implanted defibrillators can shock the heart back into a rhythm during a cardiac arrest without assistance from a bystander.

While paddles were once very common, they’ve now been overtaken by the use of adhesive patches, which are generally easier to use and perform better because they adhere to the chest without the need for gel application.

Risks of Defibrillation

Defibrillation risks are minimal when the machine is well-maintained. Defibrillators measure the patient’s heart rhythm and, if you have an automatic machine, they provide clear instructions based upon that rhythm. All you have to do is follow his instructions even if you have no medical training.

Fragile patients, like the elderly, may experience broken ribs or the aggravation of an existing injury. But defibrillation saves a life, so those are generally considered reasonable casualties of the procedure.

Shocking yourself is also possible, so when the machine says “hands off”, please make sure no one’s touching the victim.

Defibrillator burns have been reported but are rare with current equipment and a sign of a poorly-maintained machine that probably needs to be replaced. The batteries and pads of an automatic defibrillator typically last two to five years, depending on the frequency of use. So they need to be replaced to maintain proper function. An implanted defibrillator lasts up to 10 years, according to Johns Hopkins at

These same risks will apply to cardioversion plus a few more.

Energy Levels for Defibrillation

  • Monophasic – CPR algorithm recommends single shocks started at and repeated at 360 Joules (J).
  • Biphasic – CPR algorithm recommends shocks initially of 150-200 J. Then use subsequent shocks incrementally 150, 200, 300, 360 J.

A clinical trial on biphasic defibrillation showed that increasing the energy level of shocks was more likely to result in a conversion and stop ventricular fibrillation (VF) when compared with repeated shocks at 150 J. Often, the first shock did nothing, but the subsequent higher J shock did.

What Is Cardioversion?

man-lying-with-aed-padsSynchronized cardioversion is a process to achieve a sinus rhythm in a patient who has arrhythmia. It requires the delivery of a low energy shock at a specific time. Doctors achieve this timing by viewing a defibrillator monitor or using a machine that syncs to the rhythm automatically. In both cases, you’ll only find this equipment in a hospital or specialized clinic.

They sync the shock to a specific point in the QRS complex (the main spike visible on that monitor with EKG / ECG).

Through this method, atrial fibrillation can be converted to a normal sinus rhythm while preventing the prompting of ventricular fibrillation (VF). This happens because you’ve synchronized the electrical shock to the R wave. At the same time, you avoided that susceptible T-wave and averted ventricular fibrillation.

Uses & Indications

Patients who need cardioversion experience the following symptoms:

  • Breathing struggles from pulmonary edema (fluid in the lungs)
  • Hypotension (low blood pressure)
  • Chest pain
  • Syncope (loss of consciousness from low blood pressure)

When a patient has symptoms like these, their doctor will need to run tests to confirm the cause of the symptoms.

Electrical cardioversion may be indicated if those tests show:

  • Unstable pulse
  • Failed chemical cardioversion with chest pain or unlikelihood chemical cardioversion would be successful.
  • Decompensated rapid AF with rapid ventricular response. An example of this would be a hypotensive patient who isn’t responding to other medical therapies.
  • Ventricular tachycardias (VT) with pulse
  • Supraventricular tachycardias, which includes AF without decompensation but only if not in an acutely urgent situation.

The technique is used less frequently in situations such as atrial fibrillation to revert the heart rhythm to a sinus rhythm.

In a patient with atrial fibrillation, you use cardioversion to control the rhythm. However, cardioversion will not always be successful. Studies have shown as many as 50% of patients’ hearts will return atrial fibrillation within 12 months. However, a doctor could do another cardioversion at that time, if needed.

Risks of Cardioversion

In addition to the general risks of defibrillation, you should stay aware of these potential complications.

Having cardioversion may put patients at increased risk of thromboembolic disease (TED) soon after the procedure. As a precaution, doctors prescribe anti-coagulation at least three weeks before and four weeks after the procedure.

Doctors may also use a transesophageal echocardiogram during the procedure to spot thrombus, although some patients may still develop TED.

How to Cardiovert

Cardioversion is performed in a hospital with a doctor, an anesthesiologist, and cardiac nursing professionals present. The anesthesiologist will plan and supervise sedation of the patient using general anesthesia. Once sedated, the team performs the following steps:

  1. Attach the self-adhering electrodes
  2. Pick an energy level to start. Note: a higher starting energy may require fewer shocks. Levels are based on the starting type of rhythm. For Broad complex tachycardia and AF, you’d do monophasic starting at 200 or biphasic at 120-150 J. For atrial flutter and narrow complex tachycardia, the recommendation is starting at 100 for monophasic and 70-120 J for biphasic
    Achieve a visible trace on the monitor
  3. Hit the sync button. You’ll see a blip or dot marking the QRS complex. This will allow the shock to sync automatically so it shocks with correct timing that might not be achieved manually. Note: sync may not work in tachycardia where the QRS complex is variable.
  4. Charge
  5. Call “Clear” and look to see that the bed is clear.
  6. Shock. Expect a 1-2 second delay since the machine is synchronizing the shock.
  7. View the rhythm to see if you have a sinus rhythm. If yes, stop. If not, adjust the energy level up incrementally and repeat steps 5-8.
  8. Look for burns that may need treatment
  9. Get a 12-lead ECG for a more thorough reading

Special Considerations on Infants and Children

Pediatric Cardioversion and Defibrillation require attention to detail and some basic math skills. The recommendation is to start at 1J for every kilogram of weight.

You should never use an automatic or semi-automatic defibrillator on a child younger than one year. Only use a manual one with a monitor, which usually means the child is in the hospital.

Whether performing on an infant or child, note that manual defibrillators only go as low as 4J/Kg, so use specialized pediatric pads for both manual and automatic external defibrillators. This reduces the energy delivered when used in combination with the pediatric setting. You can view further details related to pediatric treatment in Pediatric Cardiac Arrest Algorithm.

Children eight and older can receive cardioversion and defibrillation at adult levels, according to the guidelines outlined in your Advanced Cardiac Life Support Training. You can now obtain this training 100% online.

Cardioversion Vs. Defibrillation

Cardioversion and defibrillation both rely on the power of shock to achieve the desired heart rhythm. But when they’re needed and how doctors use the defibrillation equipment are very different. Exploring the differences between procedures with so many similarities can help you better understand how the heart works and how doctors can apply different strategies to help the heart perform at its best.

Share your story with us about how have you applied cardioversion or defibrillation.

About Greta

Greta is a dedicated life saver and a distinguished expert in the field of medical content creation and editing. Her impressive array of certifications in ACLS, CPR, PALS, and BLS underscores her commitment to excellence in the medical field. With over four years of invaluable experience in medical education, Greta plays an indispensable role within the Advanced Medical Certification team, shaping the way healthcare professionals around the world acquire and apply vital knowledge.

Greta's profound expertise serves as the driving force behind the development and distribution of medical content that has significantly enhanced the capabilities of countless healthcare practitioners across the globe.

In addition to her medical qualifications, Greta holds a prestigious academic distinction in Marketing and Global Business from Vilnius University. Her academic journey has been enriched by immersive studies in Slovakia and Portugal during her time as an exchange student, providing her with a global perspective that complements her medical expertise.

Beyond her professional commitments, Greta possesses a genuine passion for global exploration, with a particular focus on immersing herself in diverse cultures and appreciating the intricacies of the natural world. While residing in Vilnius, Lithuania, she continues to make substantial contributions to the field of medical education, leaving an indelible mark on the sector.

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