Synchronized Cardioversion: Everything You Need to Know

Every year, 100s of thousands of people need CPR after suffering a cardiac arrest. For many, bystander CPR and defibrillation will give them a chance to receive professional emergency support and survive the ordeal. Others require more advanced cardiac techniques such as synchronized cardioversion. This professionally-performed cousin to defibrillation can help correct arrhythmias like ventricular tachycardia and atrial fibrillation.

Here’s what you’ll want to know about this advanced procedure.

What is Synchronized Cardioversion?

operating-room-countershock-equipmentSynchronized cardioversion is a life-saving procedure performed to treat certain arrhythmias when medication is unable to convert those rhythms to a normal rhythm. This method involves a low-energy shock to the front of the body. While similar to defibrillation, this shock synchronizes to administer the shock at a precise moment–when the QRS complex peaks. This high point is called the R wave.

This perfect timing is accomplished by selecting the “SYNC” option on a defibrillator. Once you push the button, you’ll note a pause. During this pause, a lot is happening. The machine synchronizes itself with the patient’s ECG rhythm. Now, the device knows when the QRS complex peaks on the R wave. As a result, it can deliver a precisely timed shock to obtain the desired effect.

When Do You Use Synchronized Cardioversion Vs. Unsynchronized

Unsynchronized cardioversion delivers a high energy shock when the rescuer presses the button. It relies on a person to try to time the shock (or not) to get the desired result. Most often, this means the shock occurs randomly in the cardiac rhythm cycle (QRS complex). And if that shock lands at a specifically bad time, it could make the situation worse.

Unsynchronized cardioversion is called for when the patient has no apparent electrical activity. In other words, they have pulseless ventricular tachycardia (VT) or ventricular fibrillation (VF). You would also use unsynchronized cardioversion if the defibrillator can’t synchronize with an unstable patient despite your efforts to use the sync function. 

On the other hand, you have synchronized cardioversion.

The goal of synchronization is to help the rescuer prevent the shock from hitting at the wrong time and leading to ventricular fibrillation (VF). This allows you to change the rhythm to a normal sinus rhythm. An unsynchronized shock doesn’t prevent this ventricular fibrillation because you have no control over where the shock hits on the QRS complex.

The T-wave is particularly vulnerable because, during this part of the wave, ventricle repolarization happens. Conversely, shock depolarizes heart cells, so hitting at this moment would basically send counteracting signals, causing instability.

The synchronization prevents this hitting of the T wave by timing the LOW ENERGY shock so that it does not happen during cardiac repolarization (t-wave), which would be very likely to cause VF (Ventricular Fibrillation). Instead, it shocks at the QRS peak to restore a normal rhythm. And it goes beyond that to precisely, hit the R wave instead.

What Is the Difference Between Cardioversion and Defibrillation?

These terms have some interchangeability, which can lead to confusion. 

Defibrillation always refers to an unsynchronized, high-energy shock. So it’s the same as unsynchronized cardioversion for all intents and purposes.

However, when professionals use the term cardioversion, it’s understood that they are talking about synchronized cardioversion rather than unsynchronized.

Apply defibrillation during an immediate life-threatening event where the heart has stopped. The patient has no pulse. There are three primary indicators for defibrillation, and these are:

  • Pulseless ventricular tachycardia
  • Ventricular fibrillation
  • Cardiac arrest that either results from or causes ventricular fibrillation 

You can view Save A Life’s Adult Cardiac Arrest Algorithm to learn more about defibrillation.

On the other hand, cardioversion (aka synchronized cardioversion) converts the rhythm from arrhythmia to normal.

What is Synchronized Shock Therapy?

Synchronized shock therapy is a less commonly used name for synchronized cardioversion. It refers to the fact that the rescuer administers a shock during the treatment. You’re unlikely to hear this in a professional setting.

When Is Synchronized Cardioversion Performed?

A licensed professional performs this procedure in some cases where the heart is beating very fast or irregularly but medications fail to restore a normal rhythm. 

When the pulse is too fast, that’s known as tachycardia, while fibrillation is an erratic heartbeat, which can also be rapid. Flutter is a type of fibrillation that is considered less serious but still something that needs correction because it causes Hemodynamic Instability (irregular blood pressure), and therefore poses a risk.

What Rhythms Require Synchronized Cardioversion?

  • Unstable Atrial fibrillation (AF)
  • Atrial flutter (types I and II)
  • Atrial tachycardia
  • Ventricular tachycardia with a pulse
  • Supraventricular tachycardia

Why Are Irregular Rhythms Corrected?

First of all, you cannot correct all abnormal rhythms. If the cause continues, so will the condition. However, medication can keep some Incessant (chronic) arrhythmias in check. And, if needed, synchronized cardioversion to correct these rhythms, although a corrected rhythm may return to arrhythmia after a period.

You may also need to correct an irregular rhythm during an emergency.

As an example, consider a severe episode of atrial or ventricular tachycardia? You may follow the Adult Tachycardia With Pulse Algorithm, which takes you through the steps to determine when synchronized cardioversion is required.  This algorithm is part of Save A Life’s Advanced Cardiac Life Support (ACLS) course you can take for free online.

The Dangers of Atrial Fibrillation and Flutter

According to Mayo Clinic (MayoClinic. org), atrial fibrillation (or its lesser cousin, atrial flutter) can cause:

  • Chest pain
  • Dizziness
  • Fatigue
  • Weakness
  • Trouble exercising
  • Shortness of breath

Left untreated, it can lead to blood clots and stroke.

The Dangers of Atrial and Ventricular Tachycardia

According to Mayo Clinic (, tachycardia causes:

  • The sensation of a racing or pounding heart
  • Fainting
  • Lightheadedness
  • Shortness of breath

In an extreme case, you’re at risk of injuring yourself during an episode, heart failure, and sudden death.

Differences Between Monophasic and Biphasic Systems

There are two basic types of cardioversion systems–monophasic and biphasic.


  • Energy flows in one direction from one paddle to the other
  • Higher energy shock (360 Joules) and greater damage to tissue
  • Only around 60% chance of restoring rhythm on the first shock


  • Energy flows in one direction between paddles, then reverses and returns several times
  • Lower energy (150 and 200 Joules) with less tissue damage
  • 90% first shock success rate

How Does Synchronized Cardioversion Work?

A specialized node in the upper right heart called a sinus node manages the heart’s pace. It’s usually influenced by environmental and emotional factors like stress, substances, hormones, and exercise. If your heart “skips a beat” when you’re surprised, your sinus node did that. Your heart speeds up when you exercise to accommodate blood flow needs and slows down when you sleep. These represent the sinus node’s responsibility.  These momentary disruptions and changes are normal and quickly balance themselves out.

However, sometimes a pattern occurs that the heart can’t get out of. Like a song you can’t get out of your head, the rhythm feeds itself, creating a loop.

It self-perpetuates.

When you shock the heart to alter the rhythm, you’re actually influencing how the sinus node manages the heart rhythm.

The intermediate delivery of an electrical shock to the heart causes a momentary depolarization of the heart cells. This split-second change of heart cells from a negative to positive charge. This gives the sinus node a chance for a fresh start. Now, the sinus node can restart the heart with a normal sinus rhythm.

A Brief History of Synchronized Cardioversion

In 1775, a Danish veterinarian by the name of Peter Christian Abildgaard first used electricity to stop and start the heart of hens, in an effort to understand how electricity might help physicians manage a stopped heart ( 

Much later, in 1947, an Ohio cardiac surgeon, named Claude Beck, who was well-known for his innovative techniques, first used direct current to successfully treat ventricular defibrillation (VF) on a 14-year old boy during open-heart surgery.

Beck “experimentally” performed the shock out of desperation after other more-accepted rhythm-restoring procedures failed to work. But after his success, the procedure underwent testing and quickly became more widely accepted ( 

According to the New England Journal of Medicine (, later in 1963, Bernard Lown M.D. et al. conducted a controlled experiment in which they used synchronized shocks to the heart to convert atrial fibrillation into ventricular tachycardia and then to a normal rhythm.

The biphasic waveform, the machine we use today to automatically synchronize with a patient’s rhythm and apply lower energy shocks, did not exist until the 1980s.

Performing a Synchronized Cardioversion

doctor-stethoscope-heart-handsBefore performing these procedures, it’s important to prepare the patient. 

If this were a cardiac arrest where you use defibrillation to restart the heart, you’d act very quickly and have little time to prepare the patient. But during cardioversion, you need to act quickly. But the patient is not in an immediate life-threatening emergency. So your rescue team should take time to prepare the patient for the highest possibility of success.

1. Get the Patient’s Rhythm

Identify the patient’s current rhythm using a 12 lead EKG unless you have no doubt about the patient’s rhythm.

2. Insert an IV and Start Sedation

You will also provide proper IV sedation to the patient since cardioversion is painful. If this isn’t possible, you also have other options such as hypnotic agents, sedative agents, and additional analgesics.

3. Make Sure Emergency Equipment Is On Hand

These include items like a suction device and a bag-mask device in case you need to provide manual ventilation as well as more advanced airway management tools, including intubation tubes.

4. Shave the Chest

Shave any chest hair to avoid interference with electrodes.

5. Provide Oxygen, if Needed

Provide supplemental oxygen according to facility guidelines.  A CO2 monitor is also recommended to track levels.

6. Place the Electrodes

The first one goes on the right side of the chest below the clavicle. The second goes two inches below the mid-axillary line. That’s to the left side of the left nipple.

7. Press the Sync Button

Now, it’s time to see synchronized cardioversion in action. The defibrillator will pause to follow the heart rhythm, attempting to sync with it.  Take a look at the rhythm strip to make sure the machine is finding and marking the R Wave.

8. Select the Appropriate Energy Level

This is determined by standard procedures.

9. Call “clear”

Everyone should be ready to expect a delay after “clear” is called.

10. Reassess the Patient’s Rhythm

If the patient’s rhythm does not convert, press SYNC again and repeats the procedure. Don’t forget to press sync as a defibrillator will always revert to defibrillation mode after the shock. Continue to monitor the patient.

What Patients Do Not Benefit from Cardioversion?

Cardioversion is not called for when a patient is known to have an incessant arrhythmia. Even though cardioversion may “fix it”. It will come right back. 

Cardioversion will not be effective if the cause of the arrhythmia remains. Some instances of this might include tachycardia caused by digitalis toxicity (or consuming Foxgloves from which the medication is made). Epinephrine, norepinephrine, and dopamine still in the system can also restart arrhythmia, whether introduced externally or internally produced. 

Patients with multifocal atrial tachycardia and clinically-induced sinus tachycardia will also not benefit.

Additionally, because of the risk of blood clots, a transesophageal echocardiographic study should be performed on those who are not taking anticoagulants because of the risk of a possible left atrial thrombus.

Finally, cardioversion is not effective for people with ventricular fibrillation. But this is only because the machine will not be able to sense and sync up with the QRS wave in these patients.

What Are the Risks and Complications?

Risks to both medical professionals and patients exist.

For example, according to MedScape, injuries to paramedics performing shock therapies have been reported at a rate of one case per 1700. Other medical professionals aren’t doing any better. Doctor and nurse injury rates are at around one for every 1000 shocks administered. In other words, if you perform defibrillation or cardioversion with any frequency, you’re likely to get shocked from time to time. 

As for the patients, hypoxia (lack of oxygen) or hypoventilation (shallow, insufficient breathing) during sedation do happen. That’s why it’s important to be ready to support breathing or intubate if needed. Most burns are superficial and will heal.

In more extreme cases, patients can develop severe bradycardia ( dangerously slow heart rate) or asystole (flatline/stopping of the mechanical or electrical function of the heart). Certain conditions can increase these risks.

Synchronized cardioversion is one of many advanced life-saving techniques you’ll learn and become confident performing within your scope of practice when you get your Advanced Cardiac Life Support Certification online.  Share your wisdom and thoughts in the comments below, or head on over to the discussion about this post on Facebook.

About Mackenzie Thompson, Life Saver, NHCPS

Mackenzie is a seasoned life saver and a multifaceted professional in the medical field. With an impressive 8-year track record in medical education, Mackenzie boasts a comprehensive set of certifications, including ACLS, PALS, BLS, and CPR, which reflect her unwavering commitment and expertise. Her significant contributions to teaching and the development of medical content underscore her profound knowledge and dedication to advancing healthcare.

Beyond her medical prowess, Mackenzie seamlessly integrates her passion for education with her proficiency in media and marketing. Her academic journey at Indiana University culminated in a degree in Media and Marketing, further solidifying her expertise in these domains.

In addition to her impressive professional achievements, Mackenzie possesses a refined taste for global exploration, photography, design aesthetics, sartorial elegance, and the culinary arts, with a particular affinity for Chinese cuisine. Currently based in the vibrant city of Manhattan, she continues to be a driving force in the medical community. She is an invaluable asset to, where she champions excellence and innovation with unwavering dedication.

Contact Mackenzie Thompson, Life Saver, NHCPS at.

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