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Cardioversion vs. Defibrillation: Everything You Need to Know

Cardioversion Vs Defibrillation

Cardioversion and defibrillation are both treatments which treat abnormal heart rhythms, called arrhythmias. Arrhythmias can occur in healthy hearts and, most of the time, they are both short-lived and harmless. However, serious arrhythmias that are prolonged and significantly disrupt heart function are very dangerous. Certain types of arrhythmia can even lead to Sudden Cardiac Arrest (SCA).

Cardioversion is used on patients who have an irregular or unstable pulse. Cardioversion shocks the heart back into a normal rhythm without interrupting the heart’s own important cycles. It syncs up with the heart to deliver the shock at the best possible moment. This is necessary for treating certain arrhythmias, but other arrhythmias can be treated through defibrillation.

Let’s take a closer look!

Cardioversion Vs Defibrillation – an overview

Cardioversion is a medical procedure used to restore a normal heart beat in people who have arrhythmia. Cardioversion involves the use of synchronized electrical shocks to stabilize an irregular heartbeat, often atrial fibrillation or atrial flutter. Cardioversion sends electric shocks to the heart through electrodes placed on the patient’s chest. Cardioversion can also sometimes be done with medication, this is called chemical cardioversion or pharmacologic cardioversion. Cardioversion is usually a scheduled procedure which takes place in the hospital and is better suited for patients with certain types of arrhythmias that benefit from a shock that comes during specific times during the heart’s electrical cycle.

Defibrillation is used to treat patients in cardiac arrest who do not have a pulse, as well as patients with certain types of arrhythmias. A defibrillation shock from an AED is a more powerful shock than those delivered during cardioversion, resetting scrambled signals and enabling the heart to beat normally. 

In a normal, healthy heart, electrical impulses trigger a coordinated sequence of muscle contractions that enable the heart to pump blood. When the heart is in ventricular fibrillation or ventricular tachycardia, the electrical signals that control the heart are scrambled. Therefore, the heart muscle is not contracting in an organized fashion. 

A “shock” of electrical current from a defibrillator depolarizes most of the heart and ends the arrhythmia. When a patient receives a “shock” from an AED, the energy from the AED passes between the electrode pads and through the patient’s heart. The sudden burst of energy a defibrillator sends through the patient’s heart causes their heart cells to depolarize, stopping the chaotic and ineffective rhythm, otherwise known as defibrillation. 

After this depolarization, there is typically a brief period where the electrical signals in the heart pause before a normal heartbeat is reestablished.

After the heart has been defibrillated, it goes back into a normal sinus rhythm wherein it starts recirculating oxygen-rich blood throughout the body. Skin color may return to normal and, in some cases, the person will start breathing on their own.

sinus rhythm

Types of Cardioversion

There are two types of cardioversion. Both aim to restore a regular rhythm to a patient’s heart.

The first type of cardioversion is chemical or pharmacologic cardioversion where medicines are used to get the heart back to a normal rhythm. This is often the initial approach upon detecting an irregular heartbeat, while taking into consideration the patient’s underlying medical conditions. Chemicals contained in antiarrhythmic medications may restore a normal rhythm. 

Electrical cardioversion uses energy shocks to restore the heart to a normal pulse. 

Synchronized & Unsynchronized Cardioversion

Synchronized cardioversions occur during a specific part of the heart’s cycle. Recognize that electric impulses control the heartbeat, and when measured on an electrocardiogram (ECG), each part of the cycle is distinctive. Synchronized cardioversion shocks are timed with the aim of minimizing risks and maximizing effectiveness.  

Unsynchronized cardioversion is not as nuanced, uses more energy, and delivers a shock at any point during the cardiac cycle, when the machine has reached full charge. For all intents and purposes, unsynchronized cardioversion is the same thing as defibrillation, and is used when the patient does not have a pulse.

Energy Levels for Cardioversion

Typically set at 100 joules, the first shock may be enough to restore a regular rhythm. If the first shock is unsuccessful, the clinician may ramp up the energy incrementally, delivering shocks until the abnormal rhythm ceases. The precise amount of energy delivered typically varies from 100 to 300 joules, with most arrhythmias stopping after an application of 120 joules. 

Risks Associated with Cardioversion

As is the case with most medical procedures, there are certain risks associated with cardioversion:

  • blood clotting issues – blood clots can form due to abnormal heart rhythms, cardioversion may dislodge them, which can lead to other complications, including stroke
  • could lead to the development of future arrhythmias
  • it may not work and further interventions could be needed

Types of Defibrillators

There are a few different types of external defibrillators:

  • Automated External Defibrillators (AEDs) are machines used to treat cardiac arrest, a life-threatening condition where the heart suddenly stops beating properly. By sending an electric shock to the heart of a person in cardiac arrest, the AED restores a normal heart rhythm. AEDs are specifically built for lay people. 

There are two types of AEDs:

  • Fully-automatic AED (FAED) — Analyzes the heart, and if a shock is warranted, the AED will say so, instruct bystanders to stand back with no physical contact with the victim, and deliver a shock on its own.
  • Semi-automatic AED (SAED) — Analyzes the heart, and if a shock is warranted, the AED will say so and instruct the responder to press the shock button to deliver a shock. If the button isn’t pressed, no matter how badly the patient needs therapy, no life-saving “shock” will be delivered.

Learn more about fully-automatic vs. semi-automatic AEDs.

  • Manual defibrillators are the machines you are accustomed to seeing on TV. They are usually in hospitals or in the back of ambulances. Manual defibs allow the operator to view the patient’s ECG and make adjustments to the clinical therapy delivered. For instance, they might decide to manually deliver electricity to the patient instead of using an automated algorithmic decision. They can help with pacing and cardioversion, unlike AEDs. Since they offer more sophisticated capabilities, they are typically used by medical professionals for monitoring patients. These devices are much larger and heavier than AEDs, making them less portable. Since they are more complex, these devices are always used by professionals and not laypeople.

Manual defibrillators

Monophasic Vs Biphasic Defibrillators — Almost all defibrillators have two paddles or pads. Each one houses an electrical lead. One lead is a positive, the other a negative. Monophasic defibrillators send the current in only one direction. Biphasic defibrillators initially send the shock in one direction and then reverse the electrical flow in the opposite direction. 

Energy Levels for Defibrillators

The amount of energy released by a defibrillator varies. Studies have examined the efficacy of different energy levels for defibrillating a patient.

Monophasic defibrillation devices deliver 200–320 joules of total energy.

Biphasic defibrillators can tailor their energy level to the patient by measuring their impedance (electrical resistance) using an internal mechanism to dampen the energy delivered when appropriate. Biphasic defibrillator devices vary by manufacturer but typically release between 120 to 200 joules of energy with each shock.

Risks Associated with Defibrillation 

You should only use an AED on a person if they are experiencing Sudden Cardiac Arrest (SCA).

Every minute that a person in Sudden Cardiac Arrest does not receive defibrillation, their chances of survival drop by 7-10%, making rapid defibrillation imperative for survival and one of the key steps in saving a life from Sudden Cardiac Arrest. Therefore, the benefits of using a defibrillator far outweigh the risks.

Complications a SCA survivor may experience post-defibrillation could include:

  • damaged cardiac and adjacent tissue
  • cardiac arrhythmias
  • skin burns

Some patients with irregular heartbeats, or in certain circumstances, should not be treated with an AED. Learn more about when not to use an AED.


Understanding cardiac arrhythmia is essential for treating it properly. Even minor atrial flutter or fibrillation could require treatment to avoid potential blood clotting and the development of more severe  arrhythmias. If you have severe tachycardia or atrial fibrillation, your healthcare provider may talk with you about cardioversion.

Fortunately, there’s also a treatment for a person with either ventricular fibrillation or pulseless ventricular tachycardia- defibrillation, a life-saving shock from an Automated External Defibrillator (AED). If used quickly, an AED can be a highly effective therapy.

*All information on the Avive website is provided in good faith. However, Avive makes no representation or warranty of any kind, express or implied, regarding the accuracy, validity, or completeness of any information on the Site. Any medical/health information on the Site is for general informational and educational purposes only and is not a substitute for professional medical/health advice.


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