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The Shocking Truth: How Defibrillator Voltage Saves Lives During Cardiac Arrest  

defibrillator voltage

Out-of-hospital cardiac arrest is a serious global health problem affecting around 3.8 million people each year, with only a small percentage surviving when they reach the hospital. Early defibrillation using electric shocks can improve survival, but getting defibrillators quickly to those who need them has been a challenge.

A defibrillator is a medical device used to detect and treat life-threatening arrhythmias. Arrhythmias are irregular heartbeats that can sometimes be fatal and sometimes be minor. An electrical shock from a defibrillator can prove to be lifesaving for someone suffering from Sudden Cardiac Arrest (SCA) if their heart is in one of two potentially fatal heart rhythms: ventricular fibrillation or ventricular tachycardia. A defibrillator treats a fatal arrhythmia by delivering a controlled electric shock to a person’s heart. 

Even though a defibrillator is a medical device, Automated External Defibrillators, or AEDs, are not reserved for medical professionals. In other words, anyone can use an AED. In fact, AEDs are public access devices designed to be used by non-medical personnel. Learn more about how defibrillation works, and how it saves lives below. 

How Does a Defibrillator Work?

During SCA, the heart suddenly stops beating normally because there is a problem with its electrical system. If it’s not treated quickly, it can lead to a deadly result. In cardiac arrest, the heartbeat is erratic and, as a result, cannot pump blood effectively throughout the body. The goal of defibrillation is to momentarily disrupt the problematic, erratic electrical activity, and allow the heart to “reboot.” Defibrillation gives the heart’s natural pacemaker a chance to regain control and reestablish its proper rhythm, potentially saving someone’s life.

The main components of an AED include:

  • Electrode pads, which attach to the patient’s chest and monitor their heartbeat. They can also deliver the lifesaving electric shock.
  • A capacitor, which stores all of the voltage and then releases energy to the patient.
  • A battery, which charges the capacitor.
  • A processor that determines whether or not a patient has a shockable rhythm. Some arrhythmias do not benefit from an AED shock.

When the electrode pads get applied to a person, the AED detects the patient and will immediately start recording and reading their heart rhythm. The AED will then classify the rhythm as either shockable or non shockable

If it determines that the rhythm is shockable, the AED will charge up the capacitor, confirm the rhythm once more, and deliver a shock to the patient through the electrode pads. The American Heart Association suggests that during a cardiac arrest rescue, the AED analysis cycle is followed by two minutes of high-quality CPR. Then, the AED analysis cycle repeats its analysis after those two minutes of CPR. 

Terms to know: Defibrillator Voltage, Current, and Joules 

When it comes to defibrillators, the terms voltage, current and joules can get confusing since people sometimes use them interchangeably. Here’s how you can keep these terms straight:

  • In defibrillation, joules (J) represent the amount of energy delivered to a patient’s heart from a shock. For example, an adult patient would receive a shock with more joules than a pediatric patient would. 
  • Voltage, measured in volts (V), determines the strength of the shock from an AED, which is measured in joules. The higher the voltage, the greater the capacity to push electric charges through a circuit. 
  • Current is the rate at which electric charges move in a circuit, or the flow of energy.

In the context of defibrillation, joules represent the energy of the shock, voltage is the intensity or “push” of the shock, and current is the actual flow of electric charges during defibrillation, working together to restore the heart to its normal rhythm. 

Importance of Defibrillator Voltage

Most AEDs have a fixed number of joules they will deliver to the patient when a shock is advised. Typically, their energy level is somewhere between 120-200 joules. For example, the capacitor in the Avive Connect AED stores 2,000 V so that it can deliver  a 150 joule shock to an adult patient and a 50 joule shock to a pediatric patient. 

Advanced life support, or ALS defibrillators, are reserved for use by medical professionals because the joules in each shock can be manually adjusted depending upon different factors that can be assessed through analyzing the ECG and taking into account patient specifics. Public access AEDs do not have this feature, though, since they are designed to be used by untrained bystanders.

Studies have shown that biphasic shocks of 150 joules are just as effective in restoring a heart rhythm as higher energy shocks, and they typically cause less harm to the patient because there is less energy being administered. All modern AEDs deliver biphasic, or double phased, shocks which allow for a safe, effective shock via lower energy levels. 


Luckily, AEDs take all of the guesswork out of defibrillation and are designed to be used by untrained users. While AED training is always a good idea, anyone can and should feel empowered to step in and use a defibrillator should they need to act in the event of a cardiac emergency. 

Key Takeaways

  • Sudden Cardiac Arrest is a common and unpredictable condition where the heart’s rhythm needs to be restored through an electric shock from a defibrillator.

  • Defibrillators are crucial for detecting and treating life-threatening irregular heartbeats, called arrhythmias, especially in cases of ventricular fibrillation or tachycardia.

  • Automated External Defibrillators (AEDs) are designed for use by non-medical personnel, offering accessibility to all, and understanding key terms like Joules, Voltage, and Current is vital.

  • Defibrillators work by momentarily depolarizing the heart muscle to restore normal rhythm, with voltage and energy level playing critical roles in successful defibrillation. Studies support the effectiveness of lower-energy biphasic shocks delivered by modern AEDs.

Voltage & Defibrillator FAQ

How many joules are delivered during a shock with the Avive Connect AED?

For adult patients who are in a shockable heart rhythm, the Avive Connect AED will deliver a 150 joule shock. For pediatric patients over the age of one year old, the AED will adjust and deliver a more appropriate, lower energy shock at 50 joules. 

Learn more about the Avive Connect AED!

Why is high voltage needed in a defibrillator?

High voltage is needed in a defibrillator to deliver a powerful enough shock to the heart in order to reset its electrical activity and restore a normal rhythm.

What is the difference between an AED and a defibrillator?

The main difference between an AED and defibrillator is the intended user. Both a defibrillator and an AED are used for the same purpose- to restore a person’s heartbeat to normal; the “D” in AED stands for “defibrillator.” 

While AEDs are intended for use by untrained bystanders in the event of cardiac arrest, defibrillators are meant for use by trained medical professionals only and are found in facilities like hospitals and ambulances. Doctors, nurses, or paramedics typically use manual defibrillators which absolutely require training to assess and treat various heart arrhythmias.

What is the recommended defibrillator voltage range for adult patients?

The energy delivered by an AED is measured in joules, not volts. The recommended range for an electric shock for adult patients typically falls between 150-360 joules. 

Most modern public access AEDs, and all automatic AEDs, deliver a predetermined, evidence-based amount of energy and do not require the user to manually select the energy level before administering a shock.

How many times can you be shocked with a defibrillator?

During a cardiac arrest, sometimes it takes multiple shocks from a defibrillator for a heart to regain its normal rhythm. Everyone is different, so the number of shocks varies from person to person. 

Will Flanary, TikTok sensation and SCA survivor, was famously shocked with a defibrillator five times before his heart regained its normal rhythm. Listen to his story here.

How many volts can a human sustain?

The energy delivered by an AED is measured in joules, not volts. It depends on the person, but AEDs are programmed to deliver a specific amount of energy, usually between 150-200 joules for each shock. The voltage delivered when using an AED is not a constant – it is a brief pulse of electrical energy. 

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