Defibrillators are instrumental in preventing sudden cardiac arrest (SCA) deaths, but how do they actually work? Though different types of defibrillators have different mechanisms behind them, they all operate on the same basic principle: using controlled electric shocks to prevent cardiac arrest or to restart a heart that no longer beats.
How Does a Defibrillator Work
A defibrillator is designed to correct or restore a heartbeat with the use of electric pulses or shocks. In a general sense, it can:
- Correct the kinds of heart arrhythmias (irregular heartbeats) causing cardiac arrest, specifically ventricular fibrillation, and pulseless ventricular tachycardia.
- Restore a normal heartbeat in a cardiac arrest sufferer
A defibrillator first checks for heart arrhythmias. If a shockable arrhythmia is detected, the device triggers a shock to the heart in an effort to restore normal function. A typical defibrillator will deliver 150 to 360 Joules of electricity at a time (less in the case of children). Subsequent shocks may be automatically adjusted with more or less electricity based on the response of the patient.
In the past, defibrillators relied exclusively on monophasic shock delivery, meaning that the electrical current flows in a single direction. In recent years, all device makers have switched to lower-energy biphasic shocks, meaning that the current flow goes in both directions. In the first phase, the current flows from one paddle to the other, and in the second phase it reverses direction. The Philips HeartStart series is one of the first AED lines to standardize biphasic waveform technology.
One key benefit of biphasic delivery is that it allows defibrillators to restore normal heart function with less energy, thereby reducing the risk to the patient and allowing a greater likelihood of success. In fact, studies have shown that 200 J biphasic shocks can be as effective as 360 J monophasic shocks.
Types of Defibrillators
There are three main types of defibrillators:
- Automated external defibrillators (AEDs)
- Wearable cardioverter defibrillators (WCDs)
- Implantable cardioverter-defibrillators (ICD)
When most people think of defibrillators, they imagine pads or paddles used externally on the chest, but not all defibrillators work this way. An automated external defibrillator (AED) is reserved for cardiac arrest emergencies, but other types of devices are designed for preventive measures.
Automated External Defibrillators (AEDs)
An AED is a small, lightweight, portable defibrillator designed for use on SCA sufferers. Though these defibrillators can vary widely in their cost and complexity, a typical device is equipped with step-by-step voice coaching so that an untrained lay user can save a life in an emergency.
An AED is equipped with pads and a battery, both of which come with an expiration date. The pads are equipped with a special gel that adheres to a patient’s bare skin, and built-in sensors allow the defibrillator to analyze the heart for a shockable rhythm. Most devices will only allow shock delivery after determining that such an intervention is safe and advisable.
The ‘Automated’ in ‘Automated External Defibrillator’ means that these devices are designed to read the heart rhythm and make a shock/no-shock decision automatically, as opposed to hospital or EMS-grade defibrillators which allow the operator to read the EKG rhythm and make their own decision as to whether to shock, and at what energy level. AEDs use a built-in algorithm to make the determination whether a shock is needed. How that shock is delivered is what makes an AED either Fully Automated or Semi Automated.
- Semi-automatic shock delivery is standard with most AEDs. After the device determines that a shock is warranted, it instructs the user to press a “Shock” button on the device. The shock button is only activated once the AED determines a shockable rhythm, which removes the risk of accidentally shocking someone inappropriately. Devices available in semi-automatic mode include the Philips OnSite and FRx, the Heartsine 350P and 450P, and the Defibtech VIEW.
- Fully automatic shock delivery requires no intervention from the user once the pads are placed on the patient, other than ensuring everyone stands clear. As soon as the device determines that a shock is warranted, it sounds verbal warnings to let all responders know to stand clear due to an imminent shock, then delivers that shock automatically. This feature is becoming popular in a growing number of defibrillators and is sometimes available as an upgrade. AEDs with this option include the Cardiac Science Powerheart AED G3 Plus and the Cardiac Science Powerheart G5. Several AED models can be purchased as either semi- or fully automated, such as the ZOLL AED Plus, and the Defibtech Lifeline.
In addition, a typical external defibrillator will include features like CPR coaching, automatic daily self-tests, child-friendliness settings, and—in many cases—bilingual functionality.
Unlike with other types of defibrillators, anyone can own an AED. In fact, these devices have become popular—and are often even required—in places like hotels, offices, gyms, and schools, because sudden cardiac arrest can strike at any time.
Implantable Cardioverter-Defibrillators (ICDs)
An ICD is a device that’s surgically implanted into the chest or abdomen. It’s commonly recommended for people with arrhythmias who are at an elevated risk of SCA. The goal of the ICD is to detect and correct arrhythmias in real time, thereby minimizing the risk of an irregular heartbeat triggering sudden cardiac arrest.
A doctor may recommend an ICD for patients who:
- Previously experienced cardiac arrest
- Live with a condition like cardiac sarcoidosis
- Live with a condition characterized by poor blood flow to the heart
- Live with a neuromuscular disorder like muscular dystrophy, which can affect heart rhythms
- Live with a genetic condition that can trigger a heart arrhythmia, such as congenital heart disease
- Have a history of arrhythmias such as ventricular fibrillation, ventricular tachycardia, atrial fibrillation, or atrial flutter
An ICD is similar to a pacemaker. But whereas a pacemaker only delivers low-energy shocks to address slow or missed heartbeats, an ICD delivers different types of shocks to address a wide range of arrhythmias. For instance, a low-energy shock can treat a slow or irregular heartbeat while a high-energy shock can address a rapid heartbeat.
Every ICD is equipped with a generator that analyzes heart rhythms and delivers shocks as needed, but not all models work the same way. Some models have wires inserted into one or multiple chambers of the heart. Other ICDs are designed to rest on the surface of the heart to monitor rhythms externally. Each device is custom-programmed based on the types of arrhythmias you’re most likely to experience.
Wearable Cardioverter Defibrillators (WCDs)
A WCD works like an external ICD. Rather than attaching the sensors directly to the heart muscle, the sensors are attached to the skin. Low- or high-energy shocks are delivered depending on the type of arrhythmia detected, and the device is pre-programmed to address your specific medical condition.
A WCD includes a belt-and-vest attachment that’s worn under your clothes. Unlike an ICD, a WCD pre-warns you of a potential arrhythmia and gives you the option of overriding the shock function. If you don’t respond, a shock is delivered automatically, usually within one minute. After a series of shocks is delivered, the sensors must be replaced—much like with an AED. The device also records heart rate and event information for your doctor.
Because a WCD isn’t as invasive as an ICD, it’s often recommended for lower-risk patients. Nevertheless, wearable cardioverter defibrillators have proven highly effective in preventing sudden cardiac arrest and in helping to keep even aggressive arrhythmias under control.
How to Use a Defibrillator
If you have an ICD implanted, you don’t have to do anything. If you’re prescribed a WCD, you need only wear it as advised by your doctor.
An AED is the only type of defibrillator with a learning curve, and even that’s pretty simple on account of the device’s intuitive nature. If you need to use an AED in an emergency, the process is pretty simple:
- Have someone call 911—or do it yourself if nobody is available
- Check to ensure that the patient is experiencing cardiac arrest—no breathing, no heartbeat, unconscious
- Power on the AED (this may be done by pressing a Power button or opening the lid)
- Follow the step-by-step voice instructions
The device will guide you through both CPR and defibrillation, prompting you to deliver an electric shock if the intervention is warranted. If a shock is unsuccessful, or if no shockable rhythm is detected, the device will prompt you to continue CPR until emergency services arrive. Remain with the patient until help arrives.
How Effective Is a Defibrillator?
When used promptly and properly, an automated external defibrillator can be extremely effective.
Every year, there are approximately 356,000 out-of-hospital cardiac arrests in the U.S., and most are fatal—this high fatality rate is partially due to delayed intervention. When an AED is used, a patient’s prognosis improves significantly.
One study examined the data from 13,769 out-of-hospital SCA incidents and found that:
- Overall survival to hospital discharge was 7%
- Overall survival with CPR but no AED was 9%
- Overall survival with AED application was 24%
- Overall survival with AED shock delivery was 38%
In other words, the immediate use of an external defibrillator can increase an SCA patient’s odds of survival by more than five times. Studies into ICDs and WCDs have similarly found these devices to be effective for prolonging the lives of high-risk individuals, even those who have previously suffered a cardiac arrest.
Defibrillators Save Lives
If you’ve experienced a cardiac arrest or you’re at a heightened risk for SCA, speak to your doctor about whether a wearable or implantable defibrillator is right for you.
If you’re simply looking to protect yourself or others in your home or place of business, consider investing in an automated external defibrillator. Anyone can establish an AED program, and it may just save a life one day.
