The electrical shock to the heart provided by an automated external defibrillator (originally measured in volts) shuts down the disordered electrical impulses that lead to cardiac arrest so that a normal heart rhythm can restart—preventing sudden cardiac death. There are several levels of electrical energy that can be used to achieve this, depending on the waveform and patient impedance.
Sometimes it takes multiple shocks to restart a normal heart rhythm, and sometimes medications need to be used. Understanding how the voltage of a defibrillator works to reverse cardiac arrest—and why different shock levels are used—can help you appreciate the nuanced nature of electrical defibrillation and the importance of delivering the right dose.
Voltage, Current, Impedance, and Energy Delivered (in Joules)
When most people think about electricity, they think about voltage. That’s because it’s the unit used to describe battery capacity and the output of power outlets around the world.
Interestingly, the first defibrillation attempts in the history of AEDs were measured in volts:
- In 1947, American cardiac surgeon Claude Beck delivered four direct-current 110-volt shocks to a 14-year-old patient through two electrodes during open cardiac surgery and the patient was revived.
- In 1956, Jewish American surgeon Paul Zoll developed a closed-chest method of safely defibrillating patients with shocks as high as 750 volts.
- In 1957, William Kouwenhoven of John Hopkins University designed a 250-pound “portable” external defibrillation machine that could safely deliver 480-volt shocks to an adult heart using alternating current (AC).
Despite the fact that the first defibrillation shocks were measured in volts (electrical potential difference), the energy used for AED shocks today is calculated and delivered in joules (energy delivered).
In the International System of Units (SI), voltage is defined as:
- Electrical potential
- Electrical potential difference
- Electromotive force
According to the Encyclopedia Britannica entry about volts:
“[One volt] is equal to the difference in potential between two points in a conductor carrying one ampere current when the power dissipated between the points is one watt. An equivalent is the potential difference across a resistance of one ohm when one ampere is flowing through it.”
In other words, the actual amount of power that can be provided by a 9-volt battery depends on the amount of resistance present between the battery and the object that will receive that power—in this case, the heart.
Now, as scientifically minded readers might have realized, the voltage doesn’t tell the whole story. The voltage is only the electrical potential of an electrical circuit. What stops the heart is the transmyocardial current, which is why a different unit of measurement is now used.
Current, measured in amperes, is the actual flow of electricity.
One ampere represents “a flow of one coulomb of electricity per second, […] produced in a resistance of one ohm by a potential difference of one volt.”
100mA (one-tenth of an ampere) is enough to stop a heart, which is why wires like electric fences that have 2,000 to 10,000 volts coursing through them have a low amperage (current) so that livestock or people who accidentally touch the fence won’t die.
Joules and Impedance
Impedance to defibrillation, measured in ohms (Ω), is inversely related to the amount of current delivered to the heart. That is, the greater the impedance (defined as resistance from all sources, including resistance and reactance), the more the flow of current will be reduced.
To ensure that sufficient current passes through the heart, automated external defibrillators calculate the amount of patient impedance between electrode pads and adjust the amount of delivered energy (in joules) to push through that impedance and achieve the necessary flow of current.
“[One] joule equals one watt-second—i.e., the energy released in one second by a current of one ampere through a resistance of one ohm.”
How Many Joules Are Delivered with Each Shock to the Heart?
Generally, the factory presets for biphasic defibrillators (in contrast to monophasic defibrillators) range between 120 J and 200 J, starting at the lowest dose and increasing to the highest dose. Research shows that shocks up to 360 J using a biphasic waveform do not damage the heart more than lower energy levels, but lower doses of current are less likely to cause skin burns  and animal studies show better cardiac results post-recovery for lower-dose biphasic shocks .
For this reason—and because biphasic shocks appear to be more effective for treating AED shockable rhythms—every internal defibrillator and all modern external defibrillators use a biphasic waveform with shocks delivered in the lower range rather than a monophasic waveform with shocks ranging from 200 to 360 joules.
Shock Energy Sequence Examples
The Philips HeartStart FRx and Defibtech Lifeline—both of which use a biphasic truncated exponential (impedance compensated) waveform—deliver a nominal amount of energy equal to 150 J for adults with 50 Ω of impedance and 50 J nominal delivered energy for children with 50 Ω of impedance.
- 150 J for the first shock, 150 J for the second shock, and 200 J for the third shock in adults
- 50 J for the first shock, 50 J for the second shock, and 50 J for the third shock in children
While all three models come with pre-configured factory settings, they adjust the exact energy delivered depending on the patient impedance.
For example, for an adult with the lowest impedance of 25 Ω, the Philips HeartStart FRx will deliver an attenuated shock of 128 J. In contrast, it will deliver a higher shock of 158 J if the patient impedance is 180 Ω. The pediatric doses are likewise optimized for impedance between 43.4 and 52.4 joules.
Impedance to defibrillation can be affected by things like excessive chest hair, body tissues, and issues with the cables of the AED itself. To provide the easiest pathway for the current to pass through, electrode pads come with a conductive adhesive backing that gives the electricity a direct path through the skin to the heart. That’s why it’s important not to use expired pads, on which the adhesive gel may have dried out. Wet skin should also be dried prior to pad placement and any excess chest hair should be shaved off.
How Long Each Shock Lasts
What may seem like a large jolt (and it is!) is actually delivered in around one-hundredth of a second with a minuscule pause between phase one and phase two.
The length of each phase depends on the patient impedance, with a minimum duration of around 3 milliseconds and a maximum duration of around 17 milliseconds for adults and times ranging from 7 to 11 milliseconds in children. When using the HeartSine Samaritan PAD 350P, the interphase pause is a constant 0.4 milliseconds for all patient impedances.
Battery Voltage, Shocks Delivered, and Maximum Operating Time
Today, most automated external defibrillators use compact 9-volt batteries, making them lightweight and small enough to carry to a victim in minutes.
The Philips HeartStart FRx, for example, uses a 9-volt, direct-current sealed lithium manganese dioxide battery that—when new—has the capacity to deliver 200 shocks or last for four hours of operating time at 25°C (77°F).
The HeartSine Samaritan PAD 350P uses an 18-volt combined lithium manganese dioxide battery and electrode cartridge that can deliver more than 60 shocks or six hours of continuous monitoring when new or more than 10 shocks when the battery is four years old.
The Defibtech Lifeline DCF-100 comes with one 9-volt lithium battery that is used for defibrillator self-checks. However, two higher capacity lithium/manganese dioxide battery packs are also available for purchase:
- The DBP-1400 battery pack has 15 volts and 1400mAh of power, a capacity of 125 shocks or eight hours of continuous operation, and a standby life of five years.
- The DBP-2800 battery pack has 15 volts and 2800mAh of power, a capacity of 300 shocks or 16 hours of continuous operation, and a standby life of seven years.
AEDs have certainly come a long way from the 250-pound “portable defibrillators” of the 1950s.
A Note About Manual Defibrillator Shock Energy…
Most modern manual defibrillators, much like automated external defibrillators, use biphasic defibrillation with shock delivery measured in joules. The main difference between manual defibrillators and AEDs lies in the user’s ability to set each shock level manually (hence “manual” defibrillator) rather than the machine calculating the energy of each shock.
Because manual defibrillators don’t deliver a pre-set shock, these devices are more appropriate for babies who need a lower dose than even pediatric pads usually provide and must only be used by professionals who are trained in advanced cardiac life support, such as doctors, paramedics, and EMS personnel.
How Defibrillators Deliver an Electrical Shock
Automated defibrillation involves a basic three-step process.
Step 1: Cardiac Rhythm Analysis
When electrode pads are attached to the bare skin of a person experiencing sudden cardiac arrest (using correct AED pad placement), the defibrillator analyzes the electrical activity of the heart to see whether a shockable rhythm is present. It’s essential not to touch the patient while the defibrillator analyzes the heart, as this could interfere with the analysis.
The two AED shockable rhythms are:
- Ventricular fibrillation (v-fib)
- Pulseless ventricular tachycardia (v-tach)
Both of these rhythms can be reset with an electrical shock because there is electrical activity, but it’s chaotic. When the defibrillator delivers an electrical shock to the heart, this shock shuts the erratic firing down so that the heart’s electrical activity can restart with a normal rhythm.
Other heart rhythms aren’t shockable, including asystole—in which there is no electrical activity at all—and pulseless electrical activity or PEA—in which the electrical activity is organized (not erratic) but there is no pulse. As neither of these rhythms involves erratic electrical activity, a “reset” shock won’t help.
Step 2: Shock Advised or Shock Not Advised
After analyzing the heart, the AED will say whether or not a shock is advised. If a shock is advised, the device will draw energy from the battery and store it ready to deliver the first shock. Some devices will give a voice or visual indication of “charging” status. It generally takes up to eight seconds for the capacitor to charge for a 150-joule shock and up to 12 seconds to charge the capacitor for a 200-joule shock.
Once the charge for the shock is stored, the device will either instruct bystanders to “stand clear” and deliver the shock automatically (for fully automatic external defibrillators) or say “stand clear of patient—press the shock button now” (for semi-automatic external defibrillators). If no shock is given within 30 seconds or the patient’s heart rhythm changes to a non-shockable rhythm, the stored energy is removed from the capacitor.
Step 3: Resume CPR
After the shock to the heart is delivered, the AED will tell you “it is safe to touch the patient” and provide voice instructions for performing cardiopulmonary resuscitation. If you are trained, you can provide high-quality CPR at a ratio of 30 compressions to two rescue breaths. If you are not trained, perform hands-only CPR.
Every two minutes, the defibrillation device will analyze the patient’s heart again and say whether a shock is advised. Even if no AED shock is advised, it’s essential to leave the electrode pads connected to the patient’s chest until emergency medical services arrive so that the device can continue to analyze the cardiac rhythm and deliver a shock if the heart changes to a shockable rhythm.
Importance of the Voltage of a Defibrillator for Lay Rescuers and AED Program Managers
Knowing how voltage, current, and energy delivered work to defibrillate the heart in the case of sudden cardiac arrest, there are a few practical things that AED program managers and lay rescuers should keep in mind:
- Do not touch the patient when the AED says “stand clear”! As the human body acts as a conductor, touching the patient during analysis or shock delivery could make the shock ineffective or shock the person touching the patient. The same applies to touching something (especially metal) that’s touching the patient.
- Remove medicated patches before placing the pads. Medicated adhesive bandages, such as nicotine patches, can impede the flow of current to the patient’s heart, raising the level of energy delivered and potentially causing skin burns. Before placing the pads, remove any patches on the skin (using gloves) and carefully wipe the area clean.
- Don’t place the pads directly over a pacemaker or implantable cardioverter defibrillator. A pacemaker or implantable cardioverter defibrillator (ICD) will impede the flow of electricity to the heart and could be damaged by the shock as well. Place the electrode pads a few inches away from an implanted device or use an alternate AED pad placement.
- Shave excessive chest hair. Excessive chest hair can prevent the electrode pads from sticking cleanly to the patient’s skin, creating more impedance and reducing the flow of current. To prevent ineffective shocks and burns, quickly shave excess chest air off before placing the pads. Most AEDs come with a razor for this purpose.
Electricity—When Properly Delivered—Can Reverse Common Arrhythmias of Cardiac Arrest
Whether you measure the power of a shock to the heart in volts and amperes or joules, the significant jolts provided by an AED can stop a lethal cardiac rhythm and triple an SCA victim’s chance of survival.
While defibrillation doesn’t work for everyone (some causes of SCA will not respond, no matter how amazing the technology) and every patient will need follow-up with high-level techniques and hospital care, early defibrillation combined with high-quality CPR gives SCA victims the best chance of coming home from the hospital alive.
- Skin Reactions Following Defibrillation or Cardioversion; Physio-Control White Paper; 1995
- The effects of biphasic and conventional monophasic defibrillation on postresuscitation myocardial function; Journal of the American College of Cardiology; September 1999