How Many Joules Are Needed to Defibrillate a Heart?

For a standard adult in cardiac arrest, external defibrillators typically deliver between 120 and 360 joules per shock, depending on the device and the clinical protocol. But that range is only part of the story. The energy required to restart a heart varies enormously based on whether the chest is open or closed, whether the patient is a child or an adult, whether the device is strapped to the outside of the body or implanted inside it, and even how much body fat sits between the electrodes and the heart. What looks like a simple number turns out to be a decision tree shaped by anatomy, waveform physics, and the specific rhythm the heart has fallen into.

Standard Energy Levels for Adult External Defibrillation

Most modern external defibrillators used in hospitals and by paramedics deliver biphasic waveform shocks. For ventricular fibrillation, the standard starting dose is around 200 joules. If that first shock fails, European and American guidelines generally call for energy escalation on subsequent attempts, stepping up to 300 joules and then 360 joules.¹ Some regional protocols use a fixed-energy approach, delivering 200 joules for every shock, while others escalate. A recent study compared escalating higher-energy defibrillation (200-300-360 J) against fixed low-energy defibrillation (200-200-200 J), reflecting the real-world split in how emergency medical systems approach the problem.

Older monophasic defibrillators, which send current in only one direction, required higher starting energies. The American Heart Association’s Advanced Cardiac Life Support guidelines historically recommended starting at 360 joules for monophasic shocks when treating ventricular fibrillation, compared to a default of 200 joules for biphasic devices when the specific waveform type was unknown.² Monophasic machines have largely been phased out of frontline use, but you still encounter them in older facilities and resource-limited settings.

Why Biphasic Waveforms Need Less Energy

The shift from monophasic to biphasic defibrillation was one of the most meaningful advances in resuscitation technology. A biphasic waveform sends current in one direction and then reverses it, which turns out to be much more efficient at resetting the chaotic electrical activity of a fibrillating heart. In a study of internal defibrillation during cardiac surgery, patients shocked with a biphasic waveform needed significantly fewer shocks and less total energy than those treated with monophasic shocks. The biphasic group averaged about 8 joules of maximum energy per patient compared to roughly 12 joules for the monophasic group, and the cumulative energy delivered was about half.³

Animal research reinforces this. In one study, all animals treated with biphasic waveform defibrillation were successfully resuscitated after both four and seven minutes of untreated ventricular fibrillation, while monophasic defibrillation achieved four out of five successful resuscitations in each time group. The monophasic groups also required higher delivered energies, with the difference becoming statistically significant after seven minutes of fibrillation.⁴ The practical upshot: biphasic waveforms defibrillate more reliably at lower energy, which matters because excessive energy comes with its own costs.

Open-Chest Defibrillation During Surgery

When paddles are placed directly on the surface of an exposed heart during cardiac surgery, the energy requirements drop dramatically. With no ribcage, lung tissue, or skin in the way, the electrical current reaches the heart with very little resistance. A landmark study of 202 patients undergoing open-chest defibrillation found that a first shock of 20 joules successfully defibrillated 80 percent of hearts, 10 joules defibrillated 70 percent, and 5 joules defibrillated only 56 percent. The researchers concluded that the optimal initial energy for internal defibrillation is 10 to 20 joules, and that repeating that dose would defibrillate over 90 percent of hearts.⁵ More recent guidelines and reviews have placed the range slightly wider, at 10 to 30 joules.⁶

That tenfold difference between open-chest and closed-chest defibrillation reveals something important: most of the energy from an external shock never reaches the heart. It dissipates through skin, fat, muscle, bone, and air-filled lungs. The joule number on the machine dial is not the joule number the heart muscle actually sees.

Pediatric Defibrillation Is Weight-Based

Children are not small adults, and their defibrillation doses reflect that. The American Heart Association recommends a first defibrillation energy of 2 joules per kilogram of body weight for ventricular fibrillation or pulseless ventricular tachycardia, escalating to 4 joules per kilogram for subsequent shocks.⁷ For a 20 kg child, that means starting at about 40 joules, far less than the 200 joules an adult receives. A study of pediatric in-hospital cardiac arrests found that survival to hospital discharge was significantly lower when the initial energy dose strayed from the recommended 2 J/kg range (defined as 1.7 to 2.5 J/kg), supporting the current guidelines.⁸

The evidence base behind pediatric defibrillation energy is thinner than many clinicians would like. The original recommendations were extrapolated from animal data, adult findings, and a single small retrospective study. That study used a broad dose range of 2 J/kg plus or minus 10 joules, which meant a much wider relative spread for smaller infants than for adolescents.⁷ Pediatric cardiac arrest is rare enough that conducting large randomized trials is extremely difficult, so clinicians still work somewhat in the dark on ideal energy dosing for very young patients.

Implantable and Wearable Defibrillators

Implantable cardioverter-defibrillators (ICDs) sit inside the body and deliver shocks at much lower energies than external devices. A traditional transvenous ICD, with a lead threaded through a vein into the heart, might deliver a first shock at 24 joules and subsequent shocks at around 37 joules of delivered energy.⁹ The newer subcutaneous ICD (S-ICD), which avoids placing leads inside the heart, delivers its shocks at a higher energy of 80 joules because the current still has to travel through tissue to reach the myocardium from outside the ribcage.

Researchers are working to bring S-ICD energy requirements down. One approach uses dual-electrode configurations to lower the electrical resistance the shock encounters. In testing, these configurations achieved shock success rates at 40 joules of 97 percent (parallel configuration) and 90 percent (transverse configuration), compared to the 80-joule standard.¹⁰ Lowering the energy per shock could extend battery life and reduce myocardial damage over a device’s lifetime.

Wearable cardioverter-defibrillators (WCDs), which are external vests worn by patients at temporary risk, operate in yet another energy range. A contemporary WCD waveform is designed to deliver 170 joules into a standard test load, using a biphasic truncated exponential waveform.¹¹ A sobering case report highlights the consequences when wearable devices deliver too little energy: a patient wearing an older WCD model that maxed out at just 6 joules received five shocks during ventricular fibrillation, none of which terminated the arrhythmia, resulting in death.¹² Energy that is too low simply does not work.

Body Size and Chest Impedance

The same shock setting on the same machine can deliver very different amounts of current to the heart depending on the patient’s body. The key variable is transthoracic impedance, the electrical resistance the shock meets as it passes through the chest wall. In patients with obesity or a large chest diameter, impedance rises because fatty tissue insulates the heart and the electrodes sit farther apart. Research consistently shows that as BMI increases, chest impedance increases along with it.¹³

One practical workaround: pushing the anterior electrode firmly into the chest wall during the shock. This compresses the tissue, shortens the distance between the electrode and the heart, and can meaningfully reduce impedance. With less resistance, the same energy setting delivers more current to the myocardium, improving the odds of success without dialing up the joules.¹⁴ This technique is simple enough to use during resuscitation and is worth knowing about for anyone trained in emergency care.

When Standard Shocks Fail

Roughly one in ten patients in ventricular fibrillation will not respond to three or more standard shocks. This condition, called refractory ventricular fibrillation, used to be nearly always fatal. Two alternative defibrillation strategies have emerged that change the geometry of the shock rather than just cranking up the energy.

Double sequential external defibrillation (DSED) uses two defibrillators simultaneously, delivering rapid back-to-back shocks through different vectors. Vector-change (VC) defibrillation switches the pad placement from the standard anterior-lateral position to anterior-posterior. A landmark trial published in the New England Journal of Medicine found that survival to hospital discharge was about 30 percent in the DSED group compared to 13 percent in the standard-defibrillation group. The vector-change group landed in between, at about 22 percent. DSED was also associated with better neurological outcomes.¹⁵ These approaches are increasingly being adopted into paramedic protocols for cases where standard shocks at maximum energy have failed repeatedly.

Where You Place the Pads Matters

Pad placement affects how much of the shock current actually passes through the heart. The two standard positions are anterior-lateral (one pad on the upper right chest, one on the left side) and anterior-posterior (one on the front of the chest, one on the back). For cardioverting atrial fibrillation specifically, a randomized trial found that the anterior-posterior position was more effective at the first-shock level. Roughly 33 percent of patients in the anterior-posterior group were cardioverted at the initial low energy, compared to 54 percent who failed at that energy in the anterior-lateral group.¹⁶

A systematic review of pad positioning for basic life support defibrillation found that switching to the anterior-posterior position during refractory ventricular fibrillation was associated with higher rates of fibrillation termination compared to the standard anterior-lateral placement, though the evidence was rated as very low certainty.¹⁷ The bottom line is that pad position can sometimes substitute for raw energy. A well-placed shock at lower joules may outperform a poorly placed shock at higher joules, because more of the current reaches the heart.

The Cost of Too Much Energy

Defibrillation is not free to the heart muscle. The shock itself can cause damage, which is why clinicians do not simply dial every defibrillator to its maximum setting and fire away. The severity of post-resuscitation heart dysfunction is directly related to the energy of the delivered shock. Higher-energy shocks cause greater impairment of the heart’s pumping ability in the critical period after resuscitation.¹⁸

This damage appears to be heavily influenced by blood flow. Research has shown that shocks delivered to hearts with reduced perfusion, as you would expect during cardiac arrest, cause significant impairment in myocardial function that worsens with increasing energy. In contrast, normally perfused hearts tolerate the same shocks with no measurable damage.¹⁹ This is one reason why high-quality CPR, which maintains some blood flow to the heart, matters so much alongside defibrillation. It is also why the original open-chest defibrillation study warned that doses larger than 20 joules, particularly if repeated, could cause tissue death in the heart.⁵

Even with implantable devices, where shock energies are much lower, defibrillation produces measurable biological effects. A prospective trial found that ICD shocks caused a small but statistically significant increase in a biomarker of cellular death (sFas) over 24 hours compared to patients whose devices were implanted without shock testing.²⁰ Another prospective randomized trial showed that ICD shock testing itself, not the brief episode of induced fibrillation, was responsible for elevations in high-sensitivity troponin, a marker of heart muscle damage.²¹ By contrast, subcutaneous ICDs at 80 joules showed no significant increase in cardiac injury biomarkers after shock testing.²² The overall takeaway is that less energy, if it gets the job done, is better for the heart.

How Defibrillation Actually Works at the Cellular Level

The goal of a defibrillation shock is not to “restart” a stopped heart in the way most people imagine. During ventricular fibrillation, the heart is not stopped at all. Its muscle cells are firing chaotically, with disorganized electrical waves circling through the tissue so that no coordinated contraction occurs. The purpose of the shock is to simultaneously depolarize enough of the heart muscle to extinguish those chaotic wavefronts. Research using isolated heart tissue has shown that the shock must depolarize refractory myocardium, cells that have recently fired and are in their recovery period, to a high enough level to prevent any residual electrical wave from continuing to propagate.²³ Once all those rogue waves are snuffed out, the heart’s natural pacemaker cells can reassert control and resume a normal rhythm.

This is why higher energy generally improves success: a stronger electrical field reaches more cells, including those deep in the tissue or at the margins of the shock field. But beyond a certain point, additional energy no longer improves depolarization coverage and only adds damage.

Atrial Fibrillation Requires Different Energy

Not all defibrillation targets ventricular rhythms. Atrial fibrillation, the most common sustained arrhythmia in adults, can also be cardioverted with a shock, though the energy levels and the urgency differ substantially. For external cardioversion of atrial fibrillation, guidelines recommend starting at 200 joules with either monophasic or biphasic waveforms, particularly for long-duration atrial fibrillation.²

When atrial fibrillation is cardioverted internally, using catheters placed inside the heart, the energy drops radically. A study of internal cardioversion found a mean defibrillation threshold of about 7.6 joules, with a range from as low as 0.5 joules to 25 joules.²⁴ That huge range highlights an underappreciated factor: the electrical characteristics of the fibrillation itself. The study found a strong positive correlation between the frequency of the fibrillatory waves and the energy required to terminate them. Faster, more disorganized atrial fibrillation needs more energy to break.

Low-Energy Approaches on the Horizon

One of the more intriguing research directions involves multistage defibrillation, delivering a sequence of different types of electrical stimuli rather than a single high-energy blast. In a canine model of atrial fibrillation, researchers tested a three-stage protocol: a few low-energy biphasic shocks, followed by a series of ultralow-energy monophasic shocks, followed by antitachycardia pacing. This three-stage approach lowered the defibrillation threshold dramatically, to about 0.19 joules compared to roughly 0.95 joules for the low-energy shocks alone.²⁵

If these findings translate to humans, the implications would be significant for patients with implantable devices. ICD shocks are painful, and many patients develop anxiety around them. A therapy that could terminate arrhythmias at fractions of a joule, below the pain threshold, would fundamentally change what it feels like to live with one of these devices. The research is still at the animal-model stage for this particular approach, but it illustrates the broader principle driving the field: the goal is not to find the biggest jolt that works, but the smallest one that still does the job reliably.