Standard adult external defibrillation starts at 120 to 200 joules depending on the device, with modern biphasic defibrillators sitting at the lower end of that range and older monophasic machines requiring more. But that number is just the opening bid. The energy needed for any given patient depends on the type of device, the waveform it delivers, where the pads are placed, how resistant the chest is to electrical flow, whether the heart is being shocked from the outside or directly during surgery, and whether the first shock even works. What looks like a simple number on a defibrillator dial conceals a surprisingly layered set of decisions.
Starting Energy for Adults
Current resuscitation guidelines recommend that the first biphasic shock be no lower than 120 joules for rectilinear biphasic waveforms and no lower than 150 joules for biphasic truncated exponential waveforms. Most automated external defibrillators (AEDs) used by the public and emergency services deliver a first shock somewhere in this range, typically 150 to 200 joules. If the first shock fails, the energy is either held steady or escalated, depending on the protocol and the manufacturer’s design. Maximum settings on external defibrillators usually top out at 360 joules.
A key detail that often gets overlooked: the joule setting on the machine is a measure of stored energy, not the energy that actually reaches the heart. Some of that energy is lost as heat in the device’s circuitry and across the chest wall. What matters biologically is the current flowing through the heart muscle, and that current depends heavily on how much the chest resists the electrical flow, a property called transthoracic impedance. For a given energy setting, a patient with high impedance receives less current, which can make the shock less effective.
Why Biphasic Devices Need Fewer Joules
Older defibrillators delivered a monophasic waveform, meaning current flowed in one direction only. Biphasic devices reverse the current partway through the pulse, sending it first one way and then the other. This two-phase approach turns out to be substantially more efficient at stopping chaotic heart rhythms. In one early human trial, the stored energy needed to defibrillate was about 26% lower with biphasic pulses compared to monophasic ones, averaging roughly 6 joules versus about 9 joules for internal (implanted-lead) shocks.1Journal of the American College of Cardiology. A prospective randomized evaluation of biphasic versus monophasic waveform pulses on defibrillation efficacy in humans A separate animal study found that a single-capacitor biphasic waveform required a mean of only about 6 joules to defibrillate, compared with roughly 17 to 18 joules for monophasic or double-capacitor biphasic waveforms of the same duration.2Journal of the American College of Cardiology. Comparison of the internal defibrillation thresholds for monophasic and double and single capacitor biphasic waveforms
For external (through-the-chest) defibrillation in real cardiac arrests, the advantage is consistent though harder to quantify precisely. A Cochrane review of out-of-hospital cardiac arrest found that the first-shock failure rate trended lower with biphasic devices, though the difference did not quite reach statistical significance in the pooled data.3PubMed Central. Biphasic versus monophasic waveforms for transthoracic defibrillation in out‐of‐hospital cardiac arrest In practice, the shift to biphasic technology has been nearly universal, and monophasic devices are rarely encountered in clinical settings today. Biphasic waveforms also handle variation in chest impedance more gracefully, meaning the same energy setting remains effective across a wider range of body types.4PubMed Central. A Systematic Review of the Transthoracic Impedance during Cardiac Defibrillation
When the First Shock Fails
A single 150-joule biphasic shock successfully terminates initial ventricular fibrillation in roughly nine out of ten patients.5PubMed. Low-energy impedance-compensating biphasic waveforms terminate ventricular fibrillation at high rates in victims of out-of-hospital cardiac arrest That sounds reassuring until you consider what happens to the remaining patients, and the fact that “terminating” fibrillation transiently does not always mean the heart stays in an organized rhythm. One large study found that while roughly 94% of initial fibrillation episodes were temporarily terminated by a 200-joule first shock, only about half remained terminated by the time of the next rhythm check.6PubMed Central. Escalating vs Fixed Energy Defibrillation in Out-of-Hospital Cardiac Arrest Ventricular Fibrillation
This is where the escalating-versus-fixed energy debate comes in. Some protocols keep every shock at the same level, while others ramp up the energy with each attempt. Evidence from an ischemic animal model showed that escalating from 150 joules up to 200 or 360 joules rescued the majority of animals that could not be defibrillated at the lower fixed dose.7Academic Emergency Medicine. A Model of Ischemically Induced Ventricular Fillbrillation for Comparison of Fixed-dose and Escalating-dose Defibrillation Strategies In human studies, the picture is mixed. The same study that tracked transient termination found that escalating higher-energy regimens were more likely to produce a sustained organized rhythm, with the advantage growing more pronounced for refractory fibrillation requiring a third shock or more.6PubMed Central. Escalating vs Fixed Energy Defibrillation in Out-of-Hospital Cardiac Arrest Ventricular Fibrillation However, a separate retrospective comparison of escalating energy (200 to 360 joules) versus fixed high energy (360 joules for every shock) found no difference in survival to hospital discharge, with both groups at about 28%.8Resuscitation. Survival to hospital discharge with biphasic fixed 360 joules versus 200 escalating to 360 joules defibrillation strategies in out-of-hospital cardiac arrest of presumed cardiac etiology
In short, escalating energy seems to help terminate stubborn rhythms in the moment, but whether that translates into more patients walking out of the hospital alive remains unclear. A large multi-agency study found that defibrillation dose on a per-kilogram basis was not independently associated with survival.
Refractory Fibrillation and Double Sequential Shocks
Some patients remain in ventricular fibrillation despite multiple standard shocks. For these refractory cases, clinicians have experimented with techniques that go beyond simply turning up the dial. Two approaches have gained attention: vector-change defibrillation, which involves moving the pads to a different position (typically from the standard front-and-side placement to a front-and-back arrangement), and double sequential external defibrillation, which uses two sets of pads and delivers shocks in rapid succession from two different vectors.
A landmark trial published in the New England Journal of Medicine randomized patients with refractory ventricular fibrillation to standard defibrillation, vector-change defibrillation, or double sequential external defibrillation. Survival to hospital discharge was about 30% in the double sequential group compared to roughly 13% with standard shocks. The vector-change group fell in between at about 22%.9PubMed Central. Pad size, orientation, and placement for defibrillation during basic life support: A systematic review Double sequential defibrillation was also the only technique associated with better neurological outcomes compared to standard care.6PubMed Central. Escalating vs Fixed Energy Defibrillation in Out-of-Hospital Cardiac Arrest Ventricular Fibrillation One analysis of this data found that the front-and-back pad position itself, rather than the amount of current delivered, was independently associated with return of a pulse, suggesting the geometry of how the shock field passes through the heart matters as much as raw energy.10Resuscitation. The impact of defibrillation current and pad position on return of spontaneous circulation during refractory ventricular fibrillation
Where You Place the Pads Changes the Equation
For routine defibrillation, not just refractory cases, pad placement makes a meaningful difference. The standard position places one pad on the upper right chest and the other on the lower left side. An alternative position puts one pad on the front of the chest (over the breastbone) and the other on the back. A large observational study of out-of-hospital cardiac arrest found that patients whose pads were placed in the front-and-back position had more than double the adjusted odds of regaining a pulse compared to those with standard front-and-side placement.11PubMed Central. Initial Defibrillator Pad Position and Outcomes for Shockable Out-of-Hospital Cardiac Arrest For cardioversion of atrial fibrillation (a less chaotic rhythm than ventricular fibrillation), the front-and-back position both improved success rates and reduced the total energy required, with mean energy needs dropping from about 450 joules to about 380 joules.12PubMed. External cardioversion of atrial fibrillation: role of paddle position on technical efficacy and energy requirements
The reason is straightforward: front-and-back placement sandwiches the heart more directly between the two electrodes, pushing current through more of the heart muscle. Front-and-side placement sends some of the current through lung tissue, which has high impedance and is less useful territory. Despite the evidence favoring front-and-back placement, front-and-side remains the default in most emergency settings, partly because placing a pad under a patient’s back is logistically harder during CPR.
Pediatric Defibrillation
Children are not small adults when it comes to defibrillation dosing. Rather than a fixed joule number, pediatric guidelines use a weight-based approach: the recommended first shock is 2 joules per kilogram of body weight. For a 20-kilogram child, that means a first shock of 40 joules, a fraction of what an adult receives.
A study of pediatric in-hospital cardiac arrest found that first energy doses outside the range of about 1.7 to 2.5 joules per kilogram were associated with lower survival in children aged 12 and under. Doses above 2.5 joules per kilogram were linked to lower survival in patients 18 and under with initial ventricular fibrillation.13PubMed. Improved survival to hospital discharge in pediatric in-hospital cardiac arrest using 2 Joules/kilogram as first defibrillation dose for initial pulseless ventricular arrhythmia A broader systematic review came to a slightly more cautious conclusion: outcomes were not significantly better or worse with initial doses below 1.5 or above 2.5 joules per kilogram compared to the standard 2 joules per kilogram, though the certainty of that evidence was very low.14PubMed Central. Energy doses for paediatric defibrillation in cardiac arrest: systematic review and meta-analysis Animal models suggest that the defibrillation threshold relative to body weight trends higher in infants, meaning the smallest patients may actually need proportionally more energy.15CJEM. LO08: Defibrillation energy dose during pediatric cardiac arrest: systematic review of human and animal model studies
If the first shock fails in a child, current guidelines recommend doubling to 4 joules per kilogram and escalating if needed, up to 10 joules per kilogram or the adult maximum. The practical challenge with pediatric defibrillation is that shockable rhythms are rare in children. Most pediatric cardiac arrests start with a non-shockable rhythm, so the opportunity to study defibrillation dosing in kids is inherently limited.
Internal and Open-Chest Defibrillation
When the heart is exposed during surgery, much less energy is needed because the shock bypasses the chest wall entirely. Paddles are placed directly on the heart, eliminating the impedance of bone, muscle, and lung tissue. The optimal energy for open-chest defibrillation has long been established at 10 to 20 joules, a range that successfully defibrillates more than 90% of hearts while avoiding tissue damage from excessive energy.16The American Journal of Cardiology. Open chest defibrillation during cardiac surgery: Energy and current requirements
Biphasic waveforms have pushed these thresholds even lower. A study comparing biphasic and monophasic shocks during open-heart surgery found that the biphasic threshold averaged about 7 joules versus 11 joules for monophasic, with the biphasic group requiring less cumulative energy and fewer total shocks.17Anesthesiology. Biphasic Shocks Compared with Monophasic Damped Sine Wave Shocks for Direct Ventricular Defibrillation during Open Heart Surgery Some surgical teams use a hybrid approach, placing one electrode directly on the heart and the other on the chest wall. One group reported successfully defibrillating 29 out of 32 patients with a single 30-joule hybrid shock during cardiac surgery.18PubMed Central. Combination of internal (epicardial) and external (transthoracic) defibrillation during heart surgery
Implantable Defibrillators
Implantable cardioverter-defibrillators (ICDs) deliver shocks internally through leads positioned inside or alongside the heart. Because the electrodes are so close to the cardiac tissue, the energy needed is dramatically lower than external defibrillation. Traditional transvenous ICDs, where the lead sits inside the heart, typically deliver shocks in the range of 25 to 40 joules.
Subcutaneous ICDs are a newer alternative that avoids placing leads inside the heart entirely. The electrode sits under the skin along the breastbone, which means the shock must travel through more tissue. At 40 joules, subcutaneous ICDs successfully terminated ventricular fibrillation in about 84% of patients during testing.19PubMed. Is 40 Joules Enough to Successfully Defibrillate With Subcutaneous Implantable Cardioverter-Defibrillators? That 16% failure rate has prompted research into what makes some patients harder to defibrillate with these devices. A computer modeling study found that the position of the generator and the amount of fat between the electrode and the heart had enormous effects. With optimal placement, the estimated threshold was around 22 to 29 joules. But moving the generator to the front of the chest instead of the side drove the threshold up to 135 joules, and adding a layer of fat over the electrode raised it further still. The worst-case combination of a poorly positioned generator and substantial subcutaneous fat yielded a modeled threshold of nearly 380 joules, far beyond what any implanted device can deliver.20PubMed. Determinants of Subcutaneous Implantable Cardioverter-Defibrillator Efficacy: A Computer Modeling Study
Several patient-level factors influence defibrillation thresholds for implanted devices. Larger body size, use of the anti-arrhythmic drug amiodarone, and an enlarged left ventricle have all been independently associated with higher thresholds.21The American Journal of Cardiology. Clinical Predictors of Transvenous Defibrillation Energy Requirements
Does Body Weight Change the Energy Needed for External Defibrillation?
Given how much body size matters for implanted devices, you might expect it to matter for external defibrillation too. The evidence says otherwise, at least within the range of body sizes seen in most emergency departments. An early prospective study found that shocks delivering roughly 194 joules (about 1.8 joules per kilogram) terminated 45 of 46 fibrillation episodes in patients weighing between about 90 and 225 kilograms. Weight did not determine success.22JAMA. Energy Levels and Patient Weight in Ventricular Defibrillation A more recent analysis of in-hospital cardiac arrest confirmed that body mass index did not interact with energy level in predicting first-shock success across weight categories.23Resuscitation. Body mass index and outcomes of in-hospital ventricular tachycardia and ventricular fibrillation arrest
The likely explanation is that standard adult defibrillation energy settings already provide a comfortable margin above the threshold for most people. Even in larger patients, the energy delivered at 200 joules is enough to generate adequate current through the heart. This is fundamentally different from the implanted-device situation, where the shock energy is much lower and closer to the threshold, making patient anatomy a bigger factor.
Can Too Many Joules Cause Harm?
Defibrillation is inherently a trade-off: too little energy fails to stop fibrillation, but excessive energy can damage the heart muscle. In the context of implanted devices, where researchers can measure cardiac injury markers after controlled shocks, there is evidence that the damage threshold is relatively low. One study found that blood markers of heart cell injury were significantly elevated two hours after a 15-joule internal shock but not after shocks of 10 joules or less.24Journal of Arrhythmia. The association between defibrillation shock energy and acute cardiac damage in patients with implantable cardioverter defibrillators For open-chest defibrillation, doses above 20 joules, especially when repeated multiple times, carry a risk of causing visible tissue damage.16The American Journal of Cardiology. Open chest defibrillation during cardiac surgery: Energy and current requirements
For external defibrillation in cardiac arrest, the calculus is different. The patient is already dead or dying. The risk of not delivering enough energy vastly outweighs the risk of myocardial injury from the shock itself. Still, accumulated unsuccessful shocks appear to take a toll. An animal study found that left ventricular function and survival after resuscitation correlated inversely with the cumulative burden of unsuccessful shocks, measured in joules per kilogram.25PubMed Central. Targeted Delivery of Electrical Shocks and Epinephrine, Guided by Ventricular Fibrillation Amplitude Spectral Area, Reduces Electrical and Adrenergic Myocardial Burden, Improving Survival in Swine The implication is not that any single shock is too dangerous, but that the goal should be making each shock as likely to succeed as possible, using optimal pad position and waveform, rather than relying on brute-force repetition.
Cardioversion Versus Defibrillation
Defibrillation targets ventricular fibrillation and pulseless ventricular tachycardia, both immediately life-threatening. Cardioversion is a related procedure used for less dire but still abnormal heart rhythms, like atrial fibrillation, and it delivers the shock synchronized to a specific point in the heart’s electrical cycle to avoid triggering worse rhythms. The energy requirements for cardioversion vary widely. For atrial fibrillation, which can be particularly stubborn, a study of synchronized cardioversion found a mean energy requirement of about 384 joules, reflecting the fact that many patients needed escalating attempts before their rhythm converted.26PubMed Central. Synchronised cardioversion for chronic atrail fibrillation Cardioversion for other rhythms like atrial flutter often succeeds at much lower energies, sometimes as little as 50 joules.
How Defibrillation Actually Works at the Cellular Level
The goal of a defibrillation shock is not to “restart” the heart in the way a reboot restarts a computer. The heart during fibrillation is not stopped; it is firing in hundreds of disorganized electrical wavelets. The shock works by simultaneously depolarizing enough of the heart muscle that all those chaotic wavelets are extinguished at once, giving the heart’s natural pacemaker cells a chance to resume coordinated rhythm. At higher energy levels, the shock can also punch temporary holes in cell membranes, a phenomenon called electroporation, which contributes to stopping the chaotic activity but also explains some of the tissue damage from excessive shocks.27PubMed Central. Mechanisms of defibrillation
Whether a shock succeeds depends on whether the electrical field it creates is strong enough throughout the entire heart. Areas of the heart that receive too weak a field can harbor surviving wavelets that immediately re-start fibrillation. This is why pad position matters so much and why the same number of joules can produce very different outcomes depending on how the electrical field is oriented relative to the heart’s anatomy. The joule number on the defibrillator is a necessary starting point, but it is only one variable in a much more complex equation that includes waveform shape, chest impedance, pad placement, and timing.