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Defibrillation

Defibrillation is a medical procedure that delivers a controlled electric shock to the heart through the chest wall to terminate life-threatening cardiac arrhythmias, such as ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT), thereby restoring a normal sinus rhythm.^[1] This intervention depolarizes the myocardium simultaneously, halting chaotic electrical activity and allowing the sinoatrial node to resume coordinated contractions.^[1] It is a cornerstone of advanced cardiac life support (ACLS) and is indicated exclusively for shockable rhythms during cardiac arrest, with no absolute contraindications, even in patients with pacemakers or implanted defibrillators.^[1]^[2] The history of defibrillation began with animal experiments by Jean-Louis Prévost and Frédéric Batelli in 1899, followed by the first successful human open-chest defibrillation by Claude Beck in 1947; closed-chest techniques emerged in the 1950s with Paul Zoll and William Kouwenhoven, and direct-current devices were pioneered by Bernard Lown in 1962.^[3]^[4] Modern defibrillation employs biphasic truncated exponential waveforms, which are more effective and require lower energy than older monophasic devices; the 2025 American Heart Association guidelines recommend an initial shock of 120–200 joules for adults, escalating as needed, with higher settings (≥200 J) preferable for cardioversion of atrial fibrillation.^[2]^[1]^[5] Types include manual defibrillators for healthcare professionals, AEDs for public and out-of-hospital settings, wearable cardioverter-defibrillators for high-risk patients, and implantable cardioverter-defibrillators (ICDs) for long-term prevention of sudden cardiac death.^[1] In pediatric cases, energy dosing starts at 2 joules per kilogram of body weight, increasing to a maximum of 10 joules per kilogram.^[1] For refractory VF, double sequential defibrillation—using two devices in rapid succession—may be considered, though evidence remains limited.^[2] The procedure's success is time-sensitive, with survival rates declining by approximately 10% per minute of delay in out-of-hospital cardiac arrest; overall survival is about 10.8% out-of-hospital and 25.5% in-hospital.^[1] Risks include skin burns, myocardial injury, or proarrhythmia if mistimed, but benefits far outweigh hazards in arrest scenarios.^[1] Early defibrillation, combined with high-quality CPR, remains the most effective intervention for improving outcomes in sudden cardiac arrest, which claims around 350,000 lives annually in the U.S.^[1]

Definition and Mechanism

Definition and Principles

Defibrillation is defined as the therapeutic delivery of an electrical shock through the heart to depolarize a critical mass of myocardial tissue, thereby terminating life-threatening arrhythmias such as ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT).^[6] This intervention aims to halt the disorganized electrical activity that prevents effective cardiac contraction, restoring the potential for normal sinus rhythm.^[7] In cardiac electrophysiology, defibrillation works by overriding the erratic electrical impulses characteristic of these arrhythmias through a high-energy shock that simultaneously depolarizes a large portion of the myocardium.^[8] VF and pulseless VT represent chaotic "electrical storms" in the ventricles, where uncoordinated wavefronts propagate randomly, leading to ineffective quivering or rapid contractions that fail to produce a pulse and can rapidly progress to cardiac arrest if untreated.^[6]^[7] The shock disrupts these reentrant circuits and wavefronts, creating a brief period of asystole that allows the sinoatrial node to reestablish organized electrical control and resume normal heart rhythm.^[8] A key principle underlying successful defibrillation is the alteration of transmembrane potential across myocardial cells, where the electric field induces depolarization (negative change in potential) and hyperpolarization, extinguishing excitable tissue and preventing reinitiation of arrhythmia.^[8] For efficacy, the shock must affect a critical mass of approximately 95% of the ventricular myocardium to ensure all fibrillation wavefronts are simultaneously rendered unexcitable, as smaller affected areas may allow residual activity to persist.^[8]^[9] This concept, first demonstrated in experimental models, highlights the need for sufficient shock strength to achieve uniform tissue quiescence.^[10]

Physiological Effects

Defibrillation shocks deliver a high-energy electrical current that induces a rapid change in transmembrane potential across cardiac cell membranes, leading to simultaneous depolarization of a critical mass of myocardial cells. This widespread depolarization extinguishes re-entrant circuits underlying ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT) by overriding disorganized electrical activity and allowing the sinoatrial node to resume control.^[11]^[12] Following the shock, the myocardium undergoes a period of recovery characterized by reperfusion, which restores oxygen delivery to ischemic tissue but can contribute to transient dysfunction. Successful defibrillation often results in restoration of sinus rhythm, though post-shock asystole or bradycardia may occur temporarily due to myocardial stunning, where contractile function is impaired without permanent necrosis. This stunning is exacerbated by ischemia-reperfusion injury and the shock itself, leading to reduced myocyte shortening and altered intracellular calcium dynamics, particularly with higher energy deliveries.^[13]^[1]^[14] The minimum effective energy dose for defibrillation varies, with external biphasic shocks typically requiring 120-200 joules initially to achieve success rates comparable to higher monophasic energies. Transthoracic impedance, averaging 70-80 ohms in adults, significantly influences current delivery and efficacy, as higher impedance reduces shock effectiveness and may necessitate energy escalation or techniques to lower it, such as firm electrode pressure. Patient body weight shows no clear correlation with required energy levels.^[15]^[1] Excessive shock energy can cause physiological complications, including myocardial injury manifested by troponin release and electroporation of cell membranes. Electroporation occurs at field strengths above 25 V/cm, creating transient pores that lead to diastolic depolarization, reduced action potential amplitude, and potential pro-arrhythmic effects, though it may also contribute to defibrillation success by extending refractoriness. High-energy shocks (e.g., 1-2 joules in experimental models) impair myocyte contractility and elevate biomarkers like troponin I, indicating subclinical damage that worsens with multiple or intense deliveries.^[16]^[14]^[17]

Clinical Use

Indications for Use

Defibrillation is primarily indicated for the treatment of shockable rhythms during cardiac arrest, specifically ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT), as these chaotic arrhythmias disrupt effective cardiac output and require electrical termination to restore organized rhythm.^[18] In hemodynamically unstable patients with non-perfusing tachycardias, such as pulseless VT, defibrillation serves as an urgent intervention to prevent deterioration into full arrest.^[5] According to the 2025 American Heart Association (AHA) Advanced Cardiovascular Life Support (ACLS) guidelines, defibrillation plays a central role in the cardiac arrest algorithm, with an immediate unsynchronized shock recommended upon identification of VF or pVT in witnessed arrests to maximize survival chances, followed by 2 minutes of high-quality CPR before rhythm reassessment.^[18] This integration emphasizes minimizing interruptions in chest compressions, with defibrillation delivered as soon as possible after CPR cycles in non-witnessed cases, and subsequent shocks using biphasic energies of 120-200 J or monophasic 360 J if needed.^[5] The European Resuscitation Council (ERC) 2025 guidelines align closely, advocating early defibrillation within the ALS algorithm for shockable rhythms alongside high-quality CPR with continuous compressions and asynchronous ventilations at 10 per minute when an advanced airway is in place.^[19] Defibrillation is contraindicated in non-shockable rhythms, including asystole and pulseless electrical activity (PEA), where electrical therapy offers no benefit and may delay other resuscitative efforts, as well as in stable, organized rhythms without hemodynamic instability.^[1] Presence of a pacemaker or implantable cardioverter-defibrillator does not preclude defibrillation, though pads should avoid direct placement over the device.^[1] In special populations, indications remain focused on VF and pVT but with tailored approaches. For patients with left ventricular assist devices (LVADs), defibrillation is indicated for VF/pVT, but prioritize assessing and attempting to restart LVAD function if a second rescuer is available, alongside standard shocks. For opioid-associated cardiac arrest, integrate naloxone administration (initial 2 mg IV/IO, repeat if needed) before or after shocks if overdose is suspected.^[20] For pediatrics, the AHA Pediatric Advanced Life Support (PALS) guidelines recommend defibrillation at lower energy doses—2 J/kg for the first shock, escalating to 4 J/kg and ≥4 J/kg thereafter (maximum 10 J/kg or adult dose)—integrated with age-appropriate CPR.^[21] During pregnancy, defibrillation is indicated without delay for shockable rhythms, with standard adult protocols applied alongside left uterine displacement to optimize CPR effectiveness and preparation for perimortem cesarean delivery if return of spontaneous circulation is not achieved within 5 minutes.^[22] In post-cardiac surgery scenarios, immediate defibrillation with up to three stacked shocks within 1 minute for VF or pVT is prioritized before initiating CPR or emergency resternotomy, to address reversible causes like graft issues while adhering to ACLS principles.^[20]

Procedure and Application

The procedure for defibrillation in clinical settings follows standardized advanced life support (ALS) protocols, such as those outlined by the American Heart Association (AHA), to ensure rapid and safe delivery during cardiac arrest or elective cardioversion.^[2] Preparation begins with confirming a shockable rhythm, such as ventricular fibrillation (VF) or pulseless ventricular tachycardia (pVT), through monitor analysis after a brief pause in high-quality cardiopulmonary resuscitation (CPR), typically every 2 minutes.^[1] The patient is positioned supine on a firm, flat surface to facilitate chest compressions and electrode application, while the team coordinates roles—one member continues compressions if possible, another prepares the defibrillator, and all communicate clearly to minimize interruptions exceeding 10 seconds.^[2] The chest is exposed by removing clothing, jewelry, and any transdermal patches to prevent arcing or burns; if the chest is wet, it is dried thoroughly to reduce transthoracic impedance.^[1] Self-adhesive defibrillation pads are applied to the bare chest in standard anterolateral positions—one over the right upper sternal border and the other on the left mid-axillary line at the level of the cardiac apex—or an anteroposterior configuration if needed for refractory cases.^[2] The defibrillator is charged to the initial energy level while CPR continues, with biphasic devices set to 120–200 joules (J) based on manufacturer recommendations, or 360 J for monophasic devices.^[1] For patients in cardiac arrest with VF or pVT, unsynchronized shocks are delivered immediately after confirming the rhythm and ensuring no one is touching the patient or bed.^[2] In contrast, for elective cardioversion of stable tachyarrhythmias like monomorphic VT with a pulse, synchronized shocks are used, timed to the R-wave on the ECG to avoid inducing VF, starting at lower energies such as 100–200 J depending on the arrhythmia.^[1] The operator announces "all clear" loudly, visually confirms team safety, and presses the shock button to deliver the energy.^[2] Defibrillation integrates seamlessly with basic life support (BLS) and ALS algorithms, where CPR resumes immediately after the shock for a full 2-minute cycle without pausing to check the rhythm or pulse, unless signs of return of spontaneous circulation (ROSC) are evident, such as rising end-tidal CO2.^[2] After the cycle, the rhythm is reassessed; if VF or pVT persists, subsequent shocks are delivered at equal or higher energy levels, with epinephrine administered every 3–5 minutes and amiodarone (300 mg IV/IO bolus) given after the third shock, followed by a 150 mg dose if needed.^[1] This sequence prioritizes minimizing peri-shock pauses to maintain coronary and cerebral perfusion.^[2] Safety protocols are paramount to protect rescuers and patients. Defibrillation is avoided in oxygen-enriched environments to prevent fire risks, with oxygen delivery tubes positioned at least 1 meter from the chest and ventilator exhaust directed away.^[23] Operators wear appropriate personal protective equipment, including insulated gloves for manual paddle use if required, to guard against arc flash or electrical leakage.^[1] In wet conditions, such as around pools or during rain, the procedure is deferred until the patient can be moved to a dry area, and all metallic objects on the patient or rescuers are removed to avoid conduction hazards.^[2]

Outcomes and Success Rates

The effectiveness of defibrillation is primarily measured by its ability to terminate shockable rhythms such as ventricular fibrillation (VF) and restore a perfusing rhythm. In clinical settings, biphasic waveform shocks achieve initial VF termination rates of 80-90% in out-of-hospital cardiac arrest (OHCA) cases when delivered early.^[24] For patients with initial shockable rhythms in OHCA, return of spontaneous circulation (ROSC) rates typically range from 30-50%, while survival to hospital discharge falls between 25-40%, depending on system-wide factors like rapid response.^[25] These outcomes highlight defibrillation's role as a cornerstone intervention, though overall OHCA survival remains modest at around 10% across all rhythms due to the minority presenting with shockable arrhythmias.^[26] Several factors significantly influence defibrillation success. The time from collapse to first shock is critical, with each additional minute of delay associated with a 6-10% reduction in the likelihood of successful rhythm termination and ROSC, emphasizing the need for immediate intervention.^[27] Bystander-initiated CPR prior to defibrillation enhances outcomes by preserving myocardial viability, mediating up to 50% of the survival benefit through improved VF waveform characteristics and higher ROSC rates (odds ratio 1.72).^[28] Additionally, biphasic waveforms outperform monophasic ones, yielding higher termination rates (e.g., 82.5% vs. 71.2% for first shocks) and better long-term survival when used at energies of 150-200 J.^[24] Recent advancements, including drone-delivered automated external defibrillators (AEDs), have shown promise in improving outcomes, particularly in rural areas where traditional response times are longer. Data from 2024-2025 registries and trials indicate that drone systems can deliver AEDs in under 5 minutes (median 4:47 minutes), arriving before emergency medical services in 67% of cases and potentially boosting survival by enabling earlier shocks in remote settings.^[29] The Cardiac Arrest Registry to Enhance Survival (CARES) 2024 report notes overall OHCA survival to discharge at 10.4%, with shockable rhythms benefiting most from such innovations, though widespread adoption requires further validation.^[30] Despite these successes, defibrillation has notable limitations in certain scenarios. In prolonged cardiac arrest exceeding 10 minutes, efficacy declines sharply, with ROSC probability dropping by up to 19% per minute of intra-arrest delay due to myocardial deterioration, often necessitating adjuncts like CPR or vasopressors.^[31] In hypothermic cardiac arrest, standard defibrillation alone yields lower success rates (under 50% ROSC), as cold-induced arrhythmias resist termination; extracorporeal membrane oxygenation (ECMO) outperforms conventional rewarming, achieving survival rates of 71% versus 29% in the cardiac arrest subgroup.^[32]

Types of Devices

Manual External Defibrillators

Manual external defibrillators are devices operated by trained healthcare professionals to deliver controlled electrical shocks for terminating life-threatening arrhythmias such as ventricular fibrillation or pulseless ventricular tachycardia. These units allow for precise control over the defibrillation process, enabling operators to select energy levels and timing based on patient needs and clinical judgment.^[33] Key design features include adjustable energy delivery, typically ranging from 2 J for pediatric patients up to 360 J for adults, which permits escalation as required during resuscitation efforts. They incorporate a synchronized cardioversion mode that times the shock to the patient's R-wave on the ECG to reduce the risk of inducing ventricular fibrillation in hemodynamically stable tachyarrhythmias. Integration with ECG monitoring is standard, providing real-time rhythm analysis directly on the device interface to guide treatment decisions.^[34]^[35]^[36] Operationally, these defibrillators require user-initiated controls for rhythm interpretation via the integrated ECG display and manual selection of shock delivery, ensuring shocks are administered only after confirming a shockable rhythm. They are commonly deployed in high-acuity environments such as emergency rooms, intensive care units, and ambulances, where skilled personnel can rapidly assess and intervene. These devices are compatible with self-adhesive electrodes for efficient application during emergencies.^[36]^[33] Manual external defibrillators offer high customization in energy selection and waveform delivery, allowing adaptation to diverse clinical scenarios, but their use demands significant operator expertise to avoid errors in rhythm recognition or timing. Modern models predominantly employ biphasic waveforms, which have been the clinical standard since the early 2000s due to their superior efficacy and ability to achieve defibrillation at lower energy levels compared to older monophasic designs, thereby reducing myocardial damage.^[35]^[37] Maintenance protocols emphasize regular calibration to verify energy output accuracy and battery checks to ensure operational readiness, with annual servicing recommended to include testing, reconditioning, and submission of tracking reports to manufacturers and the FDA as required under medical device regulations.^[38]

Automated External Defibrillators

Automated external defibrillators (AEDs) are portable devices designed for use by laypersons or minimally trained individuals in emergency situations, featuring integrated electrocardiogram (ECG) analysis algorithms that automatically detect shockable rhythms such as ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT). These algorithms process ECG signals from self-adhesive pads applied to the patient's chest, analyzing waveform characteristics like frequency and amplitude to determine if a defibrillatory shock is warranted, with sensitivity and specificity rates exceeding 95% for VF detection in validated databases.^[39]^[7] Upon detecting a shockable rhythm, the AED charges its capacitor and delivers a biphasic truncated exponential waveform shock, typically ranging from 150 to 360 joules for adults, while prompting the user to ensure safety before delivery.^[40] Many AED models incorporate pediatric modes to adapt for younger patients, reducing delivered energy to approximately 50-75 joules or 2-4 joules per kg for children under 8 years old or weighing less than 55 pounds (25 kg), often activated via a dedicated button or specialized pediatric pads that attenuate the adult dose. This mode ensures safer application during cardiac arrest in pediatric cases, where VF occurs in about 15-20% of out-of-hospital arrests, aligning with guidelines emphasizing early defibrillation to improve survival odds.^[41]^[42] Public access defibrillation programs have proliferated since the early 2000s, with over 4.5 million AED units deployed across the United States by 2024, strategically placed in high-traffic locations such as airports, schools, and sports venues to facilitate rapid access during out-of-hospital cardiac arrests. These initiatives, supported by legislation in all 50 states mandating AED placement in certain public buildings, have contributed to survival rates doubling when AEDs are used within the first few minutes of collapse. Integration with mobile applications, such as the PulsePoint AED registry, enables real-time location tracking via GPS, allowing emergency dispatchers to guide bystanders to the nearest device and alert nearby CPR-trained volunteers.^[43]^[44] The user interface of AEDs prioritizes simplicity for non-experts, providing step-by-step audio prompts that guide actions like turning on the device, applying pads in an anterior-lateral position, and delivering the shock if advised, often supplemented by visual indicators such as illuminated diagrams or flashing lights for clarity in low-light or noisy settings. Built-in self-testing features perform daily, weekly, and monthly checks on battery status, electrode integrity, and circuitry, alerting users via status indicators if maintenance is required, thereby ensuring device readiness without manual intervention.^[7]^[45] A key limitation of AEDs is their inability to deliver synchronized shocks for hemodynamically stable tachyarrhythmias, as they lack timing mechanisms to align the discharge with the R-wave, potentially risking ventricular fibrillation if used on organized rhythms like stable VT; in such cases, manual defibrillators are preferred. Recent advancements in the 2020s incorporate artificial intelligence enhancements, including machine learning algorithms for improved ECG signal processing that filter artifacts and noise from motion or environmental interference, enhancing rhythm detection accuracy during uninterrupted CPR without pausing for analysis.^[46]^[47]

Implantable Cardioverter-Defibrillators

Implantable cardioverter-defibrillators (ICDs) are surgically implanted devices designed to monitor heart rhythm continuously and deliver electrical therapy to prevent sudden cardiac death in high-risk patients. Unlike external defibrillators used for acute resuscitation, ICDs provide long-term prophylaxis by detecting life-threatening ventricular arrhythmias and responding automatically. These devices have evolved since the first human implantation in 1980, becoming a cornerstone of arrhythmia management.^[48] ICDs consist of key components including lead systems that sense cardiac signals and deliver therapy, a pulse generator with battery and capacitors, and sophisticated circuitry for arrhythmia detection. The leads, typically transvenous wires positioned in the heart chambers, transmit electrical impulses and monitor rhythms, while the battery powers the device for an average lifespan of 4-6 years, depending on therapy frequency. Modern ICDs employ tiered therapy algorithms, starting with antitachycardia pacing to terminate slower ventricular tachycardias non-invasively, escalating to low-energy cardioversion or high-energy shocks for ventricular fibrillation if needed; many also include bradycardia pacing to support slow heart rates.^[49]^[50]^[51] Indications for ICD implantation fall into primary and secondary prevention categories. Primary prevention is recommended for patients with heart failure and reduced ejection fraction (HFrEF, LVEF ≤35%) in New York Heart Association class II-III symptoms, or post-myocardial infarction with LVEF ≤30% and at least 40 days elapsed, to avert first-time sudden cardiac arrest. Secondary prevention applies to survivors of cardiac arrest due to ventricular fibrillation or hemodynamically unstable ventricular tachycardia, not attributable to reversible causes, or those with structural heart disease and syncope of suspected arrhythmic origin. These guidelines, updated in 2025, emphasize patient selection to balance benefits against procedural risks.^[52]^[53] The implantation procedure involves placing the pulse generator in a subcutaneous pocket, typically below the left clavicle, under local anesthesia and fluoroscopic guidance. Transvenous implantation, the traditional approach, inserts leads via the subclavian vein into the right ventricle and sometimes atrium for sensing and pacing; this method allows dual-chamber functionality but carries risks of vascular access complications. Subcutaneous implantation positions a single lead parallel to the sternum without entering the vascular system or heart, reducing lead-related infections and extraction challenges, though it lacks pacing capabilities in standard designs. Post-implantation, devices are programmed to patient-specific arrhythmia zones and therapies, with follow-up involving in-clinic interrogations; remote monitoring uses home transmitters that wirelessly upload data nightly via radiofrequency to central servers, enabling early detection of arrhythmias, battery depletion, or lead issues without frequent visits.^[54]^[54]^[55] By 2025, advancements include leadless ICDs, which eliminate transvenous leads entirely by integrating sensing and shocking capabilities into a fully intracardiac capsule delivered via femoral vein, significantly lowering infection risks associated with leads and pockets. Additionally, the extravascular ICD (EV-ICD), which places the lead outside the vascular system while maintaining defibrillation efficacy, and the OmniaSecure lead for pediatric applications were introduced, further reducing complications. MRI-conditional designs incorporate specialized circuitry and lead materials to minimize radiofrequency heating and induced currents, allowing safe 1.5T and 3T MRI scans under specific protocols, expanding imaging options for patients with comorbidities. These innovations enhance device longevity, reduce complications, and improve quality of life through better integration with diagnostic tools.^[56]^[57]^[58]

Electrode Systems

Paddle Electrodes

Paddle electrodes are handheld, reusable devices employed in manual external defibrillation, featuring circular metal plates with diameters typically ranging from 8 to 12 cm for adult use, mounted on insulated plastic handles equipped with discharge buttons that must be pressed simultaneously to deliver the shock. These electrodes connect to the defibrillator unit via durable, coiled cables, allowing the operator to maintain control during application. The design emphasizes direct contact with the patient's chest, requiring firm pressure—approximately 8 to 12 kg per paddle—to optimize electrical coupling and minimize transthoracic impedance.^[59]^[60]^[61] Prior to use, conductive gel must be applied liberally to the electrode surfaces to facilitate current flow and prevent arcing or thermal injury to the skin. These gels are predominantly saline-based electrolytes, such as those formulated with sodium chloride to reduce skin-electrode interface resistance from kilo-ohms to as low as tens of ohms, ensuring efficient energy delivery without excessive voltage requirements. Non-saline alternatives, like certain alpha-based pastes, have been used historically but are less common due to potential sparking risks if improperly applied; modern protocols prioritize gels that are water-soluble, bacteriostatic, and non-gritty for clean, reliable performance.^[62]^[61]^[63] Paddle electrodes find primary application in in-hospital settings, such as during advanced cardiovascular life support (ACLS) for rapid intervention in ventricular fibrillation or pulseless ventricular tachycardia, where trained personnel can quickly retrieve and deploy them from crash carts. Their design supports immediate use in controlled environments, often positioned in an apex-sternum configuration for optimal current path across the heart. Advantages include the ability to apply adjustable pressure for enhanced contact in challenging anatomies, potentially achieving lower and more consistent impedance compared to poorly adhered alternatives when gel is correctly used. However, disadvantages encompass higher variability in transthoracic impedance due to operator-dependent application, increased risk of patient skin burns from inadequate gel coverage or arcing, and potential operator exposure to the patient during shock delivery, which can interrupt chest compressions. Self-adhesive pads are preferred in modern protocols, including those from the European Resuscitation Council and American Heart Association, over paddle electrodes to improve safety and reduce CPR interruptions, though paddles remain available for specific in-hospital applications.^[64]^[60]^[65]

Self-Adhesive Electrodes

Self-adhesive electrodes, also known as disposable defibrillation pads, are pre-gelled, single-use devices designed for external defibrillation and cardioversion in both manual and automated systems. These pads facilitate rapid application and enable continuous electrocardiographic monitoring prior to shock delivery, making them a standard component in modern resuscitation protocols.^[64] These electrodes consist of multi-layer constructions optimized for conductivity and patient comfort. The backing is typically a flexible polyethylene foam layer that provides insulation and structural support, while the conductive element often incorporates silver/silver chloride (Ag/AgCl) or tin-coated materials to ensure low-offset voltage and reliable signal transmission. A key feature is the conductive hydrogel adhesive layer, which interfaces directly with the skin to enhance electrical contact and adhesion. Adult pads measure approximately 10-12 cm in diameter (or equivalent area of about 100-150 cm²), whereas pediatric versions are smaller, around 8-10 cm (or 70-90 cm²), to accommodate younger patients. Snap connectors at the edges allow secure attachment to defibrillator cables.^[66]80606-7/fulltext)^[67] Application involves a simple peel-and-stick process, where the pads are applied to the patient's chest for immediate monitoring and defibrillation readiness. This design reduces transthoracic impedance to an average of 70-100 ohms, facilitating efficient energy delivery without the need for additional conductive gel.^[68] Regulated by the FDA as Class II devices, self-adhesive electrodes are approved for single-use to ensure sterility and prevent cross-contamination. They have a shelf life of 2-5 years, depending on the manufacturer, and are compatible with biphasic waveforms commonly used in contemporary defibrillators.^[69]^[70] The primary advantages include hands-free operation, which minimizes interruptions in chest compressions during resuscitation and reduces rescuer exposure to electrical hazards. Additionally, their disposable nature lowers infection risk in scenarios requiring prolonged monitoring or repeated interventions, as opposed to reusable alternatives.^[64]^[71]^[72]

Placement Techniques

Proper electrode placement is crucial in defibrillation to ensure optimal delivery of electrical current through the heart, maximizing the likelihood of successful termination of arrhythmias such as ventricular fibrillation. The goal is to position the electrodes so that the current vector traverses a sufficient mass of the myocardium, particularly the ventricles, to achieve simultaneous depolarization. As per the 2025 American Heart Association and European Resuscitation Council guidelines, self-adhesive pads are recommended to minimize CPR interruptions, with placement techniques aimed at optimizing myocardial current flow.^[1]^[73]^[74] The standard anterior-lateral (antero-apical) position for external defibrillation involves placing one electrode pad immediately to the right of the upper sternum, below the clavicle, and the second pad on the left side of the chest in the mid-axillary line, approximately at the level of the cardiac apex (fifth intercostal space). This configuration directs the current flow from the right shoulder region across the heart toward the left lateral chest, effectively encompassing the ventricular myocardium. An alternative anterior-posterior position places one pad on the anterior chest (typically over the left mid-axillary line or apex) and the other on the posterior thorax (below the left scapula or mid-back), which is often used for implantable cardioverter-defibrillators (ICDs) during testing or in scenarios where anterior-lateral access is limited.^[75]^[1] The rationale for these positions stems from the need to minimize transthoracic impedance and align the electrical vector parallel to the heart's long axis, thereby reducing the defibrillation threshold—the minimum energy required for successful defibrillation. In the anterior-lateral setup, the vector approximates the heart's anatomical orientation, promoting uniform current distribution across the ventricles. The anterior-posterior approach can further optimize flow in cases where anterior tissue attenuates the current, as it bypasses superficial chest wall resistance.^[75]^[76] Adjustments to placement are necessary for specific patient characteristics to maintain efficacy. In obese individuals or those with large breasts, the anterior-lateral pads may be shifted laterally or under the breast tissue to improve contact and reduce impedance from adipose layers; alternatively, the anterior-posterior position is preferred to avoid excessive anterior tissue. For pediatric patients weighing more than 25 kg or older than 8 years, either anterior-lateral or anterior-posterior positions are suitable, compatible with automated external defibrillators (AEDs). In infants under 1 year or weighing less than 10 kg, where pads may overlap in the anterior-lateral position, an anterior-posterior configuration is recommended, with one pad on the center of the chest and the other on the center of the back, as posterior placement is practical due to the child's size.^[75]^[77]^[78]^[79] Verification of placement involves ensuring firm adhesion of self-adhesive pads to the bare chest, removing any excessive hair, moisture, or medicaments that could increase impedance, and confirming at least 8-10 cm distance from implanted pacemakers or ICD generators to prevent device damage or interference. Prior to shock delivery, the ECG rhythm is assessed via the defibrillator's leads to verify a shockable rhythm, indirectly confirming adequate electrode contact through signal quality.^[75]^[1] Clinical evidence supports the impact of optimal positioning on outcomes, though guidelines consider both major configurations reasonable due to historical equivalence in success rates across multiple studies. However, a 2024 cohort study of out-of-hospital cardiac arrests found anterior-posterior placement associated with a 2.64-fold higher adjusted odds of return of spontaneous circulation (ROSC) compared to anterior-lateral (95% CI 1.50-4.65), suggesting up to a 17% relative improvement in initial resuscitation success. In a porcine pediatric model, anterior-posterior placement reduced the 50% defibrillation threshold by approximately 20% compared to anterior-lateral, highlighting potential energy savings and efficacy gains in vulnerable populations. These findings underscore the value of tailoring placement to anatomical factors for enhanced defibrillation performance.^[75]^[80]^[76]

Historical Development

Early Methods and Closed-Chest Techniques

The origins of defibrillation trace back to the late 19th century, when Swiss physiologists Jean-Louis Prévost and Frédéric Batelli conducted pioneering experiments on dogs. In 1899, they demonstrated that small electric shocks could induce ventricular fibrillation, while larger alternating current (AC) shocks—delivered via electrodes placed in the mouth and rectum—could revert the arrhythmia and restore normal heart rhythm.^[3] These findings established the foundational principle that electrical countershock could depolarize fibrillating myocardium, allowing spontaneous organized activity to resume, though the work remained confined to animal models and was not immediately applied clinically.^[3] Human application of defibrillation began in the mid-20th century through invasive open-chest techniques. In 1947, American surgeon Claude S. Beck achieved the first successful defibrillation of a human heart during surgery on a 14-year-old boy whose ventricle had fibrillated intraoperatively. Beck applied 110-volt AC shocks directly to the exposed heart using internal electrodes, administering multiple pulses of 0.5 to 2 seconds each, combined with manual cardiac massage, to restore sinus rhythm. This landmark procedure, performed at Case Western Reserve University, marked the transition from animal experimentation to clinical use but was limited to surgical settings due to the need for thoracotomy.^[3] The development of closed-chest techniques emerged in the early 1950s, pioneered by cardiologist Paul M. Zoll, who adapted external AC defibrillation for non-invasive application. In 1956, Zoll reported the first successful use of transthoracic shocks on humans, delivering 480 to 1,000 volts of low-frequency (60 Hz) AC through large plate electrodes applied to the chest wall, often requiring multiple discharges to terminate ventricular fibrillation. His bulky, wheeled device—typically around 150 kg—enabled resuscitation without opening the chest, expanding potential use beyond the operating room. However, these early AC methods carried significant risks, including severe skin burns from high-energy delivery and induction of additional arrhythmias due to the prolonged, oscillating nature of the current, which could damage myocardial tissue.^[3]

Shift to Direct Current and Portable Devices

The transition from alternating current (AC) to direct current (DC) defibrillation marked a pivotal advancement in the 1960s, addressing the limitations of earlier AC methods that often caused myocardial burns and arrhythmias. In 1959, cardiologist Bernard Lown and his team developed a monophasic damped sinusoidal DC waveform defibrillator, which delivered controlled energy pulses of 100 to 400 joules, significantly reducing tissue damage while effectively terminating ventricular fibrillation.^[3] This innovation, first clinically applied in 1962, shifted clinical practice toward safer, more reliable cardioversion for both atrial and ventricular arrhythmias, establishing DC as the standard over AC by the mid-1960s.^[81] Parallel developments in portability transformed defibrillation from a hospital-bound procedure to one feasible in emergency settings. In 1966, Northern Irish cardiologist Frank Pantridge introduced the first external portable defibrillator, a 70 kg unit powered by car batteries rather than mains electricity, enabling its installation in ambulances for pre-hospital use in Belfast.^[82] By the 1970s, refinements led to more compact battery-operated models weighing around 25 kg, which facilitated widespread adoption in mobile coronary care units and further integrated defibrillation into out-of-hospital response protocols.^[3] Key contributions from researchers William B. Kouwenhoven, James R. Jude, and G. Guy Knickerbocker in the late 1950s and early 1960s integrated defibrillation with closed-chest cardiopulmonary resuscitation (CPR), enhancing overall resuscitation efficacy. Their work demonstrated that combining external chest compressions with timely DC shocks improved circulation and rhythm restoration during cardiac arrest, laying the foundation for modern basic life support protocols.^[83] These advancements enabled pre-hospital defibrillation, dramatically improving outcomes for out-of-hospital cardiac arrests; early programs with portable DC units in ambulance settings achieved survival rates up to 15-20%, compared to approximately 5% without prompt intervention.^[84] Early paddle electrodes, typically 8-12 cm in diameter and applied with conductive gel, supported this portability by allowing quick manual delivery of shocks in non-hospital environments.^[3]

Modern Advancements in Waveforms and Implantables

In the 1990s, defibrillation technology advanced significantly with the development of biphasic truncated exponential (BTE) waveforms, which reversed the direction of current flow during the shock to improve efficacy and reduce required energy levels compared to traditional monophasic damped sine waveforms.^[85] Clinical studies demonstrated that BTE shocks at 150-200 J achieved defibrillation success rates comparable to or exceeding those of 360 J monophasic shocks, with lower risks of myocardial injury and faster post-shock recovery.^[86] This shift, validated in multicenter trials, led to widespread adoption of biphasic waveforms in external and implantable defibrillators by the early 2000s, enhancing patient outcomes in both hospital and out-of-hospital settings.^[87] Implantable cardioverter-defibrillators (ICDs) represent a cornerstone of modern advancements, originating with Michel Mirowski's pioneering work; the first human implant occurred in 1980 at Johns Hopkins Hospital, successfully terminating ventricular arrhythmias in a patient with recurrent malignant episodes.^[88] Over the subsequent decades, ICDs evolved from bulky transvenous systems to subcutaneous models, which avoid intravascular leads to reduce infection and complication risks. By 2025, subcutaneous ICDs such as the EMBLEM MRI S-ICD system offer projected battery longevity of 8.7 years, enabling reliable long-term monitoring and therapy delivery for high-risk patients.^[89] These devices now incorporate advanced sensing algorithms, further minimizing inappropriate shocks while maintaining high efficacy rates. Recent innovations have integrated artificial intelligence (AI) into rhythm detection for both automated external defibrillators (AEDs) and ICDs, improving accuracy in identifying shockable arrhythmias like ventricular tachycardia and fibrillation amid noisy signals or CPR artifacts. Machine learning models, such as convolutional neural networks, achieve over 95% sensitivity and specificity in classifying rhythms from single-lead ECGs, reducing diagnostic delays and enhancing survival in cardiac arrest scenarios.^[90] Complementing this, drone delivery systems for AEDs have undergone trials across Europe from 2023 to 2025, demonstrating feasibility in rural and urban areas; for instance, a Swedish program achieved median delivery times of 5 minutes with a time advantage of approximately 3 minutes over ambulances in 67% of cases, while Danish studies reported median drone delivery of 4:47 minutes in feasible scenarios.^[91]^[92] To address global disparities, efforts in low-resource settings have focused on reusable cardiac implantable electronic devices, including defibrillators, to expand access without prohibitive costs; studies from high- and low-income contexts show no increased risks of infection or device failure when protocols for cleaning, testing, and sterilization are followed.^[93] These initiatives, supported by international health organizations, promote protocols for refurbishing explanted units, potentially extending their usability and aligning with broader goals for equitable sudden cardiac death prevention in underserved regions.^[94]

Societal and Cultural Impact

Public Access and Training Programs

Public Access Defibrillation (PAD) programs emerged in the United States during the 1990s, spearheaded by the American Heart Association (AHA), which established a task force in 1990 to promote widespread AED availability and training for lay responders.^[95] These initiatives expanded through federal funding, including the Cardiac Arrest Survival Act of 2000, which allocated grants for AED purchase and placement in public venues, and the ongoing Health Resources and Services Administration's Rural Access to Emergency Devices program, supporting AED deployment in underserved areas since the early 2000s.^[96]^[97] Training programs are central to PAD success, with the AHA offering standardized courses like Heartsaver First Aid CPR AED, which emphasize performing high-quality CPR in 2-minute cycles at a rate of 100-120 compressions per minute to minimize interruptions and integrate AED use seamlessly.^[98] These courses incorporate AED features, such as voice prompts guiding users through pad application and shock delivery.^[98] Mobile applications like PulsePoint further support bystander response by sending real-time alerts to CPR-trained individuals about nearby cardiac arrests, including AED locations from a national registry, enabling rapid intervention.^[99] Studies demonstrate the effectiveness of public AED access, with defibrillation within 3-5 minutes of collapse increasing out-of-hospital cardiac arrest survival rates to 50-70%, compared to under 10% without prompt intervention, as evidenced by analyses from the AHA and European Resuscitation Council.^[100]^[101] For instance, the Public Access Defibrillation Trial showed that trained lay responders using AEDs in community settings doubled survival to discharge rates versus CPR alone.^[102] Despite these advances, challenges persist in PAD implementation, including maintenance issues such as expired batteries, faulty pads, and inadequate monthly checks, which can render up to 20% of public AEDs inoperable and contribute to preventable failures.^[103]^[104] Urban-rural disparities exacerbate access gaps, with rural areas facing lower AED density—often fewer than half the units per capita compared to cities—and longer emergency response times, resulting in survival rates 20-30% below urban benchmarks.^[105]^[106] Addressing these requires targeted funding for rural deployment and standardized maintenance protocols to ensure equitable PAD coverage.^[107]

Legal, Ethical, and Economic Aspects

Good Samaritan laws provide legal protections for lay rescuers who administer defibrillation using automated external defibrillators (AEDs) in good faith during emergencies, shielding them from civil liability in the absence of gross negligence. In the United States, all 50 states offer such immunity, with 48 states extending protections to untrained bystanders and only three requiring prior training.^[108]^[109] Internationally, similar protections exist in numerous countries, encouraging bystander intervention without fear of lawsuits, though requirements for training vary; for instance, unrestricted use by any citizen is permitted in 15 countries, while certification is mandated in three others.^[110]^[111] Ethical considerations in defibrillation encompass dilemmas around consent and resource allocation, particularly in crises. For unconscious patients experiencing cardiac arrest, implied consent is ethically justified for immediate life-saving interventions like defibrillation, as obtaining explicit permission is impossible and delay could be fatal; healthcare providers must act in the patient's best interest under principles of beneficence and non-maleficence.^[112] During the COVID-19 pandemic (2020-2022), resource shortages strained resuscitation efforts, leading to triage protocols that prioritized patients based on survival likelihood and resource needs, sometimes limiting defibrillation or advanced care in overwhelmed settings to allocate ventilators and ICU beds equitably.^[113] These decisions raised concerns over fairness, with guidelines emphasizing transparent, evidence-based criteria to avoid discrimination.^[114] Economically, AED deployment involves upfront costs balanced against long-term health benefits. A single AED unit typically ranges from $1,200 to $3,000, depending on model and features, with home models averaging around $1,450 and professional units higher.^[115] Cost-effectiveness analyses demonstrate value in public settings, where bystander defibrillation yields an incremental cost of approximately $46,700 per quality-adjusted life year (QALY) gained, reflecting improved survival and reduced morbidity from out-of-hospital cardiac arrests.^[116] As of 2025, regulatory updates emphasize workplace mandates and global disparities. In regions like Ontario, Canada, legislation requires AEDs on construction sites with 20 or more workers lasting three months or longer, enhancing access in high-risk environments.^[117] Similarly, Minnesota mandates employer-provided AED access by January 2026, with trained personnel available.^[118] On a global scale, equity remains challenged, with fewer public AEDs in low-income neighborhoods and regions compared to affluent areas, exacerbating survival gaps in low- and middle-income countries where access is limited by infrastructure and funding.^[119] Efforts to address these include international calls for subsidized distribution to promote equitable deployment.^[120]

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https://www.sca-aware.org/sca-news/report-on-aed-failures-a-reminder-that-maintenance-is-vital
[104]
Good Samaritan Laws and CPR - Emergency First Response
In most states, provinces and countries, Good Samaritan laws protect bystanders who attempt to save someone's life using CPR or an AED.
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The Automated External Defibrillator: Heterogeneity of Legislation ...
Oct 28, 2021 · The results revealed that every citizen can use an AED in 15 countries whereas a training certificate was required in three countries. In one ...
[106]
Part 3: Ethics | Circulation - American Heart Association Journals
This section provides guidelines for healthcare providers who are faced with the difficult decision to provide or withhold emergency cardiovascular care.Ethical Principles · Dnar Orders In Ohca · Dnar Orders In Ihca
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Scarce-Resource Allocation and Patient Triage During the COVID ...
Health care systems have faced or will face difficult decisions about triage, allocation, and reallocation. These decisions should be guided by ethical ...
[108]
Part 3: Ethics: 2025 American Heart Association Guidelines for ...
Oct 22, 2025 · In this chapter, the American Heart Association provides guidance on ethical considerations relevant to cardiopulmonary resuscitation and ...<|separator|>
[109]
https://www.aed.com/aed-legislation
[110]
Evidence of Impact for Public Access Defibrillation
One study found survival rates as high as 70% if AEDs are used within 2 minutes of collapse during the cardiac arrest.
[111]
Working For Workers Seven Act, 2025 | Ontario Newsroom
May 28, 2025 · Requiring automated external defibrillators (AEDs) on construction projects with 20 or more workers that are expected to last three months or ...Missing: mandates | Show results with:mandates
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Minnesota mandates employer access to automatic external ...
Mar 20, 2025 · The bill mandates that employers provide employees with reasonable access to automatic external defibrillators (AEDs) and ensure that trained personnel are ...<|separator|>
[113]
https://pmc.ncbi.nlm.nih.gov/articles/PMC7213960/
[114]
Gaps in Public Access Defibrillation: Analysis of International ...
Feb 26, 2025 · Searches identified 419,248 legal records. Enabling Legislation regarding AED provision in public areas was present in 60% U.S. states, 8% ...