defibrillation defibrillation defibrillation is a treatment for life-threatening cardiac arrhythmias, specifically ventricular fibrillation (v-fib) and non-perfusing ventricular tachycardia (v-tach). a defibrillator delivers a dose of electric current (often called a counter-shock) to the heart. although not fully understood, this process depolarizes a large amount of the heart muscle, ending the arrhythmia. subsequently, the body's natural pacemaker in the sinoatrial node of the heart is able to re-establish normal sinus rhythm. a heart which is in asystole (flatline) cannot be restarted by a defibrillator, but would be treated by cardiopulmonary resuscitation (cpr). not to be confused with defibulation. defibrillationview of defibrillator electrode position and placement in contrast to defibrillation, synchronized electrical cardioversion is an electrical shock delivered in synchrony to the cardiac cycle. although the person may still be critically ill, cardioversion normally aims to end poorly perfusing cardiac arrhythmias, such as supraventricular tachycardia. defibrillators can be external, transvenous, or implanted (implantable cardioverter-defibrillator), depending on the type of device used or needed. some external units, known as automated external defibrillators (aeds), automate the diagnosis of treatable rhythms, meaning that lay responders or bystanders are able to use them successfully with little or no training. use of defibrillators indications defibrillation is often an important step in cardiopulmonary resuscitation (cpr). cpr is an algorithm-based intervention aimed to restore cardiac and pulmonary function. defibrillation is indicated only in certain types of cardiac dysrhythmias, specifically ventricular fibrillation (vf) and pulseless ventricular tachycardia. if the heart has completely stopped, as in asystole or pulseless electrical activity (pea), defibrillation is not indicated. defibrillation is also not indicated if the patient is conscious or has a pulse. improperly given electrical shocks can cause dangerous dysrhythmias, such as ventricular fibrillation. application method the defibrillation device that is usually available out of the medical centers is the automated external defibrillator (aed), a portable machine that can be used even by users with no previous training. that is possible because the machine produces pre-recorded voice instructions that guide to the user, and automatically checks the patient's condition and applies the correct electric shocks. there also exist written instructions of defibrillators that explain the procedure step-by-step. outcomes survival rates for out-of-hospital cardiac arrests are poor, often less than 10%. outcome for in-hospital cardiac arrests are higher at 20%. within the group of people presenting with cardiac arrest, the specific cardiac rhythm can significantly impact survival rates. compared to people presenting with a non-shockable rhythm (such as asystole or pea), people with a shockable rhythm (such as vf or pulseless ventricular tachycardia) have improved survival rates, ranging between 21 and 50%. types manual models manual external defibrillators require the expertise of a healthcare professional. they are used in conjunction with an electrocardiogram, which can be separate or built-in. a healthcare provider first diagnoses the cardiac rhythm and then manually determine the voltage and timing for the electrical shock. these units are primarily found in hospitals and on some ambulances. for instance, every nhs ambulance in the united kingdom is equipped with a manual defibrillator for use by the attending paramedics and technicians. in the united states, many advanced emts and all paramedics are trained to recognize lethal arrhythmias and deliver appropriate electrical therapy with a manual defibrillator when appropriate. an internal defibrillator is often used to defibrillate the heart during or after cardiac surgery such as a heart bypass. the electrodes consist of round metal plates that come in direct contact with the myocardium. manual internal defibrillators deliver the shock through paddles placed directly on the heart. they are mostly used in the operating room and, in rare circumstances, in the emergency room during an open heart procedure. automated external defibrillators main article: automated external defibrillator automated external defibrillators (aeds) are designed for use by untrained or briefly trained laypersons. aeds contain technology for analysis of heart rhythms. as a result, it does not require a trained health provider to determine whether or not a rhythm is shockable. by making these units publicly available, aeds have improved outcomes for sudden out-of-hospital cardiac arrests. trained health professionals have more limited use for aeds than manual external defibrillators. recent studies show that aeds does not improve outcome in patients with in-hospital cardiac arrests. aeds have set voltages and does not allow the operator to vary voltage according to need. aeds may also delay delivery of effective cpr. for diagnosis of rhythm, aeds often require the stopping of chest compressions and rescue breathing. for these reasons, certain bodies, such as the european resuscitation council, recommend using manual external defibrillators over aeds if manual external defibrillators are readily available. as early defibrillation can significantly improve vf outcomes, aeds have become publicly available in many easily accessible areas. aeds have been incorporated into the algorithm for basic life support (bls). many first responders, such as firefighters, policemen, and security guards, are equipped with them. aeds can be fully automatic or semi-automatic. a semi-automatic aed automatically diagnoses heart rhythms and determines if a shock is necessary. if a shock is advised, the user must then push a button to administer the shock. a fully automated aed automatically diagnoses the heart rhythm and advises the user to stand back while the shock is automatically given. some types of aeds come with advanced features, such as a manual override or an ecg display. cardioverter-defibrillators implantable cardioverter-defibrillators, also known as automatic internal cardiac defibrillator (aicd), are implants similar to pacemakers (and many can also perform the pacemaking function). they constantly monitor the patient's heart rhythm, and automatically administer shocks for various life-threatening arrhythmias, according to the device's programming. many modern devices can distinguish between ventricular fibrillation, ventricular tachycardia, and more benign arrhythmias like supraventricular tachycardia and atrial fibrillation. some devices may attempt overdrive pacing prior to synchronised cardioversion. when the life-threatening arrhythmia is ventricular fibrillation, the device is programmed to proceed immediately to an unsynchronized shock. there are cases where the patient's icd may fire constantly or inappropriately. this is considered a medical emergency, as it depletes the device's battery life, causes significant discomfort and anxiety to the patient, and in some cases may actually trigger life-threatening arrhythmias. some emergency medical services personnel are now equipped with a ring magnet to place over the device, which effectively disables the shock function of the device while still allowing the pacemaker to function (if the device is so equipped). if the device is shocking frequently, but appropriately, ems personnel may administer sedation. a wearable cardioverter defibrillator is a portable external defibrillator that can be worn by at-risk patients. the unit monitors the patient 24 hours a day and can automatically deliver a biphasic shock if vf or vt is detected. this device is mainly indicated in patients who are not immediate candidates for icds. interface the connection between the defibrillator and the patient consists of a pair of electrodes, each provided with electrically conductive gel in order to ensure a good connection and to minimize electrical resistance, also called chest impedance (despite the dc discharge) which would burn the patient. gel may be either wet (similar in consistency to surgical lubricant) or solid (similar to gummi candy). solid-gel is more convenient, because there is no need to clean the used gel off the person's skin after defibrillation. however, the use of solid-gel presents a higher risk of burns during defibrillation, since wet-gel electrodes more evenly conduct electricity into the body. paddle electrodes, which were the first type developed, come without gel, and must have the gel applied in a separate step. self-adhesive electrodes come prefitted with gel. there is a general division of opinion over which type of electrode is superior in hospital settings; the american heart association favors neither, and all modern manual defibrillators used in hospitals allow for swift switching between self-adhesive pads and traditional paddles. each type of electrode has its merits and demerits. paddle electrodes an automated external defibrillator: this model is a semi-automatic due to the presence of a shock buttonthe same aed with electrodes attached the most well-known type of electrode (widely depicted in films and television) is the traditional metal "hard" paddle with an insulated (usually plastic) handle. this type must be held in place on the patient's skin with approximately 25 lbs (11.3 kg) of force while a shock or a series of shocks is delivered. paddles offer a few advantages over self-adhesive pads. many hospitals in the united states continue the use of paddles, with disposable gel pads attached in most cases, due to the inherent speed with which these electrodes can be placed and used. this is critical during cardiac arrest, as each second of nonperfusion means tissue loss. modern paddles allow for monitoring (electrocardiography), though in hospital situations, separate monitoring leads are often already in place. paddles are reusable, being cleaned after use and stored for the next patient. gel is therefore not preapplied, and must be added before these paddles are used on the patient. paddles are generally only found on manual external units. self-adhesive electrodes self-adhesive electrodes of a defibrillator newer types of resuscitation electrodes are designed as an adhesive pad, which includes either solid or wet gel. these are peeled off their backing and applied to the patient's chest when deemed necessary, much the same as any other sticker. the electrodes are then connected to a defibrillator, much as the paddles would be. if defibrillation is required, the machine is charged, and the shock is delivered, without any need to apply any additional gel or to retrieve and place any paddles. most adhesive electrodes are designed to be used not only for defibrillation, but also for transcutaneous pacing and synchronized electrical cardioversion. these adhesive pads are found on most automated and semi-automated units and are replacing paddles entirely in non-hospital settings. in hospital, for cases where cardiac arrest is likely to occur (but has not yet), self-adhesive pads may be placed prophylactically. pads also offer an advantage to the untrained user, and to medics working in the sub-optimal conditions of the field. pads do not require extra leads to be attached for monitoring, and they do not require any force to be applied as the shock is delivered. thus, adhesive electrodes minimize the risk of the operator coming into physical (and thus electrical) contact with the patient as the shock is delivered by allowing the operator to be up to several feet away. (the risk of electrical shock to others remains unchanged, as does that of shock due to operator misuse.) self-adhesive electrodes are single-use only. they may be used for multiple shocks in a single course of treatment, but are replaced if (or in case) the patient recovers then reenters cardiac arrest. special pads are used for children under the age of 8 or those under 55 lbs. (22 kg). placement anterior-apex placement of electrodes for defibrillation resuscitation electrodes are placed according to one of two schemes. the anterior-posterior scheme is the preferred scheme for long-term electrode placement. one electrode is placed over the left precordium (the lower part of the chest, in front of the heart). the other electrode is placed on the back, behind the heart in the region between the scapula. this placement is preferred because it is best for non-invasive pacing. the anterior-apex scheme (anterior-lateral position) can be used when the anterior-posterior scheme is inconvenient or unnecessary. in this scheme, the anterior electrode is placed on the right, below the clavicle. the apex electrode is applied to the left side of the patient, just below and to the left of the pectoral muscle. this scheme works well for defibrillation and cardioversion, as well as for monitoring an ecg. researchers have created a software modeling system capable of mapping an individual's chest and determining the best position for an external or internal cardiac defibrillator. mechanism defibrillator with pad positions shown: the model is biphasic and either pad can be placed in each position the exact mechanism of defibrillation is not well understood. one theory is that successful defibrillation affects most of the heart, resulting in insufficient remaining heart muscle to continue the arrhythmia. recent mathematical models of defibrillation are providing new insight into how cardiac tissue responds to a strong electrical shock. history defibrillators were first demonstrated in 1899 by jean-louis prévost and frédéric batelli, two physiologists from the university of geneva, switzerland. they