defibrilation

DEFIBRILLATION

DEFIBRILLATION

      Defibrillation is a common treatment for life threatening cardiac dysrhythmias and ventricular fibrillation. Defibrillation consists of delivering a therapeutic dose of electrical current to the heart with a device called a  defibrillator. This depolarizes a critical mass of the heart muscle, terminates the dysrhythmia and allows normal sinus rhythm to be re established by the body's natural pacemaker, in the sinoatrial node of the heart.

      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.

MEDICAL USES

      In a study of CPR and defibrillation for cardiac arrest under ideal conditions, survival with normal neurological function occurred in 38%. Assuming survival without defibrillation to be zero, this is equivalent to saving the life of 2 out of 5 people using defibrillation. Furthermore, when considering only those with a heart rhythm correctable by defibrillation (ventricular fibrillation), survival rate was 59%, equivalent to saving 3 out of 5. Survival rates from cardiac arrest was less, however, in more common circumstances seen outside of the study, including among ill hospitalized persons, people without access to immediate (<4–5 minutes) CPR, and for those whose arrest is not witnessed.

Types

Manual external defibrillator

      The units are used in conjunction with electrocardiogram readers, which the healthcare provider uses to diagnose a cardiac condition. The healthcare provider will then decide what charge (in joules) to use, based on proven guidelines and experience, and will deliver the shock through paddles or pads on the patient's chest. As they require detailed medical knowledge, these units are generally only 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.

MANUAL INTERNAL DEFIBRILLATOR

      These are the direct descendants of the work of Beck and Lown. They are virtually identical to the external version, except that the charge is delivered through internal paddles in direct contact with the heart. These are almost exclusively found in operating theatres (rooms), where the chest is likely to be open, or can be opened quickly by a surgeon.

Automated external defibrillator (AED)

      These simple to use units are based on computer technology which is designed to analyze the heart rhythm itself, and then advise the user whether a shock is required. They are designed to be used by lay persons, who require little training to operate them correctly.  They are usually limited in their interventions to delivering high joule shocks for VF (ventricular fibrillation) and VT (ventricular tachycardia) rhythms, making them generally of limited use to health professionals, who could diagnose and treat a wider range of problems with a manual or semiautomatic unit.

      The automatic units also take time (generally 10–20 seconds) to diagnose the rhythm, where a professional could diagnose and treat the condition far more quickly with a manual unit. These time intervals for analysis, which require stopping chest compressions, have been shown in a number of studies to have a significant negative effect on shock success. This effect led to the recent change in the AHA defibrillation guideline (calling for two minutes of CPR after each shock without analyzing the cardiac rhythm) and some bodies recommend that AEDs should not be used when manual defibrillators and trained operators are available. Automated external defibrillators are generally either held by trained personnel who will attend incidents, or are public access units which can be found in places including corporate and government offices, shopping centres, airports, restaurants, casinos, hotels, sports stadiums, schools and universities, community centers, fitness centers and health clubs.

IMPLANTABLE CARDIOVERTERDEFIBRILLATOR (ICD)

      Also known as automatic internal cardiac defibrillator (AICD). These devices 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 lifethreatening 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 lifethreatening arrhythmia is ventricular fibrillation, the device is programmed to proceed immediately to an unsynchronized shock.

WEARABLE CARDIAC DEFIBRILLATOR

A development of the AICD is a portable external defibrillator that is worn like a vest. The unit monitors the patient 24 hours a day and will automatically deliver a biphasic shock if needed. This device is mainly indicated in patients awaiting an implantable defibrillator. As of February 2011 only one company manufactures these portable external defibrillators and they are of limited availability.

MODELLING DEFIBRILLATION

      The efficacy of a cardiac defibrillator is highly dependent on the position of its electrodes. Most internal defibrillators are implanted in octogenarians, but a few children need the devices. Implanting defibrillators in children is particularly difficult because children are small, will grow over time, and possess cardiac anatomy that differs from that of adults. Recently, researchers were able to create a software modeling system capable of mapping an individual’s thorax and determining the optimal position for an external or internal cardiac defibrillator.  With the help of pre-existing surgical planning applications, the software uses myocardial voltage gradients to predict the likelihood of successful defibrillation.

      According to the critical mass hypothesis, defibrillation is effective only if it produces a threshold voltage gradient in a large fraction of the myocardial mass. Usually, a gradient of three to five volts per centimeter is needed in 95% of the heart. Voltage gradients of over 60 V/cm can damage tissue. The modeling software seeks to obtain safe voltage gradients above the defibrillation threshold. Early simulations using the software suggest that small changes in electrode positioning can have large effects on defibrillation, and despite engineering hurdles that remain, the modeling system promises to help guide the placement of implanted defibrillators in children and adults. Recent mathematical models of defibrillation are based on the bidomain model of cardiac tissue. Calculations using a realistic heart shape and fiber geometry are required to determine how cardiac tissue responds to a strong electrical shock.

INTERFACE WITH PERSON

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). Solidgel is more convenient, because there is no need to clean the used gel off of patient's skin after defibrillation (the solid gel is easily lifted off of the patient). 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, as discussed below.

                

PADDLE ELECTRODES

The most well known type of electrode (widely depicted in films and television) is the traditional metal paddle with an insulated (usually plastic) handle. This type must be held in place on the patient's skin with approximately 25 lbs 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 non perfusion means tissue loss. Modern paddles allow for monitoring (electrocardiography), though in hospital situations, separate monitoring leads are often already in place.

SELFADHESIVE ELECTRODES

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 semiautomated units and are replacing paddles entirely in nonhospital settings. In hospital, for cases where cardiac arrest is likely to occur (but has not yet), selfadhesive pads may be placed prophylactically.

PLACEMENT

Resuscitation electrodes are placed according to one of two schemes. The anteriorposterior scheme is the preferred scheme for longterm 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 noninvasive pacing.

METHOD

CLOSED CHEST METHOD

Until the early 1950s, defibrillation of the heart was possible only when the chest cavity was open during surgery. The technique used an alternating voltage from a 300 or greater volt source derived from standard AC power, delivered to the sides of the exposed heart by "paddle" electrodes where each electrode was a flat or slightly concave metal plate of about 40 mm diameter. The closed chest defibrillator device which applied an alternating voltage of greater than 1000 volts, conducted by means of externally applied electrodes through the chest cage to the heart.

PORTABLE UNITS BECOME AVAILABLE

                                      

A major breakthrough was the introduction of portable defibrillators used out of the hospital. Already Peleška's Prema defibrillator was designed to be more portable than original  In the west this was pioneered in the early 1960s by Prof. Frank Pantridge in Belfast. Today portable defibrillators are among the many very important tools carried by ambulances. They are the only proven way to resuscitate a person who has had a cardiac arrest unwitnessed by Emergency Medical Services (EMS).

CHANGE TO A BIPHASIC WAVEFORM

Biphasic defibrillation alternates the direction of the pulses, completing one cycle in approximately 12 milliseconds. Biphasic defibrillation was originally developed and used for implantable cardioverter defibrillators. When applied to external defibrillators, biphasic defibrillation significantly decreases the energy level necessary for successful defibrillation, decreasing the risk of burns and myocardial damage.

IMPLANTABLE DEVICES

A further development in defibrillation came with the invention of the implantable device, known as an implantable cardioverter defibrillator (or ICD).

References

·         Rapid Defibrillation for Cardiac Arrest". the NNT. August 19, 2010. Retrieved 2 September 2015.

·          Soar, J.; et al., eds. (2006). Immediate Life Support: Second Edition. Resuscitation Council (UK). ISBN 1903812127.

·          Eftestol T, Sunde K, Steen PA. Effects of interrupting precordial compressions on the calculated probability of defibrillation success during outofhospital cardiac arrest. Circulation 2002;105:22703

·          http://www.foh.dhhs.gov/Whatwedo/AED/HHSAED.ASP

·         What is the LifeVest?". Zoll Lifecor. Retrieved 20090209.