User:Owain.davies/Defibrillation
Defibrillation is the medical intervention of using electricity to stop ventricular fibrillation or ventricular tachycardia in a patient's heart, in order to facilitate the return of normal sinus rhythm. Allowing the continuation of either rhythm would result in the patient's inevitable death, making defibrillation the definitive treatment for those in arrhythmia, which would lead to cardiogenic shock
This is facilitated by a unit called a Defibrillator which works by delivering a therapeutic dose of electrical current to the heart of the patient, thereby depolarizing a critical mass of the heart muscle, which terminates the life threatening arrhythmia, and allows the body's natural pacemaker (in the sinoatrial node of the heart), or any implanted pacemaker, to establish a normal heart rhythm, which is more compatible with life.
Defibrillators can be external, internal or implanted, depending on the patient, and when their condition occurs. Some external units, known as Automated External Defibrillators (AEDs), automate the diagnosis of treatable rhythms, meaning that lay responders are able to successfully use them with little (or in some cases, no) training.
History of Defibrillation
[edit]Discovery of defibrillation
[edit]Defibrillation was invented in 1899 by Prevost and Batelli, two physiologists from University of Geneva, Switzerland. They discovered that small electric shocks could induce ventricular fibrillation in dogs, and that larger charges would reverse the condition.
The first use on a human was by Claude Beck, professor of surgery at Western Reserve University. Beck's theory was that ventricular fibrillation often occurred in hearts which were fundamentally healthy, in his terms "Heart too good to die", and that there must be a way of saving them. Beck first used the technique successfully on a 14 year old boy who was being operated on for a congenital chest defect. The boy's chest was surgically opened, and manual cardiac massage was undertaken for 45 minutes until the arrival of the defibrillator. Beck used internal paddles either side of the heart, along with procaine amide, a heart drug, and achieved return of normal sinus rhythm.
These early defibrillators used the alternating current from a power socket, transformed from the 110-240 volts available in the line, up to between 300 and 1000 volts, to the exposed heart by way of 'paddle' type electrodes. The technique was often ineffective in reverting VF while morphological studies showed damage to the cells of the heart muscle post mortem. The nature of the AC machine with a large transformer also made these units very hard to transport, and they tended to be large units on wheels.
The move to Direct Current
[edit]In 1959 Bernard Lown commenced research into an alternative technique which involved charging of a bank of capacitors to 100-200 Joules then delivering the charge through an inductance such as to produce a heavily damped sinusoidal wave of finite duration (~5 msec) to the 'paddle' electrodes. The work of Lown was taken to clinical application by engineer Barouh Berkovits with his "cardioverter".
The Lown waveform, as it was known, was the standard for defibrillation until the late 1980's when numerous studies showed that a biphasic truncated waveform (BTE) was equally efficacious while requiring the delivery of lower levels of energy to produce defibrillation. A side effect was a significant reduction in weight of the machine. The BTE waveform, combined with automatic measurement of transthoracic impedance is the basis for modern defibrillators.
Defibrillators become portable
[edit]A major breakthrough was the introduction of portable defibrillators in ambulances. This was pioneered in the early 1960's by Prof. Frank Pantridge in Belfast. Today portable defibrillators are one of the most important tools carried by ambulances. They are the only proven way to resuscitate a person who has had a cardiac arrest unwitnessed by EMS who is still in persistent ventricular fibrillation or ventricular tachycardia at the arrival of prehospital providers.[1]
Gradual improvements in the design of defibrillators, and partly based on the work developing implanted versions (see below) have lead to the availability of Automated External Defibrillators, which can analyse the heart rhythm by themselves, diagnosing the shockable rhythms, and then charging to treat. This means that no clinical skill is required in their use, allowing lay people to respond to emergencies effectively.
The change to a biphasic waveform
[edit]Until recently, external defibrillators relied on monophasic shock waves. Electrical pulses are sent rapidly from one electrode to the other, only in one direction. Biphasic defibrillation, however, alternates the direction of the pulses, completing one cycle in approximately 10 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. This, in turn, decreases risk of burns and myocardial damage.
Ventricular fibrillation (VF) could persist in about 40% of cardiac arrest patients treated with a single shock from a monophasic defibrillator. Most biphasic defibrillators have a first shock success rate (VF is eliminated and a normal rhythm can be returned) of greater than 90%.[2]
Implantable devices
[edit]A further development in defibrillation came with the invention of the implantable device, known as an Implantable Cardioverter Defibrillator (or ICD). This was pioneered at Sinai Hospital in Baltimore by a team including Stephen Heilman, Alois Langer, Morton Mower, Michel Mirowski, and Mir Imran, with the help of industrial collaborator Intec Systems of Pittsburgh. Mirowski teamed up with Mower and Staewen, and together they commenced their research in 1969 but it was 11 years before they treated their first patient. Imran was the young engineer who was able to turn the benchtop idea into a working device.[citation needed] Similar developmental work was carried out by Schuder and colleagues at the University of Missouri.
The work was commenced against much skepticism even by leading experts in the field of arrhythmias and sudden death. There was doubt that their ideas would ever become a clinical reality. In 1972 Bernard Lown, the inventor of the external defibrillator, stated in the journal Circulation - "The very rare patient who has frequent bouts of ventricular fibrillation is best treated in a coronary care unit and is better served by an effective antiarrhythmic program or surgical correction of inadequate coronary blood flow or ventricular malfunction. In fact, the implanted defibrillator system represents an imperfect solution in search of a plausible and practical application".
The problems to be overcome were the design of a system which would allow detection of ventricular fibrillation or ventricular tachycardia. Despite the lack of financial backing and grants, they persisted and the first device was implanted in February 1980 at Johns Hopkins Hospital by Dr. Levi Watkins, Jr. Modern ICDs do not require a thoracotomy and possess pacing, cardioversion, and defibrillation capabilities.
The invention of implantable units is invaluable to some regular sufferers of heart problems, although they are generally only given to those people who have already had a cardiac episode.
Types of Defibrillator
[edit]Manual external defibrillator
[edit]
The units are used in conjunction with (or more often have inbuilt) electrocardiogram readers, which the clinician uses to diagnose a cardiac condition (most often fibrillation or tachycardia although there are some other rhythms which can be treated by different shocks). The clinician will then decide what charge (voltage) to use, based on their prior knowledge 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.
Manual internal defibrillator
[edit]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, where the chest is liekly to be open, or can be opened quickly by a surgeon.
Automated external defibrillator (AED)
[edit]
These simple to use units are based on computer technology which is designed to analyse the heart rhythm itself, and then advise whether a shock is required. They are designed to by used by lay persons, who require little training. They are usually limited in their interventions to delivering high joule shocks for VF and VT rhythms, making them generally limiting for use by health professionals, who could diagnose and treat a wider range of problems with a manual or semi-automatic 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 quicker with a manual unit.
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 centres, fitness centres and health clubs.
The locating of a public access AED should take in to account where large groups of people gather, and the risk category associated with these people, to ascertain whether the risk of a sudden cardiac arrest incident is high. For example, a centre for teenage children is a particularly low risk category (as children very rarely enter heart rhythms such as VF or VT, being generally young and fit, and the most common cause of paediatric cardiac arrest is trauma - where an AED is no use), whereas a large office building with a high ratio of males over 50 is a very high risk environment.
In many areas, emergency services vehicles are likely to carry AEDs, with some Ambulances carrying an AED in addition to a manual unit. In addition, some police or fire service vehicles carry an AED for first responder use. Some areas have dedicated community first responders, who are volunteers tasking with keeping an AED and taking it to any victims in their area. It is also increasingly common to find AEDs on transport such as commercial airlines and cruise ships.
In order to make them highly visible, public access AEDs often are brightly coloured, and are mounted in protective cases near the entrance of a building. When these protective cases are opened, and the defibrillator removed, some will sound a buzzer to alert nearby staff to their removal but do not necessarily summon emergency services. All trained AED operators should also know to phone for an Ambulance when sending for or using an AED, as the patient will be unconscious, which always requires ambulance attendance.
Semi-automated external defibrillators
[edit]These units are a compromise between a a full manual unit and an automated unit. They are mostly used by pre-hospital care professionals such as paramedics and emergency medical technicians. These units have the automated capabilities of the AED but also feature an ECG display, and a manual override, where the clinician can make their own decision, either before or instead of the computer. Some of these units are also able to act as a pacemaker if the heart rate is too slow (bradycardia) and perform other functions which require a skilled operator.
(Automated) Implantable Cardioverter Defibrillator (ICD or AICD)
[edit]These units are implants, similar to pacemakers (and may in fact also perform the pacemaking function. They constantly monitor the patient's heart rhythm, and automatically act to correct it. All ICDs are capable of delivering a shock as all the other units do, although they may also have features to help prevent the occurrence in the first instance. More modern devices can distinguish between ventricular fibrillation and ventricular tachycardia (VT), and may try to pace the heart faster than its intrinsic rate in the case of VT, to try to break the tachycardia before it progresses to ventricular fibrillation. This is known as fast-pacing, overdrive pacing, or anti-tachycardia pacing (ATP). ATP is only effective if the underlying rhythm is ventricular tachycardia, and is never effective if the rhythm is ventricular fibrillation.
There are cases where patient's have a problem with their AICD firing constantly or inappropriately This is a [[medical emergency], as it could have severe detrimental effects on the patient's health. Most emergency medical services personnel are now trained in dealing with this, and all modern units should shut off when a ring magnet is placed over them (external to the chest), to enable the crew to work with the patient.
Interface between defibrillator and patient
[edit]The most well-known type of electrode is the traditional metal paddle with an insulated handle. This type must be held in place on the patient's skin while a shock or a series of shocks is delivered. Before the paddle is used, a gel must be applied to the patient's skin, in order to ensure a good connection and to minimize electrical resistance, also called chest impedance (despite the DC discharge). These are generally only found on the manual external units.
Another type of resuscitation electrode is designed as an adhesive pad. When a patient has been admitted due to heart problems, and the physician or nurse has determined that he or she is at risk of arrhythmia, they may apply adhesive electrodes to the patient in anticipation of any problems that may arise. These electrodes are left connected to a defibrillator. If defibrillation is required, the machine is charged, and the shock is delivered, without any need to apply any gel or to retrieve and place any paddles. These adhesive pads are found on most automated and semi-automated units.
Both solid- and wet-gel adhesive electrodes are available. Solid-gel electrodes are more convenient, because there is no need to clean the patient's skin after removing the electrodes. However, the use of solid-gel electrodes presents a higher risk of burns during defibrillation, since wet-gel electrodes more evenly conduct electricity into the body.
Adhesive electrodes are designed to be used not only for defibrillation, but also for non-invasive pacing and electrical cardioversion.
While the paddles on a Monitor/Defibrillator may be quicker than using the patches, adhesive patches are superior due to their ability to provide appropriate EKG tracing without the artifact visible from human interference with the paddles. Many monitor defibrillators provide three, five or 12-lead EKG monitoring to compensate for this downfall of the paddles. Adhesive electrodes are also inherently safer than the paddles for the operator of the defibrillator to use, as they 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 stand several feet away. Another inconvenience of the paddles is the requirement of around 25lbs of pressure to be applied while defibrillating.
Placement
[edit]Resuscitation electrodes are placed according to one of two schemes. The anterior-posterior scheme (conf. image) 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 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.
Defibrillation in popular culture
[edit]The defibrillator was first seen on film in the 1966 movie Fantastic Voyage. Since then they have appeared in many modern television and film medical programs. Its use in the near-death experience experiments by the characters of the 1990 movie Flatliners made the defibrillator virtually one of the co-stars of that film. It was also utilized in plotlines in several films and TV series.
In the television series Emergency!, firefighters John Gage and Roy DeSoto often used defibrillators, starting the film tradition of yelling "Clear" right before applying the shock, to warn everyone around to stay away from the patient for risk of electrical shock.
In the same series, the defibrillator induces a sudden, violent jerk or convulsion by the patient; in reality, although the muscles may contract, such dramatic patient presentation is rare.
Most television shows will have the medical provider defibrillate the "flat-line" ECG rhythm (also known as asystole); this is not done in real life. Only the cardiac arrest rhythms ventricular fibrillation and pulseless ventricular tachycardia are normally defibrillated. (There are also several heart rhythms that can be "defibrillated" when the patient is not in cardiac arrest, such as supraventricular tachycardia or ventricular tachycardia that produces a pulse, though the procedure is then known as cardioversion.) However, a flatline may actually be a fibrillation that is too weak to be seen on the monitor (fine v-fib), so a shock may be delivered, but it is not regarded as the treatment of choice, as the probability of a successful conversion is very small. According to the current guidelines, in this situation, continued CPR in order to improve the oxygenation of the heart for a few minutes is preferred before defibrillation is attempted.
During the penultimate season of M*A*S*H, in the episode "Heroes", B. J. Hunnicutt saves a patient's life by using a hastily-assembled defibrillator after reading an article in a medical journal (contrary to history, Hawkeye Pierce suggests that the technique had previously only been tested on dogs). His colleague claims he's just invented a new way of saving lives.
In the 1988 film Short Circuit 2, Benjamin used a defibrillator to provide the robot Johnny 5 with temporary power when its leaking battery ran out of power. It also played a chief role in a scene in the 1989 film The Abyss, when Lindsey Brigman allowed herself to be drowned in freezing water so she and her husband, Bud, could both make it back to the Deep Core underwater drilling station.
In the computer games Battlefield 2 and Battlefield 2142, the defibrillator is a tool available for medics. Contrary to reality, they are surprisingly convenient and mobile tools that can quickly revive a fallen comrade, or kill an enemy soldier with a single jolt.
In the 1998 Bobby Farrelly and Peter Farrelly classic "There's Something about Mary" there is a use of a 'defibrillation' technique in a comedy setting. Matt Dillon's character, Pat Healey, has drugged a small dog in order to control it's aggressiveness towards him. However he goes too far and the dog dies, and he has to go into quick action to save the dog. He performs a form of CPR, and then pulls electrical cord from a nearby lamp and "defibrillates" the dog with it.
In Australia up until the 1990s, it was quite rare for an ambulance to carry a defibrillator. This changed in 1990 when Australian media mogul Kerry Packer had a heart attack and the ambulance that responded to the call did carry a defibrillator. After this, Kerry Packer donated a large sum to the New South Wales Ambulance Service in order that all ambulances in New South Wales should be fitted with a personal defibrillator, leading to the Australian colloquial term for the device, Packer Whacker.
In the 2006 James Bond movie Casino Royale, Bond uses a defibrillator from his Aston Martin DBS, to revive himself after being poisoned with digitalis during his poker game. On returning to the table he remarks "Sorry; that last hand -- nearly killed me."
See also
[edit]
- Cardiopulmonary resuscitation (CPR)
- Advanced cardiac life support (ACLS)
- Cardioversion
- Implantable cardioverter-defibrillator
- Automated external defibrillator
- Heart Attack
- Ambulance
External links
[edit]
- American Red Cross: Saving a Life is as Easy as A-E-D
- FDA Heart Health Online: Automated External Defibrillator (AED)
- Resuscitation Council (UK)
- History of defibrillation
- How an internal defibrillator is implanted from Children's Hospital Heart Center, Seattle.
