Defibrillation

Defibrillation is a medical technique used to counter the onset of ventricular fibrillation, a common cause of cardiac arrest, and pulseless ventricular tachycardia, which sometimes precedes ventricular fibrillation but can be just as dangerous on its own. In simple terms, the process uses an electric shock to stop the heart, in the hope that the heart will restart with rhythmic contractions.

Uses

Defibrillation is a technique used in emergency medicine to terminate ventricular fibrillation or pulseless ventricular tachycardia. It uses an electrical shock to reset the electrical state of the heart so that it may beat to a rhythm controlled by its own natural pacemaker cells. It is not effective for asystole (complete cessation of cardiac activity, more commonly known as "flatline") and pulseless electrical activity (PEA). Ventricular tachycardia with a pulse is treated with medication (if hemodynamically stable), or syncronized cardioversion (if hemodynamically unstable).

The equipment used in this process is called a defibrillator. The "sync" function must be deactivated, otherwise the machine will be searching for an organized R-wave to synchronize the shock with. Defibrillation is part of advanced cardiac life support (ACLS).

The purpose of defibrillation of ventricular arrhythmias is to apply a controlled electrical shock to the heart, which leads to depolarization of the entire electrical conduction system of the heart. When the heart repolarizes, the normal electrical conduction may assert itself, in which case the ventricular arrhythmia is terminated. However, if enough energy is not used for defibrillation, the heart may not be completely depolarized, in which case the ventricular tachycardia or fibrillation may not be terminated. Also, if the heart itself is not getting enough oxygen or if there is an instability of the electrolytes in the cardiac cells, the ventricular arrhythmia may recur.

The shock is generally conducted through the heart by two electrodes, in the form of two hand-held paddles or adhesive patches depending on the variety of the defibrillator. One electrode is placed on the right side of the front of the chest just below the clavicle, and the other electrode is placed on the left side of the chest just below the pectoral muscle or breast. Open-chest defibrillators also exist, which have electrodes in the form of two cup-shaped paddles that surround the sides of the heart and shock it directly. Open-chest defibrillators generally require less energy to operate due to direct contact with the heart.

Although the process of defibrillation can be repeated, the number of attempts is, in practice, limited, to a series of three or four attempts at increasing energies, as the likelihood of restoring normal heart rhythm is much less in successive attempts.

AEDs and ICDs

Increasingly, automated external defibrillators (AEDs) are being made available at airports, shopping centres, etc. These analyse the rhythm automatically and recommend the administration of shocks if necessary, enabling its use by the lay public.

Defibrillators can also be implanted (implantable cardioverter-defibrillators or ICDs) in people at high risk of developing an arrhythmia. The ICD is rather like an artificial pacemaker, and indeed performs all the functions of a pacemaker, but it can also deliver a defibrillation shock if a dangerous arrhythmia is detected.

Predicting the onset of fibrillation

An alternative patented in 2006 by Biophan Technologies utilizes chaos theory algorithms to analyze an EKG signal in order to predict when a fibrillation is about to occur. In then applies a "controlled but noisy or chaotic signal back into the heart," ^* using a fraction of the power needed for standard defibrillation and avoiding ventricular fibrillation altogether. The technique can be implemented into existing cardioverter-defibrillators and pacemakers.

History

Defibrillation was invented in 1899 by Prevost and Batelli, two Italian physiologists. They discovered that electric shocks could convert ventricular fibrillation to sinus rhythm in dogs. The first case of a human life saved by defibrillation was reported by Beck et al in 1947.

A major breakthrough was the introduction of portable defibrillators in ambulances in the 1960s. This was pioneered by Prof. Frank Pantridge in Belfast.

However, 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.

References

Prevost JL, Battelli F. La mort par les déscharges électriques (French: "Death by electrical discharges". J. Physiol 1899;1:1085-1100.
Beck CS, Pritchard WH, Feil HS. Ventricular fibrillation of long duration abolished by electric shock. J Amer Med Assoc 1947;135:985.

External links

Cardiology | Emergency medicine