DEFIBRILLATOR

Typical Number in Hospital: 20

Cost Bands: 4

References: 3,6

Heart disease, electrolyte disturbances, or electric shock, can cause the normally co-ordinated action of the ventricular heart muscle to be disrupted. The unco-ordinated contractions of different parts of the ventricles is called ventricular fibrillation which may lead to death within a few minutes. The most effective treatment for this condition is to pass a short pulse of electrical current through the ventricles. This is done after open heart surgery by applying electrodes directly to the heart. In all other situations (external defibrillation) electrodes are applied to the chest and a large current (e.g. 50 A for 5 ms) is required for this.

A defibrillator employs a high-voltage generator to charge a capacitor to a high voltage, up to 7 kV. This capacitor is discharged into the body via two large electrodes placed across the axis of the heart or between the back and front of the chest. The electrodes (called paddles) are coated with conductive electrode jelly and are spring loaded and shaped to prevent the jelly spreading to the handgrips. There is also an inductor placed between the capacitor and the electrodes to produce a damped sinusoidal waveform which has been found to give good results. Some machines produce other waveforms (e.g. trapezoidal).

The typical maximum stored energy in the capacitor is 400 J, but low energy is normally used for the first attempt and then increased at each subsequent attempt until success is achieved. This is because excessive energy may cause damage to the heart, and burn the skin at the electrode sites. For children, and for use with internal electrodes (i.e. directly applied to the heart) the stored energy required may be only 50 J. Most defibrillators have a safety interlock so that energy is limited to this level when used with internal electrodes.

Older types of defibrillator (now obsolete) used a short burst of alternating current applied directly from a large mains transformer. These are now considered to be dangerous.

The ECG may be recorded through the defibrillator paddles and it is now common for defibrillators to have an integral ECG monitor, allowing proper diagnosis of ventricular fibrillation to be made and an immediate confirmation of success or failure of the procedure. An ECG recorder may also be provided. The ECG monitor is particularly important when defibrillators are used to correct abnormal rhythms of the heart rather than ventricular fibrillation. In this case the defibrillating pulse must be applied at the correct time in the cardiac cycle and hence these types are called synchronized defibrillators or cardioverters which take the R wave of the ECG signal, apply a delay which can be set by the operator and then deliver the current pulse. Incorrect timing of the pulse can damage the heart or cause ventricular fibrillation.