Introduction
When the heart is significantly damaged for any reason, there is an increased risk that it can become irritable and go into a fast abnormal rhythm (ventricular tachycardia) which may make the heart so inefficient that it can no longer effectively function as a pump. This may happen out of the blue with no real warning even if the patient has been well. If left unchecked for even a few minutes, this will degrade into a rhythm known as ventricular fibrillation which causes cardiac arrest and sudden death. In this setting, the only thing that may help is immediate cardiac resuscitation with the aim of delivering an electrical shock to the heart as soon as possible. All research indicates that the only intervention that has any chance of reviving the patient is the electrical shock treatment which is usually delivered by a defibrillator machine. A defibrillator machine however may sometimes not be easily accessible.
Given the importance of early defibrillation in this setting, scientists started working on designing an internal defibrillator which could be inserted in a vulnerable patient and which could automatically detect the onset of ventricular tachycardia or ventricular fibrillation and automatically deliver a shock to the patient thereby saving the patient’s life.
In 1980, the first (rather bulky) implantable internal defibrillator was developed and by 1985, it had received FDA approval. Since then, rapid improvements in design and technology have meant that defibrillators have become much smaller in size and more effective not only in shocking the heart out of ventricular fibrillation but also detecting ventricular tachycardia and preventing it from degrading into ventricular fibrillation. Multiple studies have also confirmed the safety and efficacy of such devices in patients with a high risk of sudden death.
Apart from delivering shocks, defibrillators have other qualities which can be very beneficial.
They are exceptionally good monitoring devices. Patients with heart disease may also be susceptible to other heart rhythm disturbances such as Afib (which may be ‘silent’ i.e that the patient may not even know about it). In this setting the defibrillator will detect it and when the defibrillator is interrogated, episodes of Afib will be detected. This is important because the presence of any Afib points to the patient being at a higher risk of strokes and therefore the patient would be considered for long term anticoagulation.

They are able to act as pacemakers. Patients with heart disease may be liable to developing very slow heart rates and sometimes the medications that are used to stop episodes of ventricular tachycardia or even stop the heart from weakening further can paradoxically push the heart rate down to very low levels. In this situation, the defibrillator will function as a pacemaker and stop the heart rate falling too low.

They can deliver treatments other than defibrillation. Sometimes when the defibrillator detects that the patient has developed ventricular tachycardia, it tries to treat the ventricular tachycardia before the ventricular tachycardia can degrade into ventricular fibrillation. This is called anti-tachycardia pacing (ATP) and can terminate the ventricular tachycardia painlessly and therefore obviate the need for a (painful) shock.

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