A defibrillator, also called a shock generator, is a medical device used for defibrillation and cardioversion. It can terminate cardiac arrhythmias such as ventricular fibrillation and ventricular flutter (defibrillation) and ventricular tachycardia, atrial fibrillation, and atrial flutter (cardioversion) by delivering targeted electrical shocks.
Defibrillation is the treatment of life-threatening cardiac arrhythmias with direct current through the chest. This method must be performed as quickly as possible (within a few minutes) alongside cardiopulmonary resuscitation (CPR) to save the patient’s life.
Cardioversion (synonym: electrocardioversion) is ECG-synchronous defibrillation under short anesthesia. It is used, among other things, in atrial fibrillation to restore sinus rhythm.
In the following, the indications for defibrillation and cardioversion in acute therapy are presented.
Then, defibrillator therapy using the implantable cardioverter-defibrillator (ICD; earlier term AICD from automatic implantable cardioverter-defibrillator) in secondary prevention is presented.
Acute Therapy
Indications (areas of application)
- Cardiovascular arrest due to ventricular tachycardia.
- Cardiovascular arrest due to ventricular fibrillation
- Ventricular tachycardia with hemodynamic efficacy.
- Patients with unrecorded syncope (brief loss of consciousness) and a left ventricular ejection fraction (ejection fraction of the left ventricle) ≤ 40%.
Defibrillation is performed in:
- Pulseless ventricular tachycardia (VT).
- Ventricular flutter
- Ventricular fibrillation
Cardioversion (an ECG-synchronized defribrillation under brief anesthesia) is performed for the following arrhythmias:
- Ventricular tachycardia (ventricular tachycardia, i.e., rapid heart rhythms occurring exclusively in the main chambers of the heart) [unless controllable with medication].
- Atrial flutter
- Atrial fibrillation
The procedure
A defibrillator consists of an accumulator, a DC/DC converter, a capacitor, an output circuit and a control unit. Since the voltage of the accumulator is too small for defibrillation, a DC/DC converter must be used to generate a larger voltage. This is done with the help of a capacitor, which is charged to a previously set amount of energy.
At the push of a button, this delivers its stored energy (approximately 200 to 360 joules) to the patient. The voltage is applied between 1 and 20 milliseconds and is up to 750 volts. The current reaches up to about 15 amperes. The energy is delivered via large-area electrodes. This is done either by pressing them onto the patient’s chest with the hands (the so-called “paddles”) or by sticking them to the chest (“adhesive electrodes” or “fast patches”).
For details on “electrical cardioversion“, see the topic of the same name below.
Note: To avoid accidental shocks to paramedics or clinicians, they should wear protective equipment and gloves.
Secondary prevention
To prevent recurrences (the recurrence of arrhythmias), an implantable cardioverter-defibrillator (ICD; formerly called AICD from automatic implantable cardioverter-defibrillator) can be used to detect arrhythmias and convert them to sinus rhythm (normofrequency, regular heartbeat) through targeted electrical pulses (defibrillation; hyperstimulation).
For details on the “implantable cardioverter defibrillator,” see the topic of the same name below.
