- AC Defibrillator Definition: An AC defibrillator is defined as the oldest and simplest type of defibrillator that uses a 50 Hz AC shock to resynchronize the heart.
- Countershock: Countershock is the procedure of applying an electric shock to the chest to restart the heart.
- DC Defibrillator Definition: A DC defibrillator is defined as a device that delivers a high-energy shock via a discharging capacitor to normalize the heartbeat, with fewer side effects than AC defibrillators.
- Voltage Levels: AC defibrillators use voltage values ranging from 250 V to 750 V for external defibrillation and 60 V to 250 V for internal defibrillation.
- Safety and Side Effects: DC defibrillators are preferred because they do not cause the side effects associated with AC defibrillators, such as skin burns and atrium fibrillation.
Two historical circuit types of defibrillators are listed below. Early equipment applied alternating current (AC) for a set interval. Later direct-current (DC) designs stored energy in a capacitor and released it as a short pulse. Modern external defibrillators are described by their monophasic or biphasic shock waveform, with biphasic waveforms preferred in current adult resuscitation guidance.
- AC defibrillators
- DC defibrillators
AC Defibrillators
An AC defibrillator was an early design that applied mains-frequency alternating current through internal or external electrodes. The shock was intended to terminate ventricular fibrillation so that an organised rhythm could return. Defibrillation does not simply restart a stopped heart, and terminating ventricular fibrillation does not guarantee a pulse or recovery.
The historical circuit used a step-up transformer with switched primary and secondary windings. A timer limited the interval for which AC reached the electrodes. This topology is useful for explaining early defibrillator electronics, but it is not the waveform used by current capacitor-discharge devices.
A resistor-capacitor timer or a monostable multivibrator could control the switching interval after a foot switch or push button was pressed. Transformer taps selected an output. Fixed voltage ranges alone do not define a safe clinical dose because delivered current and energy also depend on waveform, electrode contact and patient impedance.
The patient circuit requires isolation and controls that meet the safety and performance standard for cardiac defibrillators. Early AC shocks could cause skin burns and myocardial damage because substantial current flowed for longer than in a capacitor-discharge pulse. AC defibrillation was therefore replaced by pulsed DC designs; it should not be presented as a current treatment option.
DC Defibrillators
A DC defibrillator charges an energy-storage capacitor and then discharges it through a waveform-shaping circuit and the electrodes. The shock can terminate ventricular fibrillation or pulseless ventricular tachycardia, but it does not guarantee a normal heartbeat and is not free of adverse effects. In the simplified circuit, an auto transformer T1 supplies the primary of a high voltage transformer T2.
A diode rectifier converts the T2 output so the storage capacitor can charge. In switch position A, the capacitor charges to the selected voltage. A controlled switch then isolates the charging path before the device delivers the pulse through the electrodes.
In switch position B, the capacitor discharges through the patient circuit. An inductor L can shape the pulse by opposing a rapid change in current and extending the capacitor discharge. Current devices use controlled monophasic or biphasic waveforms and energy settings specified by the manufacturer rather than the fixed AC voltage bands in the older circuit.
