Two types of defibrillators are showing below.
- AC defibrillators
- DC defibrillators
AC defibrillator is the oldest and simplest type. The construction of AC defibrillator is such that appropriate values are available for internal and external defibrillation. In AC defibrillation, a shock of 50 Hz a.c frequency is applied to the chest for a time of 0.25 to 1 second through electrodes. The procedure of applying electric shock to resynchronize heart is known as Countershock. Defibrillation continues until patient responds to the treatment.
AC defibrillator consists of a step-up transformer with primary and secondary winding, and two switches. A.C supply is given through switches and fuse to primary winding of the transformer. The timing circuit is connected with switch, which is used to preset the time for the defibrillator to deliver shock to the patient.
A resistive and a simple capacitor network or monostable multivibrator forms the timing circuit. It is triggered with a foot switch or a push button switch. Various tapping are available along the secondary winding. They are connected to the electrodes that delivers electric shock to the heart of the patient. Voltage value ranging between 250 V to 750 V is applied for AC external defibrillation. For safety reasons, secondary coil should be isolated from earth to avoid shock. For internal fibrillation voltage values between 60 V to 250 V is applied. To produce uniform and simultaneous contraction of heart muscles large currents are used for external defibrillation. However, this results in skin burn under electrodes and violent contraction of heart muscles. It also results in atrium fibrillation and stops ventricular fibrillation.
DC defibrillator does not produce side effects and produces normal heartbeat. Ventricular fibrillation is avoided when high-energy shock is passed through discharging capacitor that is exposed to heart or chest of the patient. DC defibrillator consists of auto transformer T1 that acts as primary of the high voltage transformer T2. A diode rectifier rectifies the output voltage from T2. It is connected to vacuum type-high voltage over switch. At position A, switch is connected to one end of the capacitor. When connected in this position capacitor charges to a voltage. A foot switch present on the handle of the electrode is used to deliver shock to the patient. Now the high voltage switch changes it position to B that makes the capacitor to discharge to the heart through electrodes. To slow down the discharge from the capacitor an inductor L is placed in one of the electrode lead. This L induces a counter voltage that reduces the capacitor discharge value.