Basic Principles, Techniques and Electrodes of DC Defibrillator
An energy storage capacitor is charged at a relatively slow rate (in the order of seconds) from the AC line by means of a step-up transformer and rectifier arrangements or from a battery and a DC to DC converter arrangement. The energy stored in the capacitor is then delivered at a relatively rapid rate (in the order of milliseconds) to the chest of the subjects.
The simplest arrangements involves the discharge of capacitor energy through the patient’s own resistance (R).
The output voltage of the transformer is rectified by a diode rectifier and is connected to a vacuum type high voltage change-over switch. In position A, the switch is connected to one end of an oil-filled 16 micro-farad capacitor. In this position, the capacitor charges to a voltage set by the positioning of the auto-transformer. When the shock is to be delivered to the patient, a foot switch or a push button mounted on the handle of the electrode is operated. The high voltage switch changes over the position ‘B’ and the capacitor is discharged across the heart through the electrodes.
In a defibrillator, an enormous voltage (approx. 4000 V) is initially applied to the patient. It has short-duration pulses (as low as 20μs).
1.1. Defibrillator Electrodes
The electrodes of external defibrillation are usually metal discs about 3-5 cm is diameter and are attached to highly insulated handles.
For internal defibrillation when the chest is open, large spoon-shaped electrodes are used. Large currents are required in external defibrillation.
The external electrodes contain safety switches inside the housing and the capacitor is dis-charged only when the electrodes are making a good, the possibility of an accidental shock to the operator and the risk of burns to the patient.
Electrode gels are usually employed to reduce contact impedance of the interface and for that the impedance of the gel itself must be very low.
1.2. DC Defibrillator with Synchronizer
Thus, the function of the synchronizer circuit is to permit placement of discharge at the right point on the patient’s electrocardiogram. With the synchronizer unit, the shock is delivered approximately 20 to 30 ms after the peak of the R wave of the patient’s ECG.
The synchronizer unit contains within it, an ECG amplifier which receives the QRS complex of the ECG and uses this to trigger a time delay circuit. After an interval of the desired delay time (approximately 30 ms), the defibrillating capacitor is discharged across the chest through the electrodes.
The electrocardiogram of the patient is simultaneously monitored on a cardioscope. The moment the discharge take place, the synchronizer unit produces a marker pulse on these monitoring instruments to show the instant where the counter-shock has occurred in the ECG cycle. The defibrillator with synchronizer unit is normally preferred in coronary care units for use in cardiac emergencies.
1.3. Portable Defibrillator
The portable defibrillator operate on rechargeable batteries and therefore make use of DC-DC converters for stepping up the voltage required for charging the storage capacitor. The maximum energy delivered by portable defibrillator is generally 300Ω delivered into 50Ω load which is equivalent to about 400 Ws of stored energy.
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