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DC defibrillator principle & defibrillator.

                          DC DEFIBRILLATOR:
  •  In almost all present-day DC defibrillators, an energy storage capacitor is charged at a relatively slow rate (seconds) from the AC line by means of a step-up transformer and rectifier arrangement or from a battery and a DC to DC converter arrangement
  • the energy stored in the capacitor is then delivered at relatively rapid rate (millisecond) to the chest of the patient. 
          
  • The basic circuit diagram of a DC defibrillator is shown in Figure.
  • A variable auto-transformer T1 forms the primary of a high voltage transformer T2. 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 µF 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 to position ‘B’ and the capacitor is discharged across the heart through the electrodes.
  • The high current electric pulse damages the contractility of the ventricles. This is overcome by inserting a current limiting inductor in series with the patient circuit. The disadvantage of using an inductor is that any practical inductor will have its own resistance and dissipates part of the energy during the discharge process.
  • The inductor also slows down the discharge from the capacitor by the induced counter voltage.
  • The discharge resistance the patient represents is of 50 to 100Ω.
  • A typical discharge pulse of the defibrillator is shown in Figure.
      
  • There are two general classes of waveforms: Mono-phasic and Biphasic.
  • Monophasic waveform use high levels of energy delivered in one direction through the patient’s heart whereas a biphasic waveform delivers energy in both directions. The biphasic waveform is preferred as it defibrillate more effectively than other types of waveforms.
  • for internal defibrillation, energies up to 100 Ws are usually required whereas higher energylevels are necessary for external defibrillation.
  • The DC defibrillator cannot be used for rapidlyrepeated shocks because it requires about 10 s to recharge the capacitor.

 

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