Needed for defibrillator?

* Ventricular fibrillation is a serious cardiac emergency resulting from asynchronous contraction of the heart muscles.

*This un coordinated movement of the ventricle walls of the heart may result from coronary occlusion , from electric shock or from abnormalities of the body chemistry

*Because of this irregular contraction of the muscles fibers, they ventricles simply quiver rather than pumping the blood effectively.

*This results in a steep fall of cardiac out put and can prove fatal if adequate steps are not taken promptly.

DefibrillatorDefibrillator *a defoliator is a device that delivers electric shock

to the heart muscle undergoing a fatal arrhythmia. Defibrillator before 1960 were ac models.

*The success rate for ac defibrillator was rather than low, however, and the technique was useless

for correcting atrial fibrillate.

*These machines applied 5 to 6 A of 60 Hz ac across the patient’s chest for 250 to 1000 ms .

*Science 1960 several different dc defibrillators have been designed.

*These machine store a dc charge that can be

delivered to the patient .

*The principle difference between dc defibrillators is in the waves shape of the charge delivered to the patient the most common forms are the lown, monopulse, tapered (dc) and trapezoildal waveforms.

The LOwn waveform is shown in the figure:

U = ½ CV²

U= the energy in joules

C= the capacitance

V= the voltage across the c

Dc defibrillatorDc defibrillator

*In almost all present-day transthoracic

defibrillator, an energy storage in the capacitor is

charged at relatively slow rate from the Ac line by

means a set-up transformer and rectifier

arrangement or from a battery and Dc to Dc

converter arrangement.

*The shape of the wave form that appears across electrodes will depend upon the value of the capacitor and inductor used in the circuit .

*The discharge resistance which the patient represents for the defibrillating pulse may be regarded as purely ohmic resistance of 50 Ω to 100 Ω approximately for a typical electrode size of 80 cm²

*When the defibrillator is charged and not fired the instrument is potential source for danger.

*In some instruments, the capacitor is automatically discharged internally through a resistance when it is not fired, say within 5 min.

*The basic circuit diagram of Dc defibrillator is showing in the figure

a variable transformer T1 forms the primary of high voltage transformer T2 The out but voltage of the transformer is rectified by a diode rectifier and is connected to a vacuum type high voltage change over switch.

*In the 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 to position B and the capacitor is discharge across the heart through the electrodes.

*In a defibrillator an enormous voltage (4000v) is initially apply to the patient .

the sequence of events is described as the

following:

1 -the operator turns the set energy control to the desired level and presses the charge button

) S2 closes (

2 -capacitor C1 begins charging and will continue to charge until

*The energy from a defibrillator is delivered through a set of high-voltage paddle electrodes.

Several popular styles are:

1 -anterior paddle. 2- posterior paddle 3- anterior

*paddle D ring types is showing in the figure it used on most current model defibrillators and has been popular on portable models for some time

Defibrillator electrodesDefibrillator electrodes::

*The electrodes for external defibrillation are usually metal discs about 3-5 cm in diameter and are attached to highly insulated handles .

*Most of the conventional electrode system are circular, a little concave with sharp rims and an insulated back-side.

*For internal defibrillation when the chest is open.

*Large spoon-shaped electrode are used.

*Usually, large currents are required in external defibrillation to produced uniform and simultaneous contraction of the heart muscle

fiber.

*The external electrodes contain safety switches inside the housings and the capacitor is discharge only one when the electrodes are making a good and firm contact with the chest of the patient .

*A number of instruments are provided with electrodes which have a spring contact .

*When these electrodes are adequately pressed on the thorax, the contacts close and defibrillator is fired.

Defibrillator with synchronousDefibrillator with synchronous: :

Cardioversion:

*In certain types of arrhythmia the patient’s ventricles maintain their ability to pump blood, as evidenced by the existence of R wave feature in the ECG wave form .

*These arrhythmias are also correctable by electrical shock during the ventricles’ refractory period (the T waveform ) or the shock intend to correct the problem will create a much more serious arrhythmia

such as ventricular fibrillation. The shock is usually timed to occur approximately 30 µs after the R

wave peak .

*Human operator cannot be trusted to time the ECG waveform properly to avoid this problem so

an automatic electronic circuit is used. *A machine equipped with the synchronize circuit is

called a cardioverter .

Figure explain how this instrument work with the Figure explain how this instrument work with the

defibrillatordefibrillator

*When the switch S2 is in the defibrillate position the circuit operates in the manner of other circuit depressing S1 energizes the really, discharging the capacitor.

*But when switch S2 is in the cardioverter position the relay is not energized unless S1 is closed and the silicon-controlled rectifier (SCR) is turn on. The SCR is turned on by an ECG R wave.

REPAIR TIPSREPAIR TIPS

Symptom: No Output *Cables of pads may be lose or not properly inserted.

*Cables may be damaged and has to be replaced.*Switch button may be damaged, replace button.

*Power board on transformer side may be damaged, check the *Transformer output and electronic components on board.

Symptom: No Synchronization *One or more controller on digital board are faulty.

* Check the controller board

Symptom: Low Energy Transfer

*Low supply voltage. *Capacitor may not be charged properly, check the connections. *Capacitor may be de-value.

Symptom: Not Fully Charging

*Battery may be weak, recharge battery or change. *Failure in Charging circuit. *Charging Voltage calibration may be de-value.

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