DESCRIBE PRINCIPLE OF DC DEFIBRILLATOR AND UNDER WHAT

DESCRIBE PRINCIPLE OF DC DEFIBRILLATOR AND UNDER WHAT CONDITION YOU NEED TO USE DC DEFIBRILATOR

PRINCIPLES • A short high amplitude defibrillation obtained by using capacitive discharge circuit. • Used a half wave rectifier driven by a step up transformer to charge the capacitor • During defibrillation, the energy stored in the capacitors then discharge or delivered to the chest through the patient own resistance (usually 50 ohms) at a relatively rapid rate (ms) • energy level of dc defibrillator: 2 -400 J • Required voltage : 1000 v to 6000 v • Current range: 1 -20 A

ü An energy storage capacitor is charged at relatively slow rate from: * Ac line by means of a step up transformer and rectifier arrangement * A battery and DC to DC converter arrangement

üThe variable auto-transformer T 1 form primary of high voltage of T 2. ü Diode D rectifies the ac to charge capacitor C (16 microfarad ) ü when the shock delivered to patient, a foot witch mounted on the handle of electrode to operated. ü the voltage switch changes over to position ‘B’ and capacitor discharged across the heart through the electrodes. ü An enormous voltage = 4000 V applied initially to the patient ü high current impairs the contractility of the ventricles although short duration pulse (20 microsecond) can affect defibrillator. üInductors used to prolong the duration of current flow. L-current limiting inductor used to protect patient.

ü Disadvantages any practical inductor have its own resistance which dissipates parts of energy during the discharge process ü Inductors also slows down the discharge from the capacitor by the induced counter voltage. ü The actual delivered energy ranges from 335 J for 35 ohms and 405 J at 125 ohms. üEnergy level of defibrillator depends on: *the Vp of defibrillator by varying the setting on the varactor * duration of the defibrillator pulse ü The energy stored in the capacitor can be determine using this equation: E= ½ C(V)^2

WAVEFORM Lown down ü Curve 1 – typical discharge pulse of defibrillator which called “lown ” waveform. ü I rises rapidly to app. 20 A, then decay to 0 with 5 ms ü a negative pulse is produces for 1 to 2 ms ü the pulse width defined as the time that elapse between the start of the impulse and the moment that the current intensity passes the zero line for the 1 st time and changes direction (5 ms or 2. 5 ms) Current versus patient impedence

MONO- PHASIC ü the delivered energy through the patient’s chest is in single direction ü current flows in one direction from 1 electrode to the other, from positive to negative electrode ü high level of energy Waveforms. Current delivered in one direction

BI-PHASIC ü the delivered energy through the patient’s chest in 2 direction. üDeliver current in 2 direction. üThe Bi-phasic waveform reverses the direction of the electrical energy near the midpoint of the waveform. üLow-energybiphasic shocks may be as effective as higher-energy monophasic shocks. üBiphasic waveform defibrillation used in implantable cardioverterdefibrillators (ICDs) and automated external defibrillators (AEDs). Current delivered in 2 direction

ü for internal defibrilator, the energy up to 100 Ws required whereas higher energy for external. ü the DC defibrillator cannot bu used for rapidlly repeated shocks because it requires about 10 s to recharge capacitors. ü the amount of energy delivers to patient can be a factor to determine a successful defibrillator. üThe delivered energy to the patient can be estimated by assuming the value of a load resistance which is placed between the electrodes & thus simulates the resistance of the patient.

üfor external patient’s resistance=50 -200 ohms are taken and for internal, patient’s resistance = 25 -50 ohms are taken üMost defibrillator deliver= 60% to 80% of stored energy to a 50 ohms load. üWhen defibrillator is charged and not fired, the instrument is a potential to danger. üIn some instrument, capacitor is automatically discharged internally through a resistance when it is not fired in about 5 mins.

DC DEFIBRILLATOR ELECTRODES • The connection between the defibrillator and the patient consists of a pair of electrodes. • Each electrode provided with electricity conductive gel in order to ensure a good connection and to minimize electrical resistance, also called chest impedance (despite the DC discharge). • The electrodes for external defibrillation are metal discs about 3 -5 cm in diameter (or rectangular flat paddle 5 x 10 cm) and attached to highly insulated handles. • Big size because of the large current, which is needed by the external defibrillation.

• The size of electrodes plays an important part in determining the chest wall impedance, which influence the efficiency of defibrillation. • Contain safety switches inside the housing. • The capacitor is discharged only when the electrodes are making a good and firm contact with the chest of the patient. • For internal defibrillation when the chest is open, large spoon-shaped electrodes are used. • Paddle electrodes, which were the first type developed, come without gel, and must have the gel applied in a separate step. • Self-adhesive electrodes come prefitted with gel.

PADDLE ELECTRODES • The traditional metal paddle with an insulated (usually plastic) handle. • Must be held in place on the patient's skin with approximately 25 lbs of force while a shock or a series of shocks is delivered. • Paddles are re-useable, being cleaned after use and stored for the next patient. • Gel is therefore not pre applied, and must be added before these paddles are used on the patient. • Paddles are generally only found on the manual external units.

SELF ADHESIVE ELECTRODES • Newer types of resuscitation electrodes are designed as an adhesive pad, which includes either solid or wet gel. • These are peeled off their backing and applied to the patient's chest when deemed necessary, much the same as any other sticker. • The electrodes are then connected to a defibrillator, much as the paddles would be. • Electrodes minimize the risk of the operator coming into physical (and thus electrical) contact with the patient as the shock is delivered by allowing the operator to be up to several feet away. • Self-adhesive electrodes are single-use only.

EXTERNA L Electrode placement for DC defibrillator INTERNAL

DC DEFRILLATOR WITH SYNCHRONIZER

� � � Used when the heart experience ventricular tachycardia, atrial fibrillation and other arrhyhtmias. Less direct risk for patients with auricular fibrillation. Pump action of ventricles still exist. Defibrillation of heart in auricular fibrillation may bring the ventricles into fibrillators. There is a period in the heart cycle where the danger is leat.

� � � The ECG is fed to the defibrillator and the shock is given at the right moment. Synchronizer circuit will permit placement of discharge at the right point on patient’s ECG. The shock pulse during vulnerable T wave is avoided(~likely to produce Vfib) The shock is delivered ~20 to 30 ms after the peak of the R wave of the patient’s ECG. For effective and efficient results, the output of the defibrillating circuit is kept isolated or floating.

� � Floating circuit: The total E is always contained between the 2 electrodes; ~no loss of E to extraneous ground and highly efficient. Moreover, there is no direct path to ground and the danger of shock by accidental contact does not exist-safety.

Comparison of DC and AC defibrillator � � A DC defibrillator can deliver several hundred watt-seconds of energy over a period of few milliseconds, but AC defibrillator can’t. DC defibrillators are usually constructed to deliver energy to a patient in a shorter period than AC defibrillators.

� Advantage of DC defibrillator over AC defibrillator. 1. 2. 3. 4. 5. � Higher peak current available. Shorter duration of impulse. Lesser degree of myocardial injury. Lower incidence of post-shock arrhythmias. More consistently effective than AC type defibrillator. From the experiences of the Vancouver General Hospital in the use of the DC defibrillator at the time of open-heart surgery, this hospital strongly suggest that the DC defibrillators superior to the AC defibrillators.

WHEN DC DEFIBRILLATOR BEING USE? � � A person having cardiac arrhythmias ventricular fibrillation/ pulseless ventricular tachycardia While DC defibrillator with synchronizer can even being use for atrial fibrillation.