In The Name of Allah The Most Beneficent The Most Merciful 1

ECE 4550: Biomedical Instrumentation Lecture: Defibrillators Engr. Ijlal Haider University of Lahore, Lahore 2

NEED FOR A DEFIBRILLATOR  Ventricular fibrillation is a serious cardiac emergency resulting from asynchronous contraction of the heart muscles.  Due to ventricular fibrillation, there is an irregular or rapid heart rhythm. Fig. Ventricular fibrillation Fig. Normal heart beat

 Ventricular fibrillation can be converted into a more efficient rhythm by applying a high energy shock to the heart.  This sudden surge across the heart causes all muscle fibres to contract simultaneously.  The instrument for administering the shock is called a DEFIBRILLATOR.  Possibly, the fibres may then respond to normal physiological pacemaking pulses. NEED FOR A DEFIBRILLATOR

5 TYPES OF DEFIBRILLATORS InternalExternal

TYPES OF DEFIBRILLATORS a) Internal defibrillator Electrodes placed directly to the heart Eg.-Pacemaker b) External defibrillator Electrodes placed directly on the heart Eg.-AED

DEFIBRILLATOR ELECTRODES  Types of Defibrillator electrodes:- a) Spoon shaped electrode Applied directly to the heart. b)Paddle type electrode Applied against the chest wall c)Pad type electrode Applied directly on chest wall

DEFIBRILLATOR ELECTRODES

Fig.- Pad electrode DEFIBRILLATOR ELECTRODES

 Electrodes have insulated handles  Designed to prevent the spread of jell from electrodes to handles for the safety and ease of operator

PRINCIPLE OF DEFIBRILLATION  Energy storage capacitor is charged at relatively slow rate from AC line.  Energy stored in capacitor is then delivered at a relatively rapid rate to chest of the patient.  Simple arrangement involve the discharge of capacitor energy through the patient’s own resistance.

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PRINCIPLE OF DEFIBRILLATION

 The discharge resistance which the patient represents as purely ohmic resistance of 50 to 100Ω approximately for a typical electrode size of 80cm2.  This particular waveform Fig 13.9(b) is called ‘ Lown’ waveform.  The pulse width of this waveform is generally 5-10 ms.

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16  Peak current of 50 A  Time 5 sec  Resistance 50 ohm  Power delivered = 320 J  To overcome losses across capacitor and inductor  25% additional energy required  Energy to be stored is 400 J  Capacitor of 16 micro farad  Voltage required is 7000V

17  For Internal electrodes only 50 J is needed

Classes of discharge waveform Monophasic pulse or waveform Bi-phasic pulse or waveform

Classes of discharge waveform  There are two general classes of waveforms: a)mono-phasic waveform Energy delivered in one direction through the patient’s heart a)Biphasic waveform Energy delivered in both direction throuth the patient’s heart

Classes of discharge waveform Fig:- Generation of bi-phasic waveform

Classes of discharge waveform  The biphasic waveform is preferred over monophasic waveform to defibrillate.why????? A monophasic type, give a high-energy shock, up to 360 to 400 joules due to which increased cardiac injury and in burns the chest around the shock pad sites. A biphasic type, give two sequential lower-energy shocks of joules, with each shock moving in an opposite polarity between the pads.

Classes of discharge waveform  Dual Peak Waveform  10 msec  Truncated Waveform  Time can be adjusted

AUTOMATIC EXTERNAL DEFIBRILLATOR

 AEDs require self-adhesive electrodes instead of hand held paddles.  AED is a type of external defibrillation process. AUTOMATIC EXTERNAL DEFIBRILLATOR  AED is a portable electronic device that automatically diagnoses the ventricular fibrillation in a patient.  Automatic refers to the ability to autonomously analyse the patient's condition.  The AED uses voice prompts, lights and text messages to tell the rescuer what steps have to take next.

ELECTRODE PLACEMENT OF AED Anterior electrode pad Apex electrode pad Fig. anterior –apex scheme of electrode placement

WORKING OF AED  turned on or opened AED.  AED will instruct the user to:- Connect the electrodes (pads) to the patient. Avoid touching the patient to avoid false readings by the unit. The AED examine the electrical output from the heart and determine the patient is in a shockable rhythm or not.

When charged, the device instructs the user to ensure no one is touching the victim and then to press a red button to deliver the shock. when device determined that shock is warranted, it will charge its internal capacitor in preparation to deliver the shock. WORKING OF AED  Many AED units have an 'event memory' which store the ECG of the patient along with details of the time the unit was activated and the number and strength of any shocks delivered.

28 Courtesy Texas Instruments

Testing Defibrillators A dummy load is connected across the defibrillator electrodes Discharge is observed 50 ohm resistance is taken as human resistance connected in series with a low value resistor. Neon light is connected with low value resistor and lights when defibrillator is discharged

Testing Defibrillators Some defibrillators have built-in test system If they do not have, it can be designed and mounted on the trolley

31 Current requirements normally range up to 20 A. Voltage ranges from 1000V to 6000V Time of discharge is kept from 5 to 10 msec Current is dependent on the body (chest) resistance For reference visit: ic-waveform/ ic-waveform/ And various other resources on internet

Thank You! 32