2. defibrillators

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D E F I B R I L L A T O R

B Y

M . W A S I M M U N I R

Bio-Instrumentation I

Defibrillation2

Defibrillation is based upon the understanding that contraction of the heart and the resulting circulation is under the control of the electrical conduction system of the heart

Defibrillation is a common treatment for life-threatening cardiac dysrhythmias, ventricular fibrillation and ventricular tachycardia

Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a device called a defibrillator

Department of Biomedical Engineering, SSUET

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NEED FOR A DEFIBRILLATOR3

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


NEED FOR A DEFIBRILLATOR4

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

Possibly, the fibres may then respond to normal physiological pacemaker pulses

The instrument for administering the shock is called a DEFIBRILLATOR


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5Department of Biomedical Engineering, SSUET

TYPES OF DEFIBRILLATORS


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Internal External

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TYPES OF DEFIBRILLATORS

a) Internal defibrillator

• Electrodes placed directly to the heart

• E.g. - Implantable Cardioverter Defibrillator (ICD)

b) External defibrillator

• Electrodes placed directly on the heart

• E.g. - Automated External Defibrillator (AED)

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DEFIBRILLATOR ELECTRODES8

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


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Paddle Electrode Pad Electrode


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DEFIBRILLATOR ELECTRODES



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DEFIBRILLATOR ELECTRODES11

Paddle-type Electrodes have insulated handles

Designed to prevent the spread of jell from electrodes to handles for the safety and ease of operator


Automated External Defibrillator Electrode



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PRINCIPLE OF DEFIBRILLATION13

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


PRINCIPLE OF DEFIBRILLATION


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A shorter high-amplitude defibrillation pulse can be obtained by using the capacitive-discharge circuit shown in figure (previous slide)

In this case, a half-wave rectifier driven by a step-up transformer is used to charge the capacitor C

The voltage to which C is charged is determined by a variable autotransformer in the primary circuit


PRINCIPLE OF DEFIBRILLATION


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A series resistance R limits the charging current to protect the circuit components, and an ac voltmeter across the primary is calibrated to indicate the energy stored in the capacitor

The resistor also helps to determine the time necessary to achieve a full charge on the capacitor

The clinician discharges the capacitor when the electrodes are firmly in place on the body by shortly changing the switch S from position 1 to position 2

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The capacitor is discharged through the electrodes and the patient's torso, which represent a primarily resistive load, and the inductor L

The inductor tends to lengthen the pulse, producing a wave shape of the type shown in figure (previous slide 14)

This situation is determined entirely by the resistance between the electrodes, which can vary from patient to patient



Once the discharge is completed, the switch automatically returns to position 1, and the process can be repeated if necessary

With a circuit such as this, 50 to 100 J is required for defibrillation, using electrodes applied directly to the heart

When external electrodes are used, energies as high as 400 J may be required


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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 through the patient’s heart

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Monophasic pulse or waveform Bi-phasic pulse or waveform


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Fig:- Generation of bi-phasic waveform



The biphasic waveform is preferred over monophasic waveform to defibrillate

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 120 - 200 joules, with each shock moving in an opposite polarity between the pads

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About Lab23

About Lab files Do not copy paste word to word from Internet

Do not make a photocopy of your friend’s lab file

Marks will be deducted in both above cases

Final viva will be conducted based upon whatever you written in your lab file, make sure you know everything


CARDIOVERTER


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When an operator applies an electric shock of the magnitude of that from a dc defibrillator to the patient's chest during the T wave of the ECG, there is a strong risk of producing ventricular fibrillation in the patient

To avoid this problem, special defibrillators are constructed that have synchronizing circuitry so that the output occurs immediately following an R wave, well before the T wave occurs

Figure in the next slide is a block diagram of such a defibrillator, which is known as a cardioverter

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CARDIOVERTER


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CARDIOVERTER


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Basically, the device is a combination of the cardiac monitor and the defibrillator

ECG electrodes are placed on the patient in the location that provides the highest R wave with respect to the T wave

The signal from these electrodes passes through a switch that is normally closed, connecting the electrodes to an appropriate amplifier

The output of the amplifier is displayed on a cardioscopeso that the operator can observe the patient's ECG to see, among other things, whether the cardioversion was successful-or, in extreme cases, whether it produced more serious arrhythmias

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AUTOMATIC EXTERNAL DEFIBRILLATOR


AED

AED is a portable electronic device that automatically diagnoses the ventricular fibrillation in a patient

Automatic refers to the ability to autonomously analyze the patient's condition

AEDs require self-adhesive electrodes instead of hand held paddles

The AED uses voice prompts, lights and text messages to tell the rescuer what steps have to take next

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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

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http://en.wikipedia.org/wiki/File:Defib_electrode_placement.JPG

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WORKING OF AED

When device determined that shock is warranted, it will charge its internal capacitor in preparation to deliver the shock

When charged, the device instructs the user to ensure no one is touching the patient and then to press a red button to deliver the shock

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

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• 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


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IMPLANTABLE DEFIBRILLATORS33

An implantable cardioverter defibrillator (ICD) resembles a pacemaker, but its circuitry is similar to that in an AED

The battery, capacitor, and electronics are enclosed in a metal case which is implanted under the skin in the chest

The typical size of the case, or ‘‘can’’ is about 50x50x15mm


ICD34

The capacitors in an ICD are slightly smaller than in an AED, but in an ICD the capacitor is charged to a voltage of only approx. 600 V, implying a charge of 0.075 C and an energy of 23 J

An ICD delivers about one tenth the energy that an AED does, but in an ICD the shock is delivered through electrodes placed within the heart and is therefore just as effective for defibrillation

Tissue impedance for an ICD is at least 50 ohms, implying a discharge time constant of 5 ms


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ICD35

Lithium batteries use in ICDs

Two batteries in series provide about 6 V

Since the capacitor voltage is 600 V, the batteries are used to power a high voltage power supply

They are implanted in the patient’s body, so changing them requires surgery, implying that battery lifetime is important the battery performance begins to decay before its total charge is exhausted

Also it must provide power for continuous monitoring of the ECG and other functions, so its observed lifetime is 5 years


ICD36

Another important property of a battery is the time required to charge the capacitor

Typically, the battery takes 10–20 sec to generate a full charge

If this time increased significantly, it would delay the delivery of the shock


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ICD37

The electrodes and their leads are critical components of an ICD

Unlike the electrodes in an AED, ICD electrodes are implanted inside a beating heart and must continue to function there for years

Many ICD malfunctions arise because of problems with the leads

A typical lead contains three electrodes: one for pacing and sensing and two for defibrillation


ICD38

The ICD recording lead senses the several-millivoltECG signal within the heart

Two parameters that the ICD uses to detect abnormal arrhythmias are heart rate and arrhythmia duration

The ICDs use sophisticated algorithms to determine from the ECG if an arrhythmia is present, and these algorithms differ between manufacturers

Sufficient memory is included in the ICD to store ECGs before, during, and after a shock


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Reference39

• For reference visit:

• Book: Encyclopaedia of Medical Devices and Instrumentation 2nd ed. J. Webster (Wiley 2006)

• http://www.resuscitationcentral.com/defibrillation/biphasic-waveform/

• http://www.nhlbi.nih.gov/health/health-topics/topics/aed/

• http://www.nhlbi.nih.gov/health/health-topics/topics/icd/

• And various other resources on internet


http://www.resuscitationcentral.com/defibrillation/biphasic-waveform/




http://www.nhlbi.nih.gov/health/health-topics/topics/aed/



http://www.nhlbi.nih.gov/health/health-topics/topics/icd/



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2. defibrillators