Lecture 4

Defibrillator

Arrhythmias: SA BlockP QRS T

Defibrillators• The defibrillator is a device that delivers electric

shock to the heart muscle undergoing a fatal arrhythmia.

• Electric shock can be used to reestablish normal activity

• Four basic types of Defibrillators– AC Defibrillator– DC Defibrillator

Defibrillators

• Before 1960 were AC model

• This machine applied 5 to 6 A of 60 Hz across the patient’s chest for 250 to 1000 ms.

• The success rate for AC defibrillator was rather low

• Since 1960, several different dc defibrillators have been devised.

• This machines store a dc charge that can be delivered to the patient.

• The different between dc types in the wave shape of the charge delivered to the patient

DC types

• 1- lown

• 2- monopulse

• 2- tapered (dc) delay

• 3- trapezoidal wave.

Lown

- The current will rise very rapidly to about 20 A under the influence of slightly less than 3 kV .- The waveform then decays back to zero within 5 ms and then produces a smaller negative pulse also about 5 ms.

Lown wave form defibrillator

• That is, the capacitor stores energy, WA, which develops a voltage, V, across its metal plates. – The amount of energy in units of joules is

given by

• where C is the value of the capacitance measured in units of farads and V is the voltage across the capacitor.

2

2VCWA

• The energy stored in the capacitor is proportional to the square of the voltage between its plates. – The amount of energy typically stored in the

capacitor of a defibrillator, so that it can be later delivered to the patient, ranges from 50 to 400 joules.

• All of this energy does not get into the patient. – Some is lost in the internal resistance of the

defibrillator circuit, RD and some is wasted in

the paddle—skin resistance, RE .

• To calculate how much of this energy gets to the patient, resistance RT,

consider the equivalent circuit. – The four resistors in this

circuit are in series.

• Therefore, the current in each of them is the same. – And the energy absorbed by any one resistor

is proportional to the total available energy, according to the voltage division principle.

• The formula for the energy absorbed by the thorax,

WT is

DTED

TT W

RRR

RW

2

EXAMPLE

• A defibrillator has an available energy, WA, of 200 joules (J).

– If the thorax resistance is 40 ohms (), the electrode—skin resistance of a paddle with sufficient electrode gel is 30 ohms and the internal resistance of the defibrillator is 10 ohms.

• Calculate the energy delivered to the thorax of the patient.

Solution

• In this case, RT = 40 ohms, RE =30 ohms, and

RD = 10 ohms. The equation for the amount

energy delivered yields

DTED

TT W

RRR

RW

2

2004030210

40

TW

JoulesWT 7.72

- Monopulse is a modified lown waveform and commonly found in certain portable defibrillator.

- It is created by the same circuit of lown but without inductor L.

- Tapered delay wave form , a lower amplitude 1.2 kV and longer duration 15 ms to a chive the energy level

- It is created by placing two L–C sections- Trapeziodal low voltage / long duration ( 800 V :

500 V & 20 ms

Defibrillator: Electrodes

• Excellent contact with the body is essential– Serious burns can occur if proper contact is

not maintained during discharge

• Sufficient insulation is required– Prevents discharge into the physician

• Three types are used:– Internal – used for direct cardiac stimulation– External – used for transthoracic stimulation– Disposable – used externally

Defibrillator: Electrodes

Cardioverters

• Special defibrillator constructed to have synchronizing circuitry so that the output occurs immediately following an R wave– In patients with atrial arrhythmia, this prevents possible discharge

during a T wave, which could cause ventricular fibrillation• The design is a combination of a cardiac monitor and a

defibrillator

ECGElectrodes

AnalogSwitch

TriggerCircuit

DefibrillatorDefibrillationElectrodes

Cardioscope

30msDelay

ThresholdDetector Filter

Operator-controlledSwitch

ECG AMP

ANDGate