Post on 21-Dec-2015
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