Lecture 4
Defibrillator
Arrhythmias: SA Block
P
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
V2
WA  C
2
• where C is the value of the capacitance
measured in units of farads and V is the voltage
across the capacitor.
• 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
RT
WT 
WD
RD  2RE  RT
EXAMPLE
• A defibrillator has an available energy,
WA, of 200 joules (J).
– If the thorax resistance is 40 ohms (W), 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
RT
WT 
WD
RD  2RE  RT
40
WT 
200
10  2  30  40
WT  72.7 Joules
- 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
Cardioscope
ECG
Electrodes
Analog
Switch
Trigger
Circuit
Defibrillation
Electrodes
Defibrillator
ECG AMP
AND
Gate
30ms
Delay
Threshold
Detector
Operator-controlled
Switch
Filter