Lecture 4 Defibrillator

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Lecture 4Defibrillator
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Arrhythmias SA Block
QRS T
P
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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

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Defibrillators

• Before 1960 were AC model
• This machine applied 5 to 6 A of 60 Hz across the
  patients chest for 250 to 1000 ms.
• The success rate for AC defibrillator was rather
  low

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• 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

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DC types

• 1- lown
• 2- monopulse
• 2- tapered (dc) delay
• 3- trapezoidal wave.

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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.
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Lown wave form defibrillator
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• 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.

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• 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.

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• 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 paddleskin resistance, RE .

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• 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.

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• 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

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EXAMPLE

• A defibrillator has an available energy, WA, of
  200 joules (J).
• If the thorax resistance is 40 ohms (W), the
  electrodeskin 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.

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Solution

• In this case, RT 40 ohms, RE 30 ohms, and RD
  10 ohms. The equation for the amount energy
  delivered yields

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• 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 LC sections
• Trapeziodal low voltage / long duration ( 800 V
  500 V 20 ms

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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

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Defibrillator Electrodes
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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