1. Lecture 4 Defibrillator

2. Arrhythmias: SA Block P
QRS T

3. 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

4. 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

5. 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

6. DC types 1- lown 2- monopulse 2- tapered (dc) delay
3- trapezoidal wave.

7. 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.

8. Lown wave form defibrillator

9. 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.

10. 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.

11. 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 .

12. 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.

13. 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

14. 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.

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

16. 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

17. 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

18. Defibrillator: Electrodes

19. 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
ECG
Electrodes
Analog Switch
Trigger
Circuit
Defibrillator
Defibrillation
Cardioscope
30ms
Delay
Threshold
Detector
Filter
Operator-controlled
Switch
ECG AMP
AND
Gate