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 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

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

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