Defibrillation is a common treatment for life-threatening cardiac dysrhythmias, ventricular fibrillation, and pulseless ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a device called a defibrillator. This depolarizes a critical mass of the heart muscle, terminates the dysrhythmia, and allows normal sinus rhythm to be reestablished by the body's natural pacemaker, in the sinoatrial node of the heart. Defibrillators can be external, transvenous, or implanted, depending on the type of device used or needed. Some external units, known as automated external defibrillators (AEDs), automate the diagnosis of treatable rhythms, meaning that lay responders or bystanders are able to use them successfully with little, or in some cases no training at all
Defibrillators are key devices in maintaining proper cardiac function. A bit of background in cardiology will aid in the understanding of the role of defibrillators. First, ventricular fibrillation is a cardiac condition where individual heart muscles contract in a random, uncoordinated way. The heart seems to shiver, and blood circulation stops. The application of an electric shock to restore normal heart function is the only way to effectively treat a ventricular fibrillation and prevent death.
What is the PHYSICS behind how a defibrillator
There are three major components to consider when studying a defibrillator: a capacitor, an inductor, and a power supply. These three components will be explored in depth. Specifically, the interaction between these three components is what allows defibrillators to effectively restore proper cardiac rhythms.
One of the key components of a defibrillator is a capacitor. The capacitor of a defibrillator stores a large amount of energy in the form of electrical charge. Then, over a short period of time, the capacitor releases the stored energy. The capacitor itself contains numerous components: a pair of metal plate conductors and an insulator. The insulator is in the middle of the conductors and does not loose electrons. On the other hand, conductors easily loose electrons and promote current flow.
To quantitatively describe a capacitor, one calculates the capacitance, the ability to store charge. The formula to calculate capacitance relates charge (Q), voltage (V), and capacitance (C):
C = Q/V
A capacitor has 1 farad of capacitance if a potential difference of 1 volt is present across its plates, when they hold a charge of 1 coulomb. Capacitors typically have values of microfarads (µF = 10–6 F). According to the equation, capacitance is directly proportional to charge and indirectly proportional to voltage.
For a parallel plate capacitor, as in the case of a defibrillator, a relationship can be established between the capacitance, the dielectric constant, the area of plate overlap, and the distance between plates. Capacitance is directly proportional to area and indirectly proportional to distance between plates:
C = (Eo x A) / d
The mechanism of action of a defibrillator is depicted below in Figure 1.
When the switch is in position 1, direct current from the power supply is applied to the capacitor and electrons flow. Therefore current flows and a charge begin to build up on each electrode of the capacitor. Specifically, the lower plate is more negative and the upper plate is more positive. The build-up of opposing charges creates a potential difference across the plates (V) that opposes the electromagnetic force of the power supply (E).
Additionally, the electromagnetic force (E) can also be related to the area of plate overlap (A), the charge (Q), and the dielectric constant (Eo) with the following equation:
E = Q / (Eo x A)
Charging a Capacitor:
Charging a capacitor is an exponential process. Specifically, the work done (W) to move charge (Q) through a potential difference V is: W = VQ. Therefore, as the voltage increases more work is required.
The charged capacitor is a store of potential energy, which may be released on discharge. Thus, the amount of energy stored in a capacitor is CV and in order to store energy, work must be done.
Discharging a Capacitor:
In a defibrillator, the circuit (depicted in Figure 2) is completed when paddles are applied to the patient’s switch. Electrons on the negative, lower plate move through the patient and then to the upper plate. Thus three key steps happen in sequence: electric current flows, electrical energy is released, and the potential energy between the upper and lower plates is zero. As the electrons are transferred from the lower plate, the potential difference decreases. The rate of discharge declines as the potential difference between the upper and lower plates falls.
A graphical image differentiates between the chagrining and discharging of a capacitor. Permission was obtained to reproduce this image.
The energy that is delivered can be calculated using the following relationship: Energy = QV/2. Thus the energy delivered is directly proportional to stored charge and voltage. Additionally, the energy stored in a capacitor can be related to the electric field, area, and distance:
U = energy = ½ (Eo) E2 (Ad)
Thus, the energy of a capacitor is directly proportional to area of the plates and distance between the plates. Additionally, if the electric field doubles, the energy will quadruple.
Defibrillators are needed to shock the heart back in regular rhythm. Thus, the current that is delivered must last for a several milliseconds. However, a discharging capacitor delivers charge and current very fast. Inductors, coils of wire that produce a magnetic field when current flows through them, prolong the duration of current flow. Specifically, inductors generate electricity that opposes the motion of current passing through it. This opposition is called inductance. Inductors typically have values of microhenries (µH).
III.) Power Supply:
Step-up transformers are transformers that increase voltage. In the case of defibrillators, step-up transformers are used to convert the main voltage of 240 V AC to 5000 VAC. A step-up transformer is used in defibrillators because this allows the doctor to choose among different amounts of charge. The control switch is calibrated in energy delivered to the patient (J), because this determines the clinical effect or physical impact that a patient will experience. As an additional energy source, many defibrillators also have internal rechargeable batteries.
What are the patient and safety issues associated with defibrillators?
I.) Patient Issues:
Successful defibrillation depends on delivery of the electrical charge to the myocardium. Only part of the total current delivered (about 35 A) flows through the heart. The rest is dissipated. First, the skin and the rest of the body counteract the flow of the current. The skin and thoracic wall act as resistors in series.
R (eq) = R1 + R2 + R3
Other intrathoracic structures act as resistors in parallel.
R (eq) = (1/R1) + (1/R2) + (1/R3)
The total impedance is about 50–150 ohms; however, repeated administration of shocks in quick succession reduces impedance.
Key safety concerns exist regarding the use of defibrillators. These concerns must be taken into account before using a defibrillator on a patient with irregular cardiac rhythms:
The patient must not already be in sinus (normal) rhythm.
The leads of the defibrillator must be properly connected, ensuring current flow.
Placement of paddles should follow specific guidelines: they should be placed along the long axis of the heart, they should not cover the transdermal patches because they are flammable, they should not be placed near metal objects because currents will travel through the metal (path of least resistance) and cause burning.
All sources of oxygen must be removed from the patient during defibrillation, because it supports combustion.
No one from the medical staff should touch the bed, patient or any equipment connected to the patient during defibrillation.
Fluids may conduct electricity; therefore it is important to ensure that the immediate area is clean and dry.
The defibrillator should not be charged until the paddles are applied to the patient’s chest, because accidental discharge from open paddles may cause injury.
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