Understanding Defibrillators and Defibrillation

Defibrillator

• A defibrillator is a device used to reverse fibrillation of the heart
• Fibrillation causes the heart to stop pumping blood, which leads to brain damage

About Defibrillation

• It is a process in which an electronic device sends an electric shock to the heart
• The purpose is to stop an extremely rapid, irregular heartbeat, and restore the normal heart rhythm
• Commonly used to treat life-threatening cardiac dysrhythmia, ventricular fibrillation, and pulse less ventricular tachycardia
• The principle involves applying a high voltage electric current to the heart muscle

How a high voltage electric current is applied:

• Directly (internal defibrillator) through the open chest
• Indirectly (external defibrillator) through the chest wall to terminate ventricular fibrillation

Need for a Defibrillator

• Used in arrhythmias, ventricular fibrillation, or pulseless ventricular tachycardia
• Ventricular fibrillation leads to irregular contraction of muscle fibres
• Irregular contraction leads to ineffective pumping of blood from the left ventricle
• Ineffective pumping of blood from the left ventricle will cause a steep fall in cardiac output
• Ventricular fibrillation is converted into a more efficient rhythm by applying a high energy shock to the heart
• A sudden surge across the heart will cause all muscle fibres to contract simultaneously
• This sudden surge allows muscle fibres to respond to normal physiological pace making pulses
• The instrument for administering the shock is a defibrillator

Atrial Fibrillation

• A very fast, irregular heart rhythm in the upper heart chambers
• The waveform is almost similar to normal waveform but for the loss of 'P' waveform
• For atrial fibrillation, avoid delivering a shock in the T period, to prevent ventricular fibrillation
• Administer the shock by using synchronous mode

Block Diagram

• The defibrillator circuit includes a user interface, communication interface, memory and data logging, sensors and signal conditioning, and RTC/GPS interface
• It also includes a display control, display, audio/voice processing, high voltage boost control switching, and a standard device.
• Power supply and power on reset are also included

Schematic Diagram

• A variable auto-transformer T1 forms the primary of a high voltage transformer T2
• Rectify the output of the transformer by a diode D (half wave rectification)
• High voltage change-over switch 1 and 2 is a vacuum type
• Uses a 16 micro-farad [µF] capacitor C
• L current limiting inductor protects the patient, but has the disadvantage of own resistance
• The resistance causes a part of energy to dissipate during the discharge process
• The voltmeter AC is to indicate the energy stored in C
• RS limits the charging current to protect the circuit and determine the time for full charge on C (T=RC)
• The discharge resistance (R) represents what the patient represents (50 to 100 Ω
• In the first position, the capacitor charges to a voltage VP set by the positioning of the autotransformer (≈4000 V)
• When the shock is to be delivered, operate a foot switch or a push button mounted on the handle of the electrode
• The high voltage switch then changes over to position 2 and the capacitor discharges across the heart through the electrodes

Physics Behind Defibrillator

• There are three major components of a defibrillator: power supply, capacitor and inductor
• Step-up transformers are transformers to increase voltage
• Allows doctors to choose among different amounts of charge

Power Supply

• Output voltage is fed to a capacitor which stores the high voltage charge
• Many defibrillators have internal rechargeable batteries as an additional energy source
• Step up transformers are used to convert 240 VAC to 3000-5000 VAC
• Converted to DC by rectifier
• In battery mode the DC is converted to AC by inverter
• Amplified AC is rectified to DC again

Capacitors

• Stores a large amount of energy in the form of electric charge
• Stored energy releases over a short period of time
• Capacitance describes a capacitor quantitatively with the formula C = Q/V
• Capacitance is directly proportional to area and inversely proportional to the distance between plates
• Capacitance is C = (€0 x A) / d

Inductors

• Coils of wire that produce a magnetic field when current flows through them
• It prolongs the current flow duration
• Used to prolong the duration of current flow
• Opposition is called inductance

Energy Level

• The Energy level can be controlled by voltage amplitude (VP) by varying the varactor setting
• Also controlled by duration of the defibrillator pulse
• The energy (W) stored in the capacitor C and is available for the defibrillation using: W = (1/2)CV^2
• The discharge resistance represents the purely ohmic resistance of 50 to 100Ω, for the electrode size of 80cm2
• Pulse range: 3-8 ms at current of 10–27A (internal) and 22–60A (external)

Waveforms

• Curve 1 shows a typical discharge pulse of defibrillator is the "Lown" waveform
• The current rises rapidly to approximately 20 A
• Then it decays to 0 with 5 ms
• A negative pulse produced for 1 to 2 ms
• The pulse width is the time that elapses between the start of the impulse and the moment that the current intensity passes the zero line for the first time and changes direction, taking 5 ms or 2.5 ms

Synchronization Time

• Timing the shock to the R wave prevents delivering the shock during the vulnerable or relative refractory period of the cardiac cycle
• This could induce ventricular fibrillation

Synchronization Circuit

• Has an ECG amplifier, which receives the QRS complex of the ECG
• This triggers a time delay circuit
• After the desired delay time (approximately 30ms), the defibrillating capacitor discharges across the chest through the electrodes

Synchronized Cardioversion

• Narrow regular QRS complex tachycardia requires a dose of 50-100J
• Narrow Irregular complex tachycardia requires 120-200 J biphasic or 200 J monophasic
• Wide regular QRS complex tachycardia requires 100 J
• Wide irregular QRS complex tachycardia requires Defibrillation dose with NOT synchronized
• The delivered energy is in the range of 50-360 Joules
• Energy depends on the intrinsic characteristics of patient, and patient’s disease
• Energy depends on the duration of arrhythmia, patient’s age, and type of arrhythmia (more energy required for VF) and the machine used

Operating Principle

• Monophasic Defibrillator and Bi-Phasic Defibrillator
• A monophasic defibrillator delivers its current in one forward direction or positive
• It requires higher escalating energy levels (200-300J) to convert VF/ pulse-less VT
• A Bi-phasic waveform delivers the energy through the patient's chest in two directions
• It delivers current in two directions
• Biphasic waveform reverses the direction of the electrical energy near the midpoint of the waveform
• Low-energy biphasic shocks may be as effective as higher-energy monophasic shocks
• Biphasic waveform defibrillation is used in implantable cardioverter-defibrillators (ICDs) and automated external defibrillators (AEDs)

Defibrillator Electrodes

• Spoon shaped electrodes: applied directly to the heart
• Paddle type electrodes: applied against the chest wall
• Pad type electrodes: applied directly on chest wall
• The electrodes for external defibrillation are metal discs about 3-5 cm in diameter or rectangular flat paddle 5x10 cm
• Handle is attached to highly insulated handles
• The big size of the paddle allows the large current, which is needed by the external defibrillation and avoids burning under the electrodes
• The size of electrodes influences the efficiency of defibrillation
• It possesses safety switches inside the housing
• The capacitor discharges only when the electrodes are making a good and firm contact with the chest of the patient
• For internal defibrillation, when the chest is open, large spoon-shaped electrodes are used

Paddle Placement

• Two notable methods of paddle placement recommended by AHA
• Anterior-Anterior: Place one paddle near the second or the third right sternal border and the other on the cardiac apex
• Anterior-Posterior: one paddle on sternum and the other on the left infra-scapular region

Important Factors with Time

• Early defibrillation allows more success or the longer period of VF, the less success of defibrillation
• Early initiation of CPR improves the success rate because neurological damage sets in after > 8 minutes
• Survival probability becomes very low after > 10 mins

Energy Factors

• AHA recommends for adults that the first shock be 200 j
• The second shock 200 j to 300 j
• The third and above shocks be 360 j
• AHA recommends children get a first shock 2 joules per Kg and subsequent shocks that doubles the energy
• Defibrillation for Ventricular fibrillation and Pulseless ventricular tachycardia have Monophasic dosage of 360 J and a Biphasic dosage of 120-200 J
• If the dosage is unknown, use manufacturer recommended dosage
• Pediatric defibrillation requires 2J per Kg and Defibrillation using INTERNAL PADS/PADDLES need a Biphasic dosage of 5J, 10J, 20J, 30J, 50J (max) and a Monophasic dosage of 50 J maximum

Paddle Factors

• Adult paddles need to be 8 to 13 cm in diameter and child paddles should be 4.5 cm in diameter
• Infants use anterior posterior position

Skin To Paddle Interface Factors

• Use the right gel because too little gel increases the possibility of burn
• Too much gel causes electric current to arc from one electrode to another
• Make sure the disposable paddles used have not expired

Thoracic Impedance

• Definition is body resistance, which opposes the energy pulse from the defibrillator
• Ranges between 15 and 150 Ω, usually 70–80 Ω
• Take into consideration when administering the necessary energy
• Take into consideration that the patient’s thoracic impedance
• Thoracic impedence is crucial to adjusting the amount of energy required
• Impedance varies to a large degree in humans so dynamic adaptation of energy pulse waveform is important
• Modern devices automatically measure thoracic impedance
• Measurement is taken into account before defibrillation to deliver energy more accurately

Types of Defibrillators

• Generally divided into manual, semiautomated, and fully automated external defibrillators, along with defibrillator implants
• Semiautomated: user sees the defibrillation recommendation but the administration of pulse is triggered by user
• Fully automated: everything is done by the device
• A manual defibrillator requires clinical expertise to interpret the heart rhythm
• Experts then decide to charge the defibrillator and deliver the shock to patient
• Energy selection and delivery is given to the patient manually
• Automatic defibrillators are small, safe, simple and lightweight using two pads applied to the patient
• This guides the operator step-by-step through a programmed protocol
• It records and analyses the rhythm and instructs the user to deliver the shock using clear voice prompts, reinforced by displayed messages

External Defibrillator

• A device which delivers the high energy shock to patients Heart externally on patient's chest by using a defibrillator paddle
• It uses a maximum energy deliver to between 360 Joules in Monophasic and 200 Joules in Biphasic waves

Internal Defibrillator

• Uses a sterilized internal Handle/Paddle through which shock is delivered directly to the heart

Auto External Defibrillator (AED)

• AED is classified as either fully automatic or semiautomatic
• With fully automatic models disposable paddles are kept connected to the patient while the AED analyzes the ECG rhythm
• Based on ECG, it decides and determines whether a defibrillation counter-shock is needed, then the device automatically charges and discharges
• Semiautomatic AED analyzes the patient's ECG and notify the operator when defibrillation is indicated who then activates the defibrillator and discharge

Implantable Cardioverter Defibrillator (ICD)

• If the ICD detects an abnormally fast heart rhythm, it either electrically paces the heart very fast or delivers a small electrical shock to the heart to convert the heart rhythm back to normal
• The rapid pacing is unfelt by the patient while an electrical shock produces is felt as a strong jolt in the chest
• The ICD device is used to treat life-threatening heart rhythms that lead to sudden death

Pacemaker

• An electronic stimulator that produces periodic electric stimulation to the heart
• It is classified into two types
• Internal and External pacemakers
• An internal pacemaker runs off a battery
• It is implanted inside the patient to pace the heart
• An external pacemaker is used to pace the heart temporally
• It may use leads or electrodes for the stimulation