Defibrillators

Defibrillators
Steven Lewis
Clinical Engineering
United Lincolnshire Hospital Trust

Defibrillation
Electrical cardioversion and defibrillation have become routine procedures in the management
of patients with cardiac arrhythmias. Cardioversion is the delivery of energy that is
synchronised to the QRS complex, while defibrillation is the non-synchronised delivery of a
shock randomly during the cardiac cycle.
Most defibrillators are energy-based, meaning that the device charges a capacitor to a selected
voltage and then rapidly delivers a pre-specified amount of energy in joules (J) to the
myocardium to treat cardiac arrhythmias. The capacitance of a capacitor is the amount of
electric charge it can store for every volt applied to it. With regard to defibrillators the amount
of energy stored in a capacitor is very important. It can be calculated using the formula
E = ½CV 2 , where E is the energy in joules, C the capacitance in farads and V the voltage
measured in volts. This energy is dissipated in the patient’s body over a small time interval,
about 10 milliseconds or one hundredth of a second.
If the capacitance is 1000 μF and the voltage is 500 V then the stored energy is 125J.
[E = ½ CV 2 ]
E= ½ (1000 × 10 -6 ) x (500 2 ) = 125 J.
Current European Society of Cardiology and AHA guidelines suggest the following initial energy
selection for specific arrhythmias:
For atrial fibrillation, 120 to 200 joules for biphasic devices and 200 joules for
monophasic devices.
For atrial flutter, 50 to 100 joules for biphasic devices and 100 joules for monophasic
devices.
For ventricular tachycardia with a pulse, 100 joules for biphasic devices and 200 joules
for monophasic devices.
For ventricular fibrillation or pulseless ventricular tachycardia, at least 150 joules for
biphasic devices and 360 joules for monophasic devices.
They also incorporate an inductor to prolong the duration of the delivered current, and a
rectifier to convert alternating current (AC) to direct current (DC). (Knight, 2014)
A defibrillator can deliver a controlled electrical shock to a heart that has a life-threatening
rhythm, such as ventricular fibrillation (VF). In VF, the heart's chaotic activity prevents blood
from pumping adequately or at all. Voltage stored by the defibrillator conducts electrical
current (a shock) through the chest by way of electrodes or paddles placed on the chest. This
brief pulse of current halts the chaotic activity of heart, by depolarising a large part of the heart
muscle terminating the dysrhythmia allowing normal sinus rhythm to be re-established by the
body’s internal pace maker located in the sinoatrial node of the heart, giving the heart a chance
to re-start with a normal rhythm.
Many factors affect the chance of defibrillation success including; placement of the electrode
pads, time elapsed before the first shock is given, and certain health conditions. Successful
defibrillation requires that enough current be delivered to the heart muscle during the shock. If
the transthoracic impedance level is high the heart may not receive enough current for
defibrillation to be successful. Impedance is the body's resistance to the flow of current; some
people naturally have higher impedance than others. Therefore, it may take more current, a
longer shock duration, and/or increased voltage to ensure success. (EBME, 2003)

The shock is delivered via two electrode pads/paddles placed as shown below.
Fig 1 – Placement of Electrode Pads/Paddles
Modern defibrillators may be manual or automated; they generally produce biphasic waveforms
as opposed to monophasic waveforms, which increase safety and efficacy. Miniature
implantable cardioverter-defibrillators (ICD) may be used in patients with recurrent lifethreatening
arrhythmias. (Chaudhari, 2005)
Monophasic Waveforms
This is a type of defibrillation waveform where current flows in one direction. In this waveform,
there is no ability to adjust for patient impedance, and it is generally recommended that all
monophasic defibrillators deliver 200 - 300 J of energy to a maximum of 360J, applied to adult
patients with the assumed average impedance of 50 ohms, to ensure maximum current is
delivered which in the graph below is ≈ 45 amps.
Biphasic Waveforms
Fig 2 – Graphical representation of a Monophasic Waveform
With biphasic shocks, the direction of current flow is reversed near the halfway point of the
electrical defibrillation cycle. Biphasic waveforms were initially developed for use in
implantable defibrillators and have since become the standard in external defibrillators. With
biphasic waveforms there is a lower defibrillation threshold (DFT) that allows reductions of the
energy levels administrated and may cause less myocardial damage.
While all biphasic waveforms have been shown to allow termination of VF at lower current than
monophasic defibrillators, there are two types of waveforms used in external defibrillators.

The waveforms are shown below and will have the desired effect at current values ranging from
approx. 15 – 35 amps.
Fig 3 – Graphical representation of two Biphasic Waveforms
Types of Defibrillator
Automated External Defibrillator (AED)
AEDs are highly sophisticated, microprocessor-based devices that analyse multiple features of
the surface ECG signal including frequency, amplitude, slope and wave morphology. They
contain various filters for QRS signals, radio transmission and other interferences, as well as for
poor electrode contact. Some devices are programmed to detect patient movement.
The typical controls on an AED include a power button, a display screen on which trained
rescuers can check the heart rhythm and a discharge button. Certain defibrillators have special
controls for internal paddles or disposable electrodes.
In AED Mode, the Defibrillator analyses the patient’s ECG and advises you whether or not to
deliver a shock. Voice prompts guide you through the defibrillation process by providing
instructions and patient information. Voice prompts are reinforced by messages/pictures that
appear on the display. (Lozano, 2013)
Manual Defibrillator
Manual defibrillators are designed to give full control to the clinical users. The defibrillator
records the patients ECG, the user then assess the ECG and selects the appropriate level of
energy for defibrillation.
Capnography
End tidal Carbon Dioxide (EtCO 2 ) is the partial pressure or maximal concentration of carbon
dioxide (CO 2) at the end of an exhaled breath, which is expressed as a percentage of CO 2 or
mmHg. The normal values are 5% to 6% CO 2, which is equivalent to 35-45 mmHg. CO 2 reflects
cardiac output and pulmonary blood flow as the gas is transported by the venous system to the
right side of the heart and then pumped to the lungs by the right ventricles. When CO 2 diffuses
out of the lungs into the exhaled air, a device called capnometer measures the partial pressure
or maximal concentration of CO 2 at the end of exhalation.

Capnography uses an EtCO 2 sensor to continuously monitor the carbon dioxide that is inspired
and exhaled by the patient. It is usually presented as a graph of expiratory CO 2 against time, or
less commonly against expired volume. The sensor employs infrared (IR) spectroscopy to
measure the concentration of CO 2 molecules that absorb infrared light. This consists of a source
of infrared radiation, a chamber containing the gas sample, and a photo-detector. When the
expired CO 2 passes between the beam of infrared light and photo-detector, the absorbance is
proportional to the concentration of CO 2 in the gas sample. The gas samples can be analysed by
the mainstream (in-line) or side-stream (diverting) techniques. (Physio-Control, 2013)
During CPR, the amount of CO 2 excreted by the lungs is proportional to the amount of
pulmonary blood flow; therefore capnography can be used to monitor the effectiveness of CPR
and as an early indication of the Return of Spontaneous Circulation (ROSC).
It has been shown that when a patient experiences ROSC the first indication is often a sudden
rise in EtCO 2 as the rush of circulation washes un-transported CO 2 from tissues, likewise a
sudden drop in EtCO 2 may indicate that the patient has lost pulse and CPR may need to be
restarted. (Paramedicine, 2000)
Maintenance & Service Procedures
Defibrillators are serviced annually, during the service functional checks of all controls, displays
and sound outputs are performed. ECG functions are checked including heart rate calibration
and lead off detection, most defibrillators can detect whether the paddles/pads are connected
or disconnected and this should also be checked.
An analyser is used to ensure output energy levels are within specification and a check of all
functions/analysis is performed when in AED mode.
Pacer function and pacer detection are both tested, a functional check of the capnography (if
applicable) and finally an electrical safety test is performed.
There is also scheduled battery and patient lead replacement, the expiry dates on the pads
should be checked to ensure they are still ok to use. If they are past there expiry date the ward
staff should be informed and the pads removed from use and replaced.
During maintenance & service procedures it is vital to ensure a defibrillator is never left alone
charged. When repairing or opening the case for any reason it is important to follow the
manufacturer’s guidelines for discharging the capacitor to ensure no harm comes to yourself or
others.

Bibliography
Chaudhari, M., 2005. Anaesthesia Journal. [Online]
Available at: http://www.anaesthesiajournal.co.uk/article/S1472-0299(06)00175-5/abstract
[Accessed September 2015].
EBME, 2003. EBME - Biphasic Defibrillator. [Online]
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[Accessed September 2015].
Knight, B. P., 2014. UpToDate. [Online]
Available at: http://www.uptodate.com/contents/basic-principles-and-technique-ofcardioversion-and-defibrillation
[Accessed September 2015].
Lozano, I. F., 2013. Principles of External defibrillators. [Online]
Available at: http://www.heartrhythmcharity.org.uk/www/media/files/InTech-
Principles_of_external_defibrillators.pdf
[Accessed September 2015].
Paramedicine, 2000. End Tidal CO2. [Online]
Available at: http://www.paramedicine.com/pmc/End_Tidal_CO2.html
[Accessed October 2015].
Phillps Medical Systems, 2005. M4735A (ELD) Heartstream XL Defibrillator Service/User Manual,
Physio-Control, 2013. Lifepak® 20e Defibrillator Service/User manual.