Defibrillators

by Frank Weithöner


A defibrillator is used to stop uncoordinated heart beats of a massive heart attack by delivering a controlled electric shock on the patient's chest.
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The defibrillator is a portable device which runs on mains voltage and on internal battery. The unit contains an adjustable high voltage source, an ECG, a printer for the ECG and the patient electrodes (paddles). Modern defibrillators do a complete analyses of the patient's condition and set shock the parameters automatically.
Defibrillators are used mostly in the operating room, emergency departments and intensive care units (ICU).
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A defibrillator is not very common in developing countries. Either hospitals do not have a defibrillators or the defibrillator does not work. The defibrillator is probably the device that is most commonly defective. This is because the medical personnel are rarely trained properly on a defibrillator and they assume the equipment is dangerous (which is not entirely wrong). And because the defibrillator is hardly used it is not kept in operational condition.


Defibrillation
When the heart beats uncoordinated or too fast it cannot pump blood effectively any more. Cardiac output and blood pressure fall to dangerous levels. This situation is life threatening and immediate action has to be taken. Within a few minutes death or irreversible brain damage are otherwise the result.
Defibrillation is the treatment which can stop the abnormal heart rhythm by applying a strong, short electric shock to the heart. The shock interrupts the uncontrolled activity of the heart cells, the cells get depolarised and the abnormal heart rhythm stops. The heart then is able to control itself so that it beats normally again.
The restart of the heart comes from the heart itself; a defibrillator can not start a heart which is not beating. It only terminates the uncontrolled beating. It resets the heart.

A uncoordinated movement of the heart is called ventricular fibrillation.
A coordinated but too fast heart movement is called ventricular tachycardia.

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Normal ECG


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Ventricular fibrillation



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Ventricular tachycardia



Defibrillation is part of cardiopulmonary resuscitation (CPR) emergency procedure. It is­ applied together with chest compression and artificial ventilation.


Defibrillator types
External defibrillators for clinical use are available as manual and synchronised defibrillators. Beside these types there are fully automated defibrillators (AED) for non-clinical use and implanted defibrillators.
From a technical point of view external defibrillators are distinguished between monophasic and biphasic defibrillators, which describes the waveform of the shock pulse. In the developed world nowadays only biphasic defibrillators are found; in developing countries the older monophasic version is still the most common type.
The internal ECG unit can also be used to trigger the shock pulse at the right moment. Such a synchronised defibrillation is also called cardioversion.


Automated External Defibrillators (AED)
An AED is a small automatic controlled defibrillator. In contrast to a synchronised defibrillator in cardioversion mode the AED also diagnoses the patient's condition and sets automatically all needed parameter for the optimal shock.
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A typical AED. It is small, lightweight and easy to use by everybody. The self-adhesive electrodes are made in such a way that a correct position is always given.






The user does not need AED equipment training, only that the self-adhesive pad has to be placed on the chest by someone, and the equipment has to be turned on. The equipment even gives audible instructions to the operator.
In developed countries AEDs are found in large public places, such as airports, train stations and shopping centres. In the developing world AEDs are hard to find.


Usage
Defibrillators are potentially dangerous devices. Never contact the electrodes unless you have confirmed that the defibrillator is completely discharged. Only trained personnel may work with defibrillators.
Defibrillation is an emergency operation in addition to cardiopulmonary resuscitation (CPR) procedure. The defibrillator may be operated by specially trained hospital personnel only. These trainings are held by medics. It is not the task of the hospital technician to conduct user training for defibrillators. But the technician can assist and can give technical information e.g. about how to conduct a self-test or charging the internal batteries.
In case of an emergency the first-aider undresses the upper body of the patient, turns on the defibrillator and takes the paddles out of their holders. Then the operator applies conductive gel to the electrodes of the paddles, distributes the gel evenly by rubbing the electrodes against each other and then places the paddles on the chest of the patient.
The paddles will deliver an ECG signal on the internal monitor. On the base of the ECG the operator decides whether or not defibrillation is needed and what energy level should be selected. The right energy amount is then adjusted.
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With the paddles on the chest the operator charges up the defibrillator by pressing the charge button at one of the paddles. This can take a few seconds. A beeping sound will appear when the capacitor is fully charged. In the meantime the operator makes sure that he/she is not touching the electrodes or the patient and that anybody else stays away from the patient. After doing so and shouting 'Stand clear!' the operator presses the paddles firmly against the patient's chest and releases the charge by pressing the two shock buttons of the paddles.
When the ECG shows a normal signal, the paddles can be put back into the holder. In the holder the paddles get completely discharged and are now safe for cleaning.
If the defibrillation was not successful the operator repeats the procedure, perhaps with a higher energy setting.
When the defibrillator is not in use, it should be switched off but connected to mains. The batteries then get recharged and kept charged up.


Paddle position
Successful defibrillation depends very much on how the paddles are placed on the patient's chest. The medic must place the paddles according to the following rules:

image The sternum electrode has to be placed on the right side of the patient, below the clavicle. The apex electrode is placed to the left side of the patient just below and to the left of the pectoral muscle.
The electrodes must also not be mixed up because the polarity of the pulse will otherwise be inverted.
In order to apply full energy, the contact resistance between paddles and chest has to be as small as possible. Therefore conductive gel is applied to the electrodes before they get pressed to the patient's chest. For the same reason the pressure has to be firm (about 10-15 kg force) so that maximum contact surface is given.
When the defibrillator is completely discharged the paddles have to be cleaned with a mild soap solution or alcohol so that they are ready for use for the next time.


Cleaning by the user
Directly after usage the user should remove all residues of conductive gel from the electrodes. Also once in a while the housing, paddles and cables should be wiped clean with soapy water. Afterwards everything should be wiped with a dry cloth.


Common problems during the usage
In developing countries user trainings on medical equipment in general are insufficient, bad or do not take place. While a self-study by trial and error might work with simpler equipment it does not work with a defibrillator. The defibrillator is known as a potentially dangerous device that delivers dangerous shocks, and thus nobody wants to get close to it. Consequently nobody takes care of the defibrillator and the internal battery never gets charged.
Additionally the defibrillator is handled as any other equipment in the hospital: It is locked away when it is not in use. So it is not unusual when in an emergency 'the man with the key' has to be found first.


Construction
The defibrillator is a portable but quite heavy equipment. Especially older types can weigh 10 kg. The reason for this is a heavy internal lead-acid battery and an ECG monitor with an old fashion cathode ray tube (CRT). Modern equipment uses smaller NiMH batteries, an LCD monitor and other energy saving components. They are not as heavy and also a bit smaller.
image Two holders for the two paddles are integrated in the plastic housing. The paddles are connected through thick coiled cords with the equipment. The biggest knob on the front panel is the energy adjusting switch. Discharge energies between 2 and 360 J (monophasic) can be adjusted. A charge button which starts charging the capacitor is also found at the front panel and additionally on one of the paddles. A beeping sound will appear when the capacitor is fully charged. Then the stored energy can be released to the patient by pressing both discharge buttons on the paddles at the same time. This series connection of two switches is done for safety reasons. In monophasic defibrillators the two discharge buttons control a big high-voltage relay which switches the capacitor over from charge to discharge. In biphasic defibrillators the relay is replaced by semiconductors, mostly thyristors.
In cardioversion mode the two shock buttons do not control the relay directly but trigger a circuit which analyses the ECG, waits for the best moment and then releases the shock.
A defibrillator also includes an ECG which is displayed with a cathode ray tube (CRT) or LCD monitor. Depending on the patient's heart activity the operator decides if a defibrillation is needed and how high the shock energy should be.
The ECG signal is picked up directly by the two paddles. Alternatively, additional ECG electrodes can be connected.
A defibrillator also has an inbuilt printer which documents the patient's ECG before, during and after defibrillation.
A defibrillators is portable equipment and thus runs on rechargeable battery. Since the defibrillator has to be ready for use at any time it has to be connected to mains whenever it is not in use in order to keep the battery always fully charged.


Physical principle
The defibrillator delivers a controlled electric pulse of high energy through two paddles (electrodes) which are placed on the patient's chest.
The treatment depends largely on the energy of the pulse. The energy is expressed in Joule (J) and is a product of voltage V, current I and time t.
Energy must not be confused with power (V x I).
The electric shock can reach 5 000 V at a current of about 20 A. These extreme high values are needed because the pulse length is only a few milliseconds and per definition a defibrillator should be able to deliver 360 J. But since maximum power is not always needed the output value can be set by the operator to smaller energy values.


Wave forms
The defibrillator discharges the stored DC voltage of the capacitor across the body resistance. For a very short time a very high current flows. Because of the low body resistance the capacitor discharges very quickly.
In the simplest case a single high-voltage capacitor gets charged and discharged. The discharge current then flows in one direction only. Defibrillators which operate on this working principle are called monophasic defibrillators. The discharge energy of monophasic defibrillators depends on the energy which was stored before in the capacitor. If a small amount of energy is wanted the capacitor is charged up with only a small amount of energy. Assuming that the body resistance is always the same the discharge energy amount corresponds to a certain charge voltage. In this case the energy control is nothing else but a power supply with an adjustable output voltage.
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A typical 360 J discharge curve of a monophasic defibrillator connected to a 50 Ω test load. The peak voltage is reached after approximately 1.4 ms and lasts for 8 ms.






A monophasic defibrillator provides shocks between 2 J and 360 J. The needed energy depends on the capacity and the voltage across the capacitor. Or in other words: The shock voltage can be calculated when the energy is known and the capacity.
The energy is equal to half of the capacitance multiplied by the voltage squared.
If we want to apply a shock of 360 J by using a capacitor of 35μF the capacitor has to be charged up to approximately 4 540 V.

E = ½ C x V2           360 J = ½ x 35 μF x V2           V ≈ 4 540 V

Modern defibrillators work differently. They do not use the discharge curve of the capacitor for the shock but a truncated signal. The discharge curve is cut in order to get quick rise and fall edges. This cutting is done by high-voltage thyristors.
Additionally a negative pulse is added to the output signal when the positive one has ended. This energy is provided by a second capacitor which is charged and discharged in reverse polarity. Also this pulse is shaped by thyristors which get switched on and off.
Because the energy now can not be determined anymore just by setting the charge voltage, the control circuit gets more complex. Now the actual capacitor voltage is measured during discharge as well as the real current through the patient. A microprocessor then calculates the effective energy and controls the thyristors which switch the output signal.
This waveform is called biphasic. Biphasic defibrillators have a positive and a negative component. They were invented because this waveform is considered to be more effective. Nowadays all new defibrillators are biphasic. In developing countries were a lot of donated older equipment is used, the majority of defibrillators are still monophasic.
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A biphasic discharge curve.
The formerly discharge curves of two capacitors (one for the positive and one for the negative part) is cut into two pieces. The areas of the two parts correspond to the set energy.








Because the physiological efficiency of a biphasic defibrillator is better, less output energy is needed. As a consequence the maximum setting of a biphasic defibrillator is typically just 200 J compare to 360 J of a monophasic one. This decreases risk for the patient of burns and myocardial damage. For the defibrillator it means that the power supply becomes smaller and the internal battery lighter. As the maximum voltage gets smaller, the proof voltages of the components can be lower and the components get cheaper.
In biphasic defibrillators there is no patient relay any more. All the switching is done contact-less with thyristors. They are faster, more precise and cheaper than the charge-discharge relay of a monophasic defibrillator.


Cardioversion
All defibrillators contain an ECG monitor for diagnosing the patient's condition before and after defibrillation. But the ECG function can also be used for synchronising the discharge. This synchronised mode of defibrillation is called cardioversion. The advantage of a controlled shock at the right time is, that the defibrillation becomes more efficient by using less energy.
image When the defibrillator is set to cardioversion mode the user still pushes the shock buttons but the defibrillator releases the shock only when the defibrillator recognises the best moment. This is 20 - 30 ms after the large R-peak of the QRS complex has appeared. Another positive effect is that a shock during repolarization period (T-wave) is also avoided.
The needed ECG signal is taken trough the paddles which act as ECG electrodes. In addition all defibrillators have a connector for external 3, 5 or 12-lead ECG electrodes.


Block diagram
The basic circuit of a defibrillator consists of a high voltage power supply, a large capacitor as an energy storage, a relay for switching over from charge to discharge, a control unit, an ECG and the two paddles.
A high voltage power supply HVPS starts to charge up the high voltage capacitor (typically 15 μF - 40 μF) when the push button S1 is pushed.
The charge voltage depends on the position of the energy rotary switch S2. Usually energies between 2 J and 360 J can be set. This corresponds with a charge voltage of between 300 V and 5 000 V.
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When the capacitor is fully charged it can be discharged by pressing push button S3. Then the high voltage relay R switches over and connects the capacitor through the coil L and the paddles to the patient. Due to the low impedance of the human body (50 Ω - 150 Ω) the capacitors discharge quickly. The coil (typically 50 mH) is used to create a better physiologically waveform and to prolong the duration of current flow (3 - 10 ms).
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Capacitor in the back.
Front left: Relay.
Front right: Coil








In addition to these basic functions the defibrillator has some safety features. First, there is a circuit that monitors the charge process and blocks the discharge switch S3 when the capacitor is not fully charged. Also S3 is not just one switch but there are two in series, one in each paddle. Also for safety reasons the paddles get bypassed by an internal power resistor when returned to the holder. This ensures that the capacitor is really discharged and the paddle electrodes are safe and can be touched.
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The internal power resistor which discharges the capacitor when the paddles are put back into their holders.





With an improved circuit design the defibrillator can measure the real energy which is given to the patient. The shock voltage (at the capacitor) is monitored by the voltage divider R1, R2 and the patient current during the shock is measured with the modified coil L. The output coil now has a secondary winding which delivers a voltage which corresponds to the current through the patient. Both measurements get to a microcontroller in the control unit. Together with the shock time the microcontroller calculates the energy and switches off relay R when the preset energy amount is delivered.
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Modern biphasic defibrillators need a second capacitor in order to create the negative part of the output signal. Additionally the typical discharge curve of the capacitors are shaped in order to improve the effectiveness.
image In practice the two discharge curve are cut and put together. This is achieved by fast switching power thyristors or MOSFETs. They also switch over the capacitors from charging to discharging.
Together with a voltage and current monitoring circuit and the possibility of trimming the discharge curve, the delivered energy can now be precisely delivered to the patient.
The following simplified circuit diagram shows how thyristors are used to trim the signal and to charge and discharge the two capacitors.
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Capacitor C1 gets charged up when Thy1 is switched through. C2 is charged when Thy2 is switched. When the capacitors are fully charged (voltage divider R1/R2 is not shown here) they can now get discharged. First C1 delivers the positive parts of the signal. This happens when Thy3 is switched and the sternum electrode is connected to positive terminal of C1, and Thy6 is controlled and connects the negative terminal of C1 to the apex electrode. When the right energy amount is delivered the two thyristors switch off and at the same time thy4 and thy5 are switched on. Then the negative potential of C2 gets to the sternum via thy4 and the positive potential to the apex paddle via thy5. When the preset energy is reached the thyristors are switched off. The result is a biphasic signal with steep signal edges.


Power supply
The power supply in a defibrillator is a combination of three different power supplies. They all work as switch mode power supplies (SMPS). One delivers all needed low voltages for the control electronics, one delivers the high voltage DC for charging the capacitor and one charges the internal battery.
image While two of the power supplies work as common step-down switch mode power supplies, the one for charging the capacitor is a step-up power supply. Depending on what energy is set, the voltage can reach approximately 5 000V. Such a high voltage needs extraordinary safety measures and an excellent design.
Also all components around the capacitor (relay, coil...) have to be heavy duty components which are able to withstand these high voltages and high currents. Contacts and conductors are generously dimensioned and well insulated to prevent voltage flashover. Blank connections are sealed with plastic or silicon. This also applies to the PCB. The conductor tracks have to be protected against contact and flashovers.


Paddles
Due to the high current through the paddles and the low contact resistance which is needed to let this high current flow, the electrodes of the two paddles must have a certain size.
Also, they have to be well insulated and built that way, the operator can not touch the electrodes accidentally while applying the shock.
In practice the paddles are massive plastic handles with an electrode surface with the size of a smart phone. The paddles are connected through a rugged spiral cable with the defibrillator. For defibrillating children smaller paddles are available or suitable adapters are already integrated in the paddles.
Each paddle has a shock release button which have to be pressed at the same time. Also a charge button is usually integrated in one of the paddles.

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For safety reasons both paddles have discharge buttons (orange) which have to be pressed at the same time. One paddle also provides a charge button (yellow) and a control LED which lights up when the capacitor is fully charged.







To ensure a good connection and to minimise the electrical resistance the user has to apply conductive gel on the electrodes before placing the paddles on the patient's chest. Skin resistance can be several kilo ohms with dry skin but with gel it drops to a few ten ohms. If no gel is used serious burns can occur. Still, even with gel the paddles have be pressed firmly on the patient chest. Therefore a pressure of 10-15 kg is necessary.
During surgical operations special internal electrodes can be used. They do not have rugged handles and no integrated shock buttons. The surface of the electrodes are much smaller because the current which is used for defibrillating the heart directly is also much smaller.
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Electrodes for internal use. They do not have switches and are autoclaveable.


For clinical use self-adhesive pad electrodes are used instead of paddles. These pads are stuck on the patient as a precaution when the patient is at risk of arrhythmia. The electrodes will remained connected to the defibrillator and in case of an emergency the system is immediately ready for use. Self-adhesive electrodes are safer for the operator, as they minimise the risk for the operator coming into contact with the patient when the shock is delivered.
Self-adhesive pads are also used in conjunction with an automated external defibrillators (AED).
Adhesive electrodes are disposable items and can not be reused. They also have an expiration date. Expired electrodes should not be used because the adhesive also acts as a conductive gel and it looses its properties overtime. As a result serious burns may occur.


Myths
image Often in a TV series a patient is seen with a 'flat-line' ECG signal and medics bring the patient back to life with a defibrillator. This unfortunately works only in TV and not in real life.
A defibrillator can stop uncoordinated heart beatings like ventricular fibrillation and ventricular tachycardia, but it can not start a heart which does not beat anymore. Only continuing CPR (Cardiopulmonary Resuscitation), a combination of chess compression with artificial ventilation can help.
Also often in movies operators are seen which rub the paddles together rapidly before placing the paddles. Rapid movements are useless. Rubbing does not charge the capacitor. The paddles are only rubbed together gently to spread the conductive gel on the electrodes.


Installation
image The defibrillator should be placed where everybody in the hospital has free and fast excess and on the other hand the equipment is not in the way.
A wall socket should be near by where the defibrillator can be connected when it is not in use so that the batteries are always charged.
Also, there should be space for ECG cables, conductive gel and a roll of spare printer paper.
Additionally, signs should be fixed on the hospital walls showing where the next defibrillator is located.

In developing countries a few more considerations should be made.
  Again, there should be really free and fast access to the defibrillator. It makes no sense
    to lock up the defibrillator when in case of an emergency the person with the key has to
    be found.
  The wall socket should be exclusively for the defibrillator and not be used for other
    purposes. This applies in particular for mobile phone chargers.
    It is a not a bad idea to colour mark the wall socket and label it.
    The wall socket should have no switch as is common with the UK wall socket type.
  A bottle of conductive gel should be placed close to the defibrillator or better attached to
    the equipment in that way that it can not disappear.
  If the instruction are not clearly shown on the defibrillator a quick start guide should be
    attached to the wall close to the defibrillator. This guide does not replace a proper user
    training but it might help when defibrillators of different manufacturers are available in
    the hospital.

When the defibrillator has to be used without an internal battery the following should be regarded (only in exceptional cases):
  Make sure that the defibrillator really works without the internal battery.
  The power plug and all wall sockets in the department must have the same standard.
  All wall sockets must function and have connection to the back-up system. Note that
    maybe not all wall sockets get power from a generator in case of a power cut.
  The power cord should be long enough. If this is a problem replace the cable against a
    longer one. Do not leave an extension cable with the defibrillator. It will disappear
    one day.


Repair
A defibrillator is a potentially dangerous device. It runs with very high voltages and even when it is switched off the capacitor may keep its charge. Only trained technicians should maintain and repair defibrillators.
There is hardly any other medical equipment found in developing countries which is more often not operational than the defibrillator. Reason for this is not a sophisticated and sensitive technology. In fact a defibrillator is a quite robust and reliable equipment. In fact, almost all problems with defibrillators are due to problems with their internal batteries. They are either weak, defective or simply missing.
Before working on an open defibrillator make sure the capacitor is discharged. Generally, the paddles only have to be in their holders and the defibrillator have to be switched on (just switched on - not charged up). The correct discharge procedure is explained in the service manual of the defibrillator.
When the repair is finished a complete maintenance according to the PPM procedure should be done.


Special tools and measurement equipment
For the repair and maintenance of a defibrillator standard tools including a good multimeter with capacitor testing is needed. In case of repairs on the SMPS a component tester including an ESR meter is helpful.
For performing calibration and a performance tests a calibrated defibrillator analyser is needed. Since this is an expensive measurement equipment which is not often used, it is hardly found in hospital workshops in developing countries.
For hospital workshops in countries with limited resources which can not afford a defibrillator analyser a simple load resistor would be already helpful. With such a load resistor a function test and a battery performance tests can be made.
Tip!   Building a defibrillator test load
Defibrillator testing requires an external load resistor is needed. The resistor simulates the body resistance of the patient. Such a dummy load is in principle just a power resistor of 50 Ω. Because the output power of the defibrillator is extremely high, the resistor therefore has to be big. The exact power dissipation is difficult to calculate because the discharge time is very short. Commercial defibrillator analysers however contain a 80 W - 120 W resistor.
With such a power dissipation the resistor does not get very hot even after several discharges. Half the wattage would be also fine, when only a few discharges with maximum energy are made.
Unfortunately such a big resistor is not easy to get. But instead of using one big resistor, a combination of several resistors in series or in parallel can be an option. In fact, the series connection is the ideal configuration because the voltages across the resistors get smaller as more resistors are used. This is important because resistors with a proof voltage of 5 000 V and more are not easy to find. But when 5 resistors of the same value are connected in series the proof voltage of a single resistor has 'only' to be 1 000 V at 360 J.
The solution for a test load could be: 5 x 10 Ω resistors in series, each 17 W. 11 W types are also fine when they are mounted between two metal sheets which act as heat sinks.
For calibration and serious testing such load resistor does not help much because we still can not measure the output power. But for a function test, using the defibrillator's Joule display and for testing the battery such a test load is enough.

Typical technical problems

Battery
The biggest problem with defibrillators is the internal battery. These rechargeable batteries have to be always charged even if the defibrillator is not used. If this does not happen the battery loses its charge due to self-discharge and once dropping under the discharge cut-off voltage the battery gets damaged. That is why defibrillators and other rechargeable equipment always have to be connected to mains. Due to lack of user training on medical equipment (and missing responsibilities) in developing countries most equipment with internal batteries suffer defective batteries.
As most defibrillators are of older type, we usually find sealed lead-acid (gel) batteries and sometimes even very old nickel-cadmium (NiCd) batteries. Newer defibrillators run on nickel metal hydride (NiMH) or lithium-ion (Li-ion) cells. AEDs usually work with lithium batteries and thus are not rechargeable.
The state of the batteries can be checked by doing a series of discharges as described in the ↓ Manual performance test. Special attention should be given when an older defibrillator still runs on nickel-cadmium batteries (NiCd). NiCd batteries are sensitive to partially discharges and incomplete and irregular charging. They need regular discharging and charging, otherwise they rapidly loose capacity (memory effect). Because of these drawbacks NiCd batteries are not suitable any more for medical equipment these days and should always be replaced with nickel metal hydride (NiMH) batteries.
In developing counties defibrillators run mostly on lead-acid (gel) batteries. Theses batteries are more robust than NiCd batteries but even with these batteries the manufacturers suggest replacement every two years. Therefore batteries of defibrillators have a label which shows the date of last exchange. When you replace a battery please do not forget to note the date of replacement on the battery.
Battery replacement every other year is not realistic in developing countries. Instead a regular performance test can be done, so that the battery is only replaced when it gets significantly weaker.
In general, batteries should be replaced only with the same type and size (Ah). If this is not possible or not wanted (in case of NiCd batteries) make sure that the end-of-charge voltage of the battery is similar. If the battery voltages (nominal and end-of-charge voltage) are different the new battery would not get fully charged or gets overcharged.
The charging characteristics of NiCd and NiMH are similar and a NiCd battery can be exchanged with a modern NiMH battery. But a lead-acid battery behaves differently during charging and can only be replaced by another lead-acid battery. After replacing a battery always do a ↓ Manual performance test.
If you plan to run a defibrillator without the internal battery because a spare is not available, please note that a defibrillator without a battery might work but the time for charging the capacitor can take much longer.

Capacitor
image Even when the paddles are in their holders and the capacitor should be discharged, discharge the capacitor manually before working on it. Use a discharge-cable, a cable with a small load resistor.
When the capacitor is discharged it can be tested using a quality multimeter. Such a high-voltage capacitor has a capacity of 15-100 μF. The proof voltage is about 2 000 V (biphasic) and up to 5 000 V (monophasic).
When a capacitor has to be replaced because of capacity loss it is very unlikely to find a spare in the next electronic shop. Because of their gigantic proof voltage they are only found in defibrillators. But if you can get another broken defibrillator it might contain an identical one. Especially with older monophasic defibrillators that often work with a 35 μF type. And if the capacity is the same, also the proof voltage will be the same because they all work with the same energy at the same load.

Power supply
The power supply unit of a defibrillator consists of different independent power supplies for different purposes. They are all designed as switch-mode power supplies (SMPS).
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First, there is a standard power supply which supplies all electronic stages. It is a common design and delivers low voltages (e.g. 5 V, 12 V) for all electronic stages.
Then there is the high-voltage power supply which delivers the charge voltage for the capacitor(s). This time the SMPS is a step-up type, meaning the output voltage is higher than the input voltage. This also means that the components have much higher proof voltages (2 000 - 5 000 V).
The last SMPS is the battery charger which delivers the charge current for the internal battery. It also monitors the state of charge.
Due to the life-threatening voltages and for shielding reasons, the power supply is completely covered by a metal housing.
Before working on the power supply make sure that the shock capacitor is completely discharged. Only then the power supply can be opened. But keep in mind that the mains filter capacitors of the power supply are remain charged. Discharge them all by a discharge cable.
Problems with SMPS in medical equipment can happen but are not very common. The components are of much better quality and e.g. the leaking capacitors in consumer electronics are rarely used. Problems with power transistors and MOS-FETs are more likely to arise. More about SMPSs and their repair in the SMPS chapter. (→ Switched-Mode Power Supplies)

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Paddles
The function of the paddles can be checked by doing a test run in self-test mode or with an external defibrillator tester. When the paddles do not work properly or just infrequently, do not forget to inspect the spiral cable and the plug and socket. When in doubt the cable and the connection can be checked with an ohm-meter. The resistance should be no more than 0.15 Ω.

Printer
In developing countries it is not uncommon that internal printers are not used simply due to missing printer paper. In fact the printer itself rarely has a technical problem.
The printer in a defibrillator is generally a thermal printer. That means that it does not use ink and thus does not have a printer head which can get clogged or dried up. That is why they are so robust.
The printer head consists of small heating elements which are controlled by a microprocessors while a thermal-sensitive paper is moved by a rubber roll under the printer head. Whenever a heating element is controlled it blackens the paper.
The drawback of thermal printers is the thermal paper itself. Thermal paper is often difficult to get, especially for older equipment.
After long use it can happen that dust and paper fibres accumulate on the printer head and the print out gets difficult to read. Then the print head can be cleaned with a cotton swab and a drop of alcohol. Also the rubber roller should be wiped off with alcohol in order to ensure the paper feed.
TIP!     Thermal paper is also used at supermarket checkouts and for ticket machines. It is not a bad idea to check if these locally available paper rolls perhaps also fit the defibrillator printer.

Planned preventive maintenance (PPM)
Since the battery is the weakest component of the defibrillator, checking the state of the battery is recommended in addition to the a normal function test.
Please remember that a defibrillators is a lifesaving device for an emergency situation. When doing PPM on a defibrillator make sure that a spare defibrillator is available and functioning.
For the same reason it is also not advisable doing PPM of all defibrillators the same day. Also note that for a battery capacity test the battery has to be discharged and again recharged which can take 24 hours.
Planned preventive maintenance on defibrillators should be done twice a year and after every repair.


Inspection from the outside
  Defibrillator housing and paddles should not be damaged.
  Power cord, mains plug and strain relief should be firmly connected and not damaged.
  Paddles should be clean and should not show pitting, corrosion and residues of conductive
    gel.
  Coiled cables of the paddles should be in good condition and not damaged. Unplug the
    paddle connector. The contacts must be clean.
  Printer should be in good condition. No signs of wear and tear on drive gear and rollers.
  Accessories (defibrillator gel, ECG electrodes) should be available at the location of the
    defibrillator. Also the wall socket should functioning properly.


Inspection from the inside
  Take off the defibrillator housing.
  Remove dust if necessary.
  Check for corrosion of the metal parts. All cables are tightly connected.
  Check the electronic components for burns, breaks and capacitors for deformation and
    leakages.
  Search the solder side of the board for cold soldering points and loose connections.
  Check all fuses. No fuse must be bypassed.


Function check
Close the defibrillator and connect it to mains. The battery charge LED should come on.

Safety function check
  Turn the energy select switch to 20 J. Make sure that paddles are placed in their holder.
    Press charge button.
  With the paddles in their holders, press Apex shock button. Defibrillator must not
    discharge.
  With the paddles in their holders, press Sternum shock button. Defibrillator must not
    discharge.
  Press both shock buttons. Defibrillator should discharge now. The internal printer should
    print the test result.

ECG function check
  Connect an ECG tester and check if the ECG is displayed.
  Check the ECG alarm functions. There should be alarms when the heart rate is too low or
    too high (e.g. 40 and 120).
  Print out the ECG. It has to be good print quality.

Cardioversion test
  Place paddles on the defibrillator analyser. Switch to sync (cardioversion) mode. Turn the
    energy select switch to 20 J. Press charge button.
  Press both shock buttons. Defibrillator must not discharge.
  Turn on the ECG simulator of the analyser. Press both shock buttons. Defibrillator should
    discharge now.


Automated self-test
Most defibrillators provide an automated self-test function. During a self-test the defibrillator gets charged up and a controlled discharge follows. The process is monitored, the energy measured and the result displayed on the monitor or can be printed out. How this test exactly works and what tests are covered is explained in the user manual.
It is possible that the performance of the battery is not tested during this self-test. Sometimes another menu item has to be selected to check also the battery performance. Check the user or service manual for more information.
Some manufacturers recommend a weekly self-test and in many hospital in the developed world the self-test has to be done even on a daily base. In many hospitals in the developing countries the defibrillator is rarely or never used. Thus hardly anyone knows the self-test function. Discuss this topic with the responsible doctors and offer instruction in how to perform the self-test.


Manual performance test
The self-test is done with reduced energy and thus is not a performance test. Only a test under real conditions can give information about the performance of the defibrillator and its internal battery. For such a performance test a defibrillator analyser is needed.
image

image
For safety reasons both paddles have discharge buttons (orange) which have to be pressed at the same time. One paddle also provides a charge button (yellow) and a control LED which lights up when the capacitor is fully charged.








Performance test with a defibrillator analyser. The analyser measures the energy of the shock and can also create different types of ECG signals including tachycardia and fibrillation.

A typical manual performance test covers a charge up test, an output energy measurement and a battery test. In practise you run the defibrillator on battery power and do a series of charging and discharging while you observe the charge time and the discharge energy. The idea is that the battery should last a number of charges/discharges without a reduced output power or increased charge time.
  Make sure the internal battery is fully charged. In case of a doubt charge the defibrillator     for24 hours before you start.
  Connect a defibrillator analyser. Select 360 J.
  Charge the capacitor. The charge time should be no longer than 15 s.
  Deliver a shock. The analyser displays the delivered energy. It should be not less than
    306 J (-15%).
  Repeat charging and discharging 10 times. Make sure that the test load does not get
    overheated.
    The charge time should never be longer than 15 s and the output power never less than
    306 J.
When a defibrillator has problems delivering 10 discharges the battery is weak (25-50 discharges can be expected with a new battery). The battery will not last for much longer and the defibrillator will no longer work reliably. The battery should be replaced.

Capacitor test
Once the capacitor is charged up it should not loose its charge too quickly. This can be checked as followed:
  Connect a defibrillator analyser. Select 360 J.
  Charge up the defibrillator but do not release a shock.
  Wait 1 minute and then discharge. The output energy should not be less than
    85% of the set value.

Paddle continuity
All patient cables and the wires inside defibrillator cables can break or tear off at the connection points. Also the pins in the plug can get corroded. For this reason a continuity check of paddles should be done during every maintenance. The continuity is measured with an ohmmeter (milli-ohmmeter, good multimeter) between the paddle surface and the corresponding pin of the connector. The resistance should not exceed 0.15 Ω. Wiggle the cable, especially close to the the paddle and the plug and observe the ohmmeter.


Calibration
image An adjustment of the output energy is needed when the measurement result of a connected defibrillator analyser differs by more than 15%. That means for example, when the output energy at 360 J is less than 306 J.
It is important that the measurement is done with an external and calibrated analyser. The energy display of the defibrillator is not relevant.
Adjustment procedure differ from manufacturer to manufacturer. Sometimes it has to be done through the equipment software and sometimes it needs manual adjustment on the PCB. The correct procedure is described in the service manual.


Cleaning by the technician
  Clean the housing, paddles and cables. Use soapy water. Wipe with a cloth and let
    everything dry.
  Remove all residues of conductive gel from the electrodes. The surface must be
    completely clean and shiny. If soapy water does not help try it with polishing paste. Avoid
    abrasive products. The surface must not be scratched.
  De-dust the interior if necessary.
  Clean the printer unit. Take out the paper roll and check for dust and paper residues.
    De-dust the printer if needed. Clean the printer head and rubber rollers with alcohol and
    cotton swaps.


Electrical safety test
The last step of the maintenance procedure is the electrical safety test. The defibrillator should be connected to a electrical safety tester now and the following tests performed. The measurements can also be done manually step-by-step according to the procedure explained in the Electrical safety test procedures chapter.

Ground resistance
The ground resistance is measured between the grounding pin of the power cord and exposed metal on the chassis. It should be less than 0.5 Ω.

Earth leakage current
The earth leakage current is the current through the ground wire to earth.
The measurements are made when the defibrillator is switched on, in normal and reverse polarity. The current should not exceed 500 μA.
Then the same measurements should be made in single fault condition (SFC) when neutral is open, also in on and off mode in normal and reverse connection. The current should not exceed 1 000 μA .

Chassis leakage current
The chassis leakage current is measured from chassis to ground with the PE connection of the defibrillator temporarily open.
The measurements are made when the defibrillator is switched on, in normal and reverse polarity. The leakage current should not exceed 100 μA.
Then the same measurements should be made in single fault condition (SFC) when neutral is open, also in on and off mode in normal and reverse connection. The current should not exceed 500 μA.

Patient leakage test
The patient leakage current is measured between the paddles and ground. The two paddles should be measured separately.
The defibrillator has to be switched on and measurements should be done with normal mains polarity and in reverse. The current should not exceed 10 μA.
Then the same measurements should be made in single fault condition (SFC) with open neutral and also with open PE, also in on and off mode in normal and reverse connection. The leakage current should not exceed 50 μA.


Manufacturers
Important manufacturer of defibrillators are:
Cardiac Science, GE, HP, Nihon-Kohden, Philips, Physio-Control, Schiller, Welch Allyn, Zoll


Further literature
On Wikipedia you can find further articles about these topics:

     Defibrillation
     Automated external defibrillator
     Cardiac arrest
     Ventricular fibrillation
     Ventricular tachycardia
     Cardioversion
     Cardiopulmonary resuscitation
     Battery (electricity)
     Switched-mode power supply