PHYSICS AND CLINICAL
MEASUREMENT FOR THE
PRIMARY FRCA
Ian Wrench - 2002
Electricity
Source material:
1.Basic physics and measurement in anaesthesia - Davis,
Parbrook and Kenny, fourth edition, Butterworth and
Heineman.
2. Electrical safety – JA Langton, Royal College of
Anaesthetists Newsletter January 2000 – Issue 50 page
290 – 292.
3. Key Topics in Anaesthesia – TM Craft and PM Upton,
second edition, Bios Scientific Publishers.
4. Anaesthesia for patients with pacemakers and similar
devices. P Diprose and JM Pierce, BJA CEPD reviews –
volume 1, no 6 pages 166 – 170.
5. Electrical safety in the operating theatre. S
Boumphrey, JA Langton, BJA CEPD reviews – 2003,
volume 3, no1 pages 10-14.
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CLINICAL MEASUREMENT FOR THE
PRIMARY FRCA
For the primary fellowship it is necessary to understand some of the principles on
which clinical measurement is based. This part of the handout serves as an
introduction to this subject area.
ELECTRICITY, MAGNETISM AND MONITORING
Monitors are designed with a view to producing the best recording possible of a
physiological process. For this reason many of the principles of monitoring are
concerned with the production of an amplified signal which is as free as possible of
interference. Whilst the following account is relevant to all theatre monitoring, the
ECG is used as a specific example. First, however it is necessary to understand some
basic principles of electricity and magnetism:1) Capacitance - the ability of an object to hold electrical charge (e.g. a capacitor figure 1). Capacitance is measured in farads.
A capacitor consists of two plates which are separated by an insulator. When a direct
current (DC) is applied, one of the plates becomes positive and the other negative and
thus charge is stored (e.g. a defibrillator). A capacitor will not allow the passage of
DC across it, however alternating current (AC) may flow. In an operating theatre a
capacitor may be formed from an electrical device acting as one plate, the patient as
the other and with the air inbetween as the insulator. AC may flow across such a
capacitor and act as a source of interference to monitors.
2) Inductance - Electrical current is induced in a wire by movement of the wire
relative to a magnetic field (Fleming’s right hand rule). For example an electric
generator uses this principle to generate electricity. In theatre many components of
electrical apparatus have strong magnetic fields around them and the passage of
electrical current through a conductor also causes a magnetic field. As these fields
change or as wiring is moved through them so current is induced which may cause
electrical interference.
3) Impedance - Whenever the resistance to current flowing varies with frequency it is
known as impedance and is measured, as with other forms of electrical resistance, in
ohms. When AC flows across a capacitor the impedance diminishes as the frequency
of the AC increases. In contrast for an inductor the impedance becomes greater as the
frequency increases.
Working principle of the ECG
1) The ECG electrode - The ECG signal is picked up by an electrode. Interference
may arise at this site in a number of ways. Firstly the signal may produce a chemical
change in the electrode which causes it to become polarised altering its impedance and
generating a potential which may interfere with the signal. Another possible source of
interference is the formation of a battery between the metal electrode and skin
moisture. Both of these problems are minimised by the use of the
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BATTERY
CAPACITOR
++++++++++
INSULATOR
----------
Figure 1 – A capacitor. The capacitor plates are charged positively and negatively by the
potential difference produced by the battery. The quantity of charge stored is measured in
coulombs in the SI system and the capacitance is measured in farads.
LAYER OF SILVER
CHLORIDE
SILVER ELECTRODE
GEL WITH CHLORIDE IONS
SKIN
Figure 2 – The ECG electrode
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silver/ silver chloride electrode in which a silver electrode is in contact with a silver
chloride solution which is in turn in contact with a chloride solution in a gel which is
next to the skin (figure 2). The junction between the silver and the silver chloride
gives a stable DC potential which does not cause interference. The impedance of the
electrode may also be greatly increased by poor contact with the skin which may
attenuate the signal. To avoid this problem the electrodes are securely fixed with a
sticky pad.
2) The ECG leads - The leads conduct the signal to the ECG machine. Both
capacitance and inductance effects may give rise to electrical currents in the leads
which may overlie the signal. This interference can be reduced by covering the leads
with a metallic mesh which is earthed, a system known as screening. In this way the
currents occur in the mesh rather than the ECG leads and are conducted to earth and
not to the machine (figure 3).
3) The Amplifier - The biological signal is small and must be increased in size by
means of an amplifier. The degree of amplification is known as the gain (figure 4), for
example an amplifier which delivers an output power 1000 times greater than the
input power is said to have a gain of 1000. For an amplifier as the power of the input
increases the power of the output should be similarly raised, this is known as a linear
response (see figure 5). The monitoring device contains many different components
but provided they all display linearity it is possible to predict the response of the
system to a complex signal by breaking the signal down, seeing what effect each part
of the signal produces and then adding them up. This simple relationship may
however become distorted in the amplifier so that it is not linear over the whole range.
It may also happen that the response to an increasing power is different to that of a
decreasing one, an effect known as hysteresis (figure 6). In order that the correct
response is recorded it is important that allowance for these variations is made in the
design of the ECG machine.
Amplifiers usually measure the difference in potential between two sources looking at
the signal from two different directions (differential amplifiers). In this case the
biological signal will be different from each source whereas any interference will be
the same. By disregarding the common signal, only the biological one is amplified,
this is known as common mode rejection ratio.
For the ECG the biological signal lies within a range of frequencies which is 0.5 to
100 Hz. In order to produce the best image, the amplifier should have a bandwidth
which responds to this range of frequencies. The bandwidth of the amplifier is the
range of frequencies over which the amplification is relatively constant. Sometimes
the bandwidth of the amplifier is restricted so that frequencies where interference is
likely to occur are not included. For example, high frequency interference from
muscle movement for the EEG is minimised by excluding these frequencies from the
amplifier bandwidth.
It is important to have an amplifier with a high input impedance relative to the signal
source. If the input impedance of the amplifier is low then this results in attenuation of
the input signal. The skin impedance is reduced by using electrode gel
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Screening
Mesh
ECG
Monitor
ECG
Electrode
Earth
Figure 3 – A mesh surrounds the ECG
electrode and conducts away
interference to earth (screening).
Amplifier
Figure 4 - The degree of amplification is the gain and the range of frequencies to which the
amplifier responds is the bandwidth. The power of the output increase with the power of the
input although the relationship is not always simple (see figures 5 & 6). The amplifier must
have a high input impedance relative to the signal source or the signal may be attenuated.
The amplifier converts the physiological analogue signal to digital information via an
analogue to digital converter so that the signal may be processed. Sufficient data is stored in
the random access memory to fill the ECG screen.
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Power of
input
Power of output
Figure 5 – An amplifier for which there is a proportionate increase in
output for an increase in input. This is an example of a linear
response.
Power of
input
Power of
output
Figure 6 – An amplifier which displays hysteresis. The response to an
increasing power is different to a decreasing power.
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and ensuring that there is good contact with the skin. Additionally most modern
amplifiers have a very high input impedance to get around this problem.
Amplifiers contain semiconductors that are used in components such as transistors
and diodes. The conductivity of semiconductors varies with temperature leading to
inaccuracy, for example if equipment becomes heated. This phenomenon is known as
drift. Using an amplifier that is designed for AC rather than a DC potential
considerably reduces this problem.
4) The display - Most modern monitors make use of microprocessor technology
which uses digital information. The physiological signal is analogue (a continuous
scale) and must be converted digital (a discontinuous scale – “yes” or “no” data) by
means of an analogue to digital converter. Sufficient ECG data is then stored in the
random access memory to fill the screen. As the latest information is stored so the
oldest is lost to produce a rolling display. If the display is via a cathode ray tube then
the data must be converted back to analogue using a digital to analogue converter.
However, this is not necessary for a liquid crystal display that can use digital
information. In more sophisticated monitors further analysis of the trace may be
performed such as ST segment or arrhythmia analysis. Algorithms may be
programmed in so that the machine alarms if certain parameters are exceeded.
Despite the measures to reduce interference, this may still occur and is referred to as
noise and the amount of noise the noise to signal ratio. The noise to signal ratio can
be improved by averaging a repetitive waveform. The interference is random and is
averaged to zero leaving the signal alone. Diathermy is a particularly troublesome
source of noise the effects of which may be reduced by the use of a high frequency
electronic filter.
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ELECTRICAL SAFETY FOR THE PRIMARY
FRCA
How electricity is supplied:
Substation
Wall socket and plug
Live
Wall
Neutral
Earth
Apparatus
Earth
Figure 7 – Electricity emerges from the substation and flows to apparatus via the wall socket/
plug. The neutral wire completes the circuit back to the substation. Electricity may flow back to
the substation via an alternative route if a connection is made from the live wire back to earth.
This will only happen if there is a fault with the apparatus or the apparatus is not used
correctly. If this route to earth is via a person then they are electrocuted.
Electrocution
When faulty apparatus allows electricity to flow through the anaesthetist to earth, the
greater the impedance the smaller the current (current = potential/ impedance). The
impedance is mostly due to foot wear and the floor. With the standard 240 volt supply,
if the impedance is 240 kilo ohms or more then the current is 1 mA - i.e. relatively
small and harmless. If the impedance is only 10 kilo ohms or less then the current is
24 mA which is enough to stop the anaesthetist letting go and may cause ventricular
fibrillation (VF). The impedance of footwear is recommended to be between 75 kilo
ohms and 10 mega ohms to reduce the risk of electrocution whilst allowing the
dissipation of static electricity. The set limits for electrical resistance for anaesthetic
flooring (defined as the resistance between 2 electrodes 60 cm apart) are 20 kilo ohms
to 5 mega ohms.
Skin impedance is very low and made lower by moisture and a large surface area of
contact with the electrical source. These factors become important if for some reason
the impedance of the footwear is low. With regard to a patient in theatre, if their skin
is in contact with earthed equipment this provides a route from faulty live apparatus to
earth. The impedance is likely to be low so that a dangerously high current may flow
in the patient.
The risk of VF with an electric shock is increased by - timing with the T wave of the
ECG, a relatively low frequency (e.g. mains - 50 Hz) and CVS disease.
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Classification of electrical equipment (British Standard 5724).
Class I - Any accessible conducting parts are connected to earth. If the live supply
connects to these parts an abnormally high current flows from live to earth resulting in
melting of the fuses in the live wire.
Class II - The equipment is double insulated so that it is impossible to touch the live
wire. There is no earth wire.
Class III - The equipment is internally powered (e.g. batteries). Any electric shock
would not be as harmful as from the mains.
Any of the above class of apparatus may contain floating circuits that are not earthed
and are electrically isolated from the rest of the equipment by transformers. Even so
the alternating current of mains electricity can produce small electrical currents
through inductive and capacitative effects. These small currents are known as leakage
currents - they will usually be small but may lead to microshock.
Leakage currents may lead to microshock to the heart if conducted down a wire in
close contact with it. This may lead to VF or damage to the heart. This may occur with
cardiac pacemakers, cardiac catheters or a temperature probe in the lower third of the
oesophagus. Damage is caused by these small shocks (around 100 μA) because the
current density in the myocardium is equal to that of a generalised 24 mA shock. The
current may arise from faulty equipment and find a route to earth through the
endocardial wire.
Classification of equipment for leakage currents
All equipment is tested for leakage currents in the event of a fault. For equipment that
contacts the heart the classification is CF (F is for floating circuit) and the leakage
current should be less than 10 μA. For other medical monitoring equipment the
classification is B or BF if it contains a floating circuit. For B or BF the maximum
permitted leakage current if 100 μA
Equipotentiality
Different pieces of equipment may operate at different potentials, electricity may
therefore flow from the higher to the lower potential via a user causing an electrical
shock. To avoid this, the terminals of each piece of equipment in a stack are
connected so they are all at the same potential.
Pacemakers
There are 2 types of pacemaker - temporary with an external wire and permanent that
are contained within the patient and are battery powered. Usually pacemakers have a
demand mode. Theoretically interference from an external source of electromagnetic
radiation may be misinterpreted as a QRS complex causing the pacemaker not to
produce a stimulus. As a general rule with pacemakers no diathermy is best followed
by bipolar which is in turn theoretically safer than unipolar although the risks are
small. Many pacemakers switch to a fixed mode if there is excessive interference as a
safety measure. However, most modern pacemakers will function normally during
surgery provided the diathermy plate is kept well away from them so that the
diathermy current does not flow towards them. Important exceptions are those
pacemakers that cardiovert or defibrillate. These pacemakers may interpret diathermy
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as ventricular fibrillation or a tachyarhythmia and shock the heart unnecessarily. Such
pacemakers should have this facility discontinued prior to surgery.
Formerly it was the case that an external magnet could be applied to a pacemaker to
convert it to a fixed mode of 70 beats per minute. However, the application of an
external magnet may be harmful to a modern pacemaker and should no longer be
used. Radiotherapy may also damage pacemaker circuits if directed at them and the
use of flecainide may elevate the threshold required to trigger depolarisation so that
the pacemaker no longer works. Other factors which may rarely be sources of
interference include lithotripsy, factors which may cause lead displacement (IPPV,
shivering, patient position), factors which may confuse the rate modulator (shivering
and fasciculation) peripheral nerve stimulators, and TENS.
Should a patient with a pacemaker in situ require defibrillation then it is important
that the pads are positioned appropriately:
Pacemaker
Defibrillator pads
Figure 8: Correct positioning of defibrillator pads with the pacemaker sited
below the left clavicle (taken from manufacturers recommendations). The pads
should be reversed if the pacemaker is on the other side. However, some texts
suggest that defibrillation should be at 90 degrees to the pacing wire (paddles
front and back).
Mobile phones
All medical electronic equipment may behave as a radio receiver at a number of
frequencies. These frequencies are difficult to predict. Mobile phones, even in standby
mode, send signals to the base station. If the frequencies that the mobile phone is
using match those of medical equipment in the vicinity then interference may occur.
To prevent this it is recommended that mobile phones should not be switched on
within 10 metres of medical equipment.
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DIATHERMY
Diathermy involves the passage of a high frequency current (usually 1 megaHz)
through the patient in order to cause cutting/ coagulation. A sine wave is used for
cutting diathermy and a modulated waveform for coagulation diathermy. The
frequency must be high as low frequency current may cause muscle contraction and
VF. In unipolar diathermy the cutting probe in contact with the patient has a small tip
and thus the current density is high and so is the degree of tissue damage. Also
attached to the patient is a patient plate that completes the circuit back to the
diathermy machine. The area of the patient plate is large so that the current density is
small and there is no tissue damage.
The patient plate is attached within the diathermy machine to an isolating capacitor.
The diathermy current which is of high frequency flows easily across the capacitor
(impedance decreases with increasing frequency of current) whilst the lower mains
frequency is subject to a much higher impedance. In this way the patient plate is
prevented from becoming a route to earth through which a current from faulty
equipment may be conducted.
Diathermy Machine
Isolating capacitor
Patient
Earth
Patient plate
If the patient plate is not properly attached then the current may flow to any point
where the patient is in contact with a conducting surface. In this circumstance the
current may also flow by capacitance effects in the absence of direct contact.
In an alternative arrangement there is no isolating capacitor and the diathermy is not
earthed at all. This is called a floating or isolated circuit. There is still a risk of burns
with stray capacitance linkages however. Modern ECG’s also contain floating or
isolated circuits to avoid providing a route to earth for any stray currents. The
amplified signal then passes into the rest of the equipment through isolating
transformers which ensure electrical isolation. In older models the reference or neutral
lead was earthed through the casing and electrocution could occur.
In bipolar diathermy there is no patient plate as the current is conducted between the
two prongs of the diathermy forceps. Bipolar diathermy is relatively low power and is
therefore used for delicate work only.
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ELECTRICAL SYMBOLS
Capacitor
Battery
Inductor
Resistor
Diode (rectifier) – converts AC to DC e.g.
used in a defibrillator to allow AC mains
current to charge the capacitor.
Amplifier
Earth
Transformer
Class 2 equipment
(double insulation)
Floating circuit
Switch
On/ Off switch
High Voltage
equipment
Hazard – read instructions
before use
Equipment which is safe to
be connected to patient
during defibrillation
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