Lecture 1a- Intro to electrodes and functions

Question 1

What produces electrical potentials

Answer 1

Almost every part of our bodies e.g. neural tissue, muscles, organs and skin

They can be by products or essential control signals

Question 2

What is the size of the electrical signals produced by the body

Answer 2

They are very small.
ECG and EOG are the largest, with amplitudes in the order of 1 mV.
* Focal ERGs as low as a few nV.
* Generally far smaller than interfering signals from outside and inside the body

Question 3

What are electrodes and recording systems

Answer 3

Common to all electrophysiology is a need to
detect and record, often very small, electrical
signals under fairly hostile conditions.
* To do this, we need some form of electrode, an
amplifier, a filter, and a display or recording
device.
* We may also need a stimulator of some sort…
* Our hearts obligingly beat once every second,
but other parts of our bodies may need some
persuasion in order to produce a recordable
signal.

Question 4

What are electrodes

Answer 4

These come in many forms – skin surface, needle, or highly specialised types for a particular
organ, cavity, or application method.
* Used to convert ionic flow of current in the body to electronic flow along a wire
* Usually metal, but not always.

Question 5

What are microelectrodes

Answer 5

They are not clinical and they are used to record from within or close to a single cell.

A traditional glass tube, open at the tip which contains saline (top). New tungsten-in-glass electrode (bottom).

Question 6

What do needle electrodes record

Answer 6

They are used to record from small areas such as motor units, deep within muscles
(Electromyography or EMG)

Question 7

What are surface electrodes

Answer 7

It’s used to record gross potentials such as ECG & EEG from the skin surface

It’s also used for some EMG studies looking at whole or large sections of muscle

Question 8

What is needed to record electrophysiological signals

Answer 8

Electrodes- this is because they make electrical contact with the body

Question 9

Are electrodes always made of metals

Answer 9

Not always and there are advantages suchas reduced skin reaction and better recordings if non metals are used

Question 10

Can electrodes be thought of as a transducer

Answer 10

Yes, as it has to convert the ionic flow of current in the body to an electronic flow along a wire to a recorder

Question 11

What are the types of electrodes

Answer 11

Microelectrodes
Needle electrodes
Surface electrodes

Question 12

What is a microelectrode

Answer 12

electrodes which are used to measure the potential either inside or very close to a
single cell

Question 13

Do needle electrodes pass through the skin and record potentials from small areas such as a motor unit within a muscle

Answer 13

Yes

Question 14

What is a surface electrode

what tests is it used to record

Answer 14

electrodes applied to the surface of the body and used to record signals such
as the ECG and EEG.

Question 15

Why are the features of microelectrodes and why are they not routinely used in medical departments

Answer 15

-small tip
-can only penetrate a single cell
-can only be applied to samples of neural tissue
- very fine wire can be used
- the smallest electrodes consist of a tube of glass which has been stretched and polished to give a tip size as small as 0.5 µm - - the tube is filled with an electrolyte, such as KCl, to which a silver wire makes contact

Question 16

Why must microelectrodes be handled with care

Answer 16

Microelectrodes must be handled with great care and special recording amplifiers used in order to allow for the very high impedance of tiny electrodes.

Question 17

What are the forms needle electrodes can come in

design name and test used on

Answer 17

There are many forms but one type is a concentric design used for electromyography

Question 18

How are needle electrodes used for electromyography

Answer 18

A fine platinum wire is passed down the centre of a hypodermic needle with a coating of epoxy resin to insulate the wire from the needle.

The needle is connected to a differential amplifier, to record the potential between the
tip of the platinum wire and the shaft of the needle. The platinum wire tip may be as small as 200 µm in diameter.

Question 19

Why is a needle electrode used for needle electromyography

Answer 19

it allows the potentials from only a small
group of motor units to be recorded.

Question 20

Why is sterilising needle electrodes important

Answer 20

They must be sterile and clean to work satisfactorily and be sterilised before every use.

Question 21

What are the forms of sterilisation

A&E

Answer 21

Autoclaving and Ethylene oxide gas

Question 22

What is the method of sterilisaton using ethylene oxide gas

Answer 22

• needle needs to be placed in the ethylene oxide gas at 20 psi (140 kPa)
• 1.5 hours at a temperature of 55-66°C.
• the needles must not be used for 48 hours following sterilisation
• allowing for spore tests to be completed and any absorbed gas to be cleared.

Question 23

What were the early types of surface electrode like

Answer 23

Buckets of saline into which the subject placed their arms or legs. A wire was placed in the bucket to make electrical contact with the recording system.
There are now hundreds of different types of surface electrode, most of which can give good recordings if correctly used

Question 24

What is the most important factor in using any type of electrode

Answer 24

Skin preparation

Question 25

What are some of the problems with nearly all surface electrodes

Answer 25

They are subject to movement arefacts;
movement of the electrode disturbs the electrochemical equilibrium between the electrode and the skin and
results in contact potential changes

Question 26

Which part of the electrode needs to be cleaned

Answer 26

The metal tip where a film of dirt can change the electrical performance of the electrode;

Dirt on the tip can cause rectification of radiofrequency interference, with the result that radiobroadcasts can be recorded through
the electromyograph

Question 27

Are there other types of electrode

Answer 27

Yes there are many other types e.g. conductive polymer electrodes which can make contact with the cornea and record electrical signals from the eye.

Question 28

What must an electrode be

(3) F,N,S

Answer 28

Flexible, non toxic and sufficiently conductive to make contact to a wire and then to an amplifier.

Question 29

Where are electrical signals produced and what do they allows us to do

Answer 29

Almost every part of the body and they allow us to percieve our environment, move within it and interact with it

Question 30

What do all functions of the human body rely on

Answer 30

The propagation of electrical signals through conductive tissues

Question 31

Abnormal v normal physiology

Answer 31

Understanding this will allow us to make differential diagnosis’, a diagnostic tool, identifying disease and dysfunction,

Question 32

What can monitoring do

Answer 32

Inform treatment strategies and reduce off target effects

Question 33

What size of electrical signals does the body produce

Answer 33

SMALL
-The largest ECG has an amplitude of 1 mV
-This is too small to drive any recorder directly or to input into a computer
-This is why amplifiers are needed

Question 34

What can amplification be

V,C

Answer 34

Of either voltage or current

Its taken for granted that voltage amplification is required, and that the amplifier will supply sufficient current to drive a recording device.

Question 35

What happens if the signal to be amplified is small

Answer 35

The amplifier gain required will be large
The EEG is only 100 uV in amplitude so an EEG amplifier will have a higher gain than an ECG amplifier

Question 36

Is the gain of an amplifier the ratio of the output and input voltages

Answer 36

YES
Ideally- the gain is independent of frequency.

Real life- the frequency response of the amplifier must be
matched to the frequency content of the signal.

Question 37

What has any signal got

Answer 37

A frequency content

Question 38

What is an ECG

Answer 38

The ECG is a periodic signal of which the lowest frequency component is the heart rate. As such, if the heart rate is 60 per minute, then the lowest frequency component is 1 Hz.

Fourier analysis reveals that the complete
ECG waveform can be produced by adding together sine waves of 1 Hz, 2 Hz, 3 Hz and so on. The amplitude of the components will determine the shape of the ECG.

Question 39

What does frequency analysis dictate

Answer 39

ECG largest components- frequency of 17 Hz

-Components above 100 Hz are of negligible amplitude

-An ECG amplifier must be able to handle frequency components between 1 Hz and 100 Hz; it must amplify the components equally and preserve the relative phase
of all the components.

it examines behavior of a system, signal, or function by looking at its frequency components

Question 40

Is the bandwidth of an amplifier the frequency range over which the gain remains constant

Answer 40

YES

In practice “-3 dB bandwidth”; that is the frequency range over which the gain is not less than 3 dB below the maximum gain

Question 41

What must the bandwidth of an amplifier be able to do

Answer 41

be sufficient to handle all the frequency
components in the signal of interest.

Question 42

Some examples of phsyiological signals and their frequency

ECG and Body temp in Hz

Answer 42

Echocardiogram (ECG) 0.5 – 100 Hz
Electroencephalogram (EEG) 0.5 – 75 Hz
Arterial pressure wave DC – 40 Hz
Body temperature DC – 1 Hz
Electromyograph (EMG) 0.01 – 5 KHz
Nerve action potentials 0.01 – 10 KHz
Smooth muscle potential (e.g. GI tract) 0.05 – 10 Hz

Question 43

What does interfacing signals require

Answer 43

It requires great care because the computer can only handle digital signals

Question 44

What is the sample rate

Answer 44

The computer will only handle digital signals; the voltage must be sampled and recorded at regular intervals (this is the sample rate).

recording and sampling of digital signals at regular intervals

Question 45

Why must the rate of sampling be high

Answer 45

to handle all the frequency components of the desired signal

Question 46

What is noise

Answer 46

It is any unwanted signal and it can be a hiss on a recording of music, snow on a TV picture etc.

Question 47

What can cause noise

Answer 47

The random motion of electrons in the recording system or from an interfering signal.

Electrical interference can be a problem when making almost any physiological
measurement.

Question 48

Why are ECG, EEG and EMG suscepicble to interference

Answer 48

Due to their small amplitude

Question 49

What else is subject to interference

L,N,C

Answer 49

-Laboratory instruments
-Nuclear medicine equipment
-Computing equipment

(subject to interference from nearby electrical machinery such as lifts, air conditioners, and cleaning equipment)

Question 50

What is differential amplification and where are differential amplifiers found

Answer 50

Where- Differential amplifiers are found in practically all systems for recording small physiological signals.

What- will only pass signals that differ between its two inputs

Question 51

What can differential amplifiers block

Answer 51

Any signal that is the same at both inputs will be blocked.

Thus, if the two inputs are connected to a patient via electrodes, any potential difference between them (such as an ECG) will be amplified, but any interfering signal picked up by the electrode leads such as mains interference will be blocked.
(diagram)

Question 52

Why must electrode leads be kept together

Answer 52

interfering signals they pick up will differ and will not be blocked