Lecture 1a- Intro to electrodes and functions Flashcards
What produces electrical potentials
Almost every part of our bodies e.g. neural tissue, muscles, organs and skin
They can be by products or essential control signals
What is the size of the electrical signals produced by the body
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
What are electrodes and recording systems
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.
What are electrodes
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.
What are microelectrodes
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).
What do needle electrodes record
They are used to record from small areas such as motor units, deep within muscles
(Electromyography or EMG)
What are surface electrodes
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
What is needed to record electrophysiological signals
Electrodes- this is because they make electrical contact with the body
Are electrodes always made of metals
Not always and there are advantages suchas reduced skin reaction and better recordings if non metals are used
Can electrodes be thought of as a transducer
Yes, as it has to convert the ionic flow of current in the body to an electronic flow along a wire to a recorder
What are the types of electrodes
Microelectrodes
Needle electrodes
Surface electrodes
What is a microelectrode
electrodes which are used to measure the potential either inside or very close to a
single cell
Do needle electrodes pass through the skin and record potentials from small areas such as a motor unit within a muscle
Yes
What is a surface electrode
what tests is it used to record
electrodes applied to the surface of the body and used to record signals such
as the ECG and EEG.
Why are the features of microelectrodes and why are they not routinely used in medical departments
-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
Why must microelectrodes be handled with care
Microelectrodes must be handled with great care and special recording amplifiers used in order to allow for the very high impedance of tiny electrodes.
What are the forms needle electrodes can come in
design name and test used on
There are many forms but one type is a concentric design used for electromyography
How are needle electrodes used for electromyography
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.
Why is a needle electrode used for needle electromyography
it allows the potentials from only a small
group of motor units to be recorded.
Why is sterilising needle electrodes important
They must be sterile and clean to work satisfactorily and be sterilised before every use.
What are the forms of sterilisation
A&E
Autoclaving and Ethylene oxide gas
What is the method of sterilisaton using ethylene oxide gas
- 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.
What were the early types of surface electrode like
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
What is the most important factor in using any type of electrode
Skin preparation
What are some of the problems with nearly all surface electrodes
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
Which part of the electrode needs to be cleaned
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
Are there other types of electrode
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.
What must an electrode be
(3) F,N,S
Flexible, non toxic and sufficiently conductive to make contact to a wire and then to an amplifier.
Where are electrical signals produced and what do they allows us to do
Almost every part of the body and they allow us to percieve our environment, move within it and interact with it
What do all functions of the human body rely on
The propagation of electrical signals through conductive tissues
Abnormal v normal physiology
Understanding this will allow us to make differential diagnosis’, a diagnostic tool, identifying disease and dysfunction,
What can monitoring do
Inform treatment strategies and reduce off target effects
What size of electrical signals does the body produce
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
What can amplification be
V,C
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.
What happens if the signal to be amplified is small
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
Is the gain of an amplifier the ratio of the output and input voltages
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.
What has any signal got
A frequency content
What is an ECG
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.
What does frequency analysis dictate
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
Is the bandwidth of an amplifier the frequency range over which the gain remains constant
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
What must the bandwidth of an amplifier be able to do
be sufficient to handle all the frequency
components in the signal of interest.
Some examples of phsyiological signals and their frequency
ECG and Body temp in Hz
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
What does interfacing signals require
It requires great care because the computer can only handle digital signals
What is the sample rate
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
Why must the rate of sampling be high
to handle all the frequency components of the desired signal
What is noise
It is any unwanted signal and it can be a hiss on a recording of music, snow on a TV picture etc.
What can cause noise
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.
Why are ECG, EEG and EMG suscepicble to interference
Due to their small amplitude
What else is subject to interference
L,N,C
-Laboratory instruments
-Nuclear medicine equipment
-Computing equipment
(subject to interference from nearby electrical machinery such as lifts, air conditioners, and cleaning equipment)
What is differential amplification and where are differential amplifiers found
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
What can differential amplifiers block
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)
Why must electrode leads be kept together
interfering signals they pick up will differ and will not be blocked
