MEDICAL ELECTRONICS.pdf

Phonocardiogram
• Phonocardiography - study of different heart
sounds.
• Phonocardiograph - instrument used to pick
up the different heart sound, filter the
required and display.
• Phonocardiogram- graphic record of heart
sounds.

Two categories
• Heart sound:
• Transient characteristics with short duration occurs due
to opening and closing of the heart valves.
• In abnormal heart additional sounds are heard between
the normal heart sound.
• Murmurs:
• additional sounds
• Noisy characteristics with long duration occurs due to
turbulent blood flow in heart.
• Caused by improper opening of the valves

Heart Sound
• Mechanical working processes of the heart
produce sound - health status.
• Relationship between blood volumes, pressures
and flows within the heart determines the
opening and closing of the heart valves.
• Normal heart sounds- lub and dub
• Valvular theory - heart sounds originate from a
point sources located near the valves.
• Cardiohemic theory -the heart and the blood
represent an interdependent system that vibrates
as a whole and propagates sound as waves of
alternate pressure.

CLASSIFICATION OF HEART
SOUND
• It is divided into four types:
• Valve closure sound
• Ventricular filling sound
• Valve opening sound
• Extra cardiac sound

Types of Heart Sound
• Valve closure sound - occurs at the beginning
of systole and at the beginning of diastole.
• Ventricular filling sound - occurred at the
time of filling of the ventricles.
• Valve opening sound - occurs at the time of
opening of atrio- ventricular valves and semi
lunar valves.
• Extra cardiac sound - occur in mid systole or
late systole or early diastole

• Systole:
• Contraction of the heart muscle
• Systolic pressure: 120 mm
• Diastole:
• Relaxation of the heart muscle
• Diastolic pressure is 80 mm

First Heart Sound
Initial vibrations
occur when first
contraction of
ventricle move
blood towards
atria, closing
AV valves
Abrupt tension
of closed AV
valves,
decelerating
the blood
Oscillation of
blood between
root of aorta
and
ventricular walls
Vibrations
caused by
turbulence
in ejected
blood flowing
into aorta

Second Sound S2
• The second sound (S2) signals the end of systole
and the beginning of diastole
• Heard at the time of the closing of the aortic
and pulmonary valves
• S2 - the result of oscillations in the cardiohemic
system caused by deceleration and reversal of
flow into the aorta and the pulmonary artery

S3 and S4
Third heart sound (S3)
• connected with the diastolic filling period. The
rapid filling phase starts with the opening of the
semilunar valves.
• attributes energy released with the sudden
deceleration of blood that enters the ventricle
throughout this period
Fourth heart sound (S4)
• connected with the late diastolic filling period
• occur during atrial systole where blood is forced
into the ventricles.

Basic Heart Sounds in a
Phonocardiogram Recording

PCG Signal Characteristics
• 1 st Heart sound:
• Closure of mitral and tricuspid valves.
• Frequency- 30 to 100Hz and duration 50 to
100ms.
• 2 nd Heart sound :
• Closure of aortic and pulmonary valves.
• Frequency- 30 to 100Hz and duration 25 to
50ms.

PCG Signal Characteristics
• 3 rd heart sound:
• Blood rapid movement into relaxed ventricular
chambers
• Frequency – 10 to 100 Hz and duration 0.04 to
0.08 S.
• 4 th heart sound :
• Atrial contraction.
• Frequency 10 to 50HZ and duration 0.03 to 0.06S

Heart Murmers
• Murmurs are extra heart sounds that are produced as a
result of turbulent blood flow which is sufficient to
produce audible noise.
• Innocent murmurs - present in normal hearts without any
heart disease.
• Pathologic Murmurs - result of various problems, such
as narrowing or leaking of valves, or the presence of
abnormal passages through which blood flows in or near
the heart.
• Graded by intensity from I to VI.
• Grade I - very faint and heard only with special effort
• Grade VI - extremely loud

Factors involved in production of
Murmers
Heart Murmurs
High rate of
flow through
valves
Flow through
constricted
valves
(stenosis)
Backward flow
through
incompetent
valve
Septal defects
Decreased
viscosity,
which causes
increased
turbulence

Block Diagram of PCG recording

PCG – Recording set up
(i) Microphone :
• used to convert heart sound into the electrical
signals.
• Certain positions are recommended to pick up the
heart sound by using microphone.
(ii) Amplifier:
• Electrical signal picked up by the microphone is
amplified by the amplifier block.
• The amplified output is given to filter block.

PCG – Recording set up
(iii) Filter:
• Permit selection of suitable frequency bands
• Avoid aliasing
• Separate louder low frequency signals from
lower intensity, much informative high
frequency murmurs.
• ECG electrode system and ECG amplifiers are
used for reference for PCG.
• ECG and PCG outputs are connected to FM
tape recorder and output display unit.

Sensors
• Sensors used when recording sound:
 Microphones
 Accelerometers
• Microphone - air coupled sensor that measure
pressure waves induced by chest-wall movements
• Accelerometers - contact sensors which directly
measures chest-wall movements
• For recording of body sounds,
 condenser microphones
 piezoelectric accelerometers

Acquisition of Phonocardiographic
Signals
• Microphones picks up
(i). Heart sounds
(ii). Heart murmurs
(iii). Extraneous noise in the immediate vicinity of
the patient
• Group 1-
(i) . Contact microphone
(ii). Air coupled microphone
• Group 2-
(i) Crystal microphone
(ii) Dynamic microphone

Contact Microphone
• also known as a pickup or
a piezo microphone
• made of a thin
piezoelectric ceramic
round
• designed to transmit audio
vibrations through solid
objects.
• contact mics act as
transducers which pick up
vibrations and convert
them into a voltage which
can then be made audible

Air coupled Microphone
• shows a low-pass frequency response because
of its air-chamber compliance.
• Movement of chest is transferred through the
air cushion.
• It provides low mechanical impedance to the
chest.
• The sound pressure, or normal stress exerted
on the chamber should be constant to keep a
flat response.

Crystal and Dynamic Microphones
• Crystal microphone :
• contains wafer of piezo-electric material
• which generates potentials when subjected to
mechanical stresses due to heart sound.
• Smaller in size, high sensitivity.
• Dynamic microphone :
• consists of a moving coil having a fixed magnetic core
inside it.
• coil moves with the heart sound and produce a voltage
because of interaction with the magnetic flux.

Writing methods for phonocardiography
• Requires a writing system capable of responding
to 2000 Hz.
• Types of writing methods:
(i). Mechanical Recorders
(ii). Optical Galvanometric Recorders
(iii). Envelope detection
(iv). Direct recording using Ink Jet
Recorders
(v). Electrostatic Recorder
(vi). Thermal Recorder

Introduction
• Electromyogram (EMG) - technique for evaluating and
recording the activation signal of muscles.
• EMG is performed by an electromyograph, which
records an electromyogram.
• Electromyograph detects the electrical potential
generated by muscle cells when these cells contract
and relax.
• Electrical signals associated with the contraction of a
muscular is called an electromyogram (EMG).
• The study of EMG’s is called electromyography

EMG Signal
Factors, which can influence the EMG signal:
• Velocity of shortening or lengthening of the
muscle
• Fatigue - extreme tiredness resulting from
mental or physical exertion or illness.
• Reflex activity - involuntary and immediate
movement in response to a stimulus

Electrodes used for EMG
(i)Surface electrodes - used when signals are
recorded on the surface and it cannot detect
deep potential from within the cell.
(ii)Needle electrodes - inserted deep into the
tissue, which records the muscle potential.
(iii)Microelectrode - used to record action
potential from single nerve.

carpal tunnel
• The carpal tunnel is a narrow passageway
surrounded by bones and ligaments on the
palm side of your hand.
• When the median nerve is compressed, the
symptoms can include insensible, pricking and
weakness in the hand and arm
• causes: Repetitive motions, like typing, or any
wrist movements

Electrical Characteristics
• The electrical source is the muscle membrane
potential of about -70mV.
• Measured EMG potentials range between < 50
μV up to 20 to 30 mV, depending on the
muscle under observation.
• Typical repetition rate of muscle- 7-20 Hz.
• Repetition rate - rate at which recurrent
signals are produced or transmitted.

Electrodes
• Electrodes record EMG potentials from the tissue.
• Disc shaped surface electrodes made of Ag/AgCl
are used to pick the signals.
• skin surface is cleaned and electrode gel is
applied on the skin surface.
• Elastic bands - to attach the electrodes to the skin.
• When electrodes are affixed tight on the skin,
it reduces the skin contact impedance to below
10kΩ.

Amplitude of EMG signal
Depends on various factors like :
• Type of electrode used
• Placement of electrode on the muscle and
• The degree of muscular exertions.
• When surface electrodes are used, it picks
signals
• from nearby spikes and produces average voltage
value from the muscles and motor units.
• If needle electrodes- pick signal from single
nerve fiber and it produces one voltage value.

Differential Amplifier
• EMG potentials are taken from the tissue by using
electrodes.
• potentials are given to differential amplifier.
• High gain amplifier and frequency range is 10 Hz
to 10 KHz.
• Bandwidth of EMG is large.
• CMRR (Common mode Rejection Ratio) of this
differential amplifier is 80 to 100 db.
• Input Impedance of this amplifier is 10 MΩ.

Power Amplifier and CRO
• Before giving the output of differential amplifier
to loudspeaker, it is given to power amplifier.
• Power amplifier - amplifies the signal that is
received by loudspeaker.
• The amplified signal from the output of the
differential amplifier is displayed by using CRO.
• Storage oscilloscope - Output cab be displayed
and the same can be stored in CRO
• signal from the differential amplifier is recorded
by using tape recorder.

Cntd..
• In the motor nerves, to indicate the location and
type of nerve lesions, conduction velocity is
measured.
• An electric shock of duration 0.2 – 0.5 ms is
stimulated on the nerves.
• Conduction velocity along the peripheral nerve is
measured to calculate the latency.
• Latency - difference in time between stimulating
impulse and action potential of the muscle.

Steps involved in measurement of
conduction velocity
• Stimulate is applied at point A
• Electrical activity of muscle is
measured at point B
• Space between A and - l1
meters.
• Latency - Time delay between
applying stimulus and
receiving action potential
• Time delay - t1 second.
• Now change the position of A
into C. Now the space is
reduced.
• It is noted as l2meters.
• Time delay - t2 second.

Electrodes
• Small metal discs usually made of stainless steel, tin,
gold or silver covered with a silver chloride coating .
• Placed on the scalp in special position
• position of electrodes is specified by international
10/20 system.
• Each electrode site is labeled with a letter and a
number
• Each letter refer to the area of brain underlying the
electrode .
• Even numbers denote the right side of the head and
odd numbers denote the left side of the head .

Electrodes
• Electrodes are attached to channel selector in
groups of eight called a montage of
electrodes.
• The right ear electrode acts as reference
electrode for the right brain electrodes
• left ear electrode acts as reference electrode
for the left brain electrodes.

Amplifier
• An electronic device that increases the power of
a signal.
• It takes energy from a power supply and
controlling the output to match the input signal
shape but with a larger amplitude.
• Human brain wave activity is too subtle to read
unless the signal is amplified.
• These units available now usually connect
through a USB port and transmit signals to the
therapist computer.

Filters
• Use of filters in recording and displaying EEG
data is an indispensable tool in producing
interpretable EEG tracings.
• Without filters, many segments of EEG would
be essentially unreadable
• The main benefit of filters is that they can
appear to “clean up” the EEG tracing
• More pleasing to the eye

Writing unit
• Final link between the
patient and a legible EEG
tracing is the writer.
• A pen-ink-paper system is
employed
• The speed of the paper
mechanism should include
30 mm/s with at least the
additional speeds of 15 mm
• 60 mm/s selectable during
operation.
• Writing unit may be
replaced by a digital screen
in modern EEG devices

Mechanism Of EEG
• Billions of nerve cells in brain produce very small
electrical signals that form patterns called brain
waves.
• During an EEG, small electrodes and wires are
attached to your head.
• Electrodes detect your brain waves and the EEG
machine amplifies the signals and records them in a
wave pattern on graph paper or a computer screen.
• EEG waveforms are generally classified according to
their frequency, amplitude, and shape, as well as the
sites on the scalp at which they are recorded.
• Classification of EEG waveform- alpha, beta, theta,
and delta

Artifact correction
• Artifacts are signals recorded by EEG but not
generated by brain.
• Physiologic artifacts originate from the patient
• Non-physiologic artifacts originate from the
environment of the patient
• Independent component analysis techniques
have been used to correct or remove EEG
contaminants
• This would result in clean EEG by nullifying
(zeroing) the weight of unwanted components
• Surface Laplacian has been shown to be
effective in eliminating muscle artifact

Why is an EEG performed ?
• An EEG is used to detect problems in the electrical
activity of the brain that may be associated with
certain brain disorders.
• Measurements given by an EEG are used to confirm or
rule out various conditions, including:
• Disorders (such as epilepsy)
• Head injury
• Brain tumor
• Encephalopathy (disease that causes brain dysfunction)
• Memory problems
• Sleep disorders
• When someone is in a coma, an EEG may be performed
to determine the level of brain activity.

Risks of EEG (Disadvantages)
• EEG can cause seizures in a person with a seizure disorder.
• This is due to the flashing lights or the deep breathing that
may be involved during the test.
• seizure - a sudden, uncontrolled electrical disturbance in
the brain.
• It can cause changes in your behavior, movements or
feelings, and in levels of consciousness
• Certain factors or conditions may interfere with the
reading of an EEG test. These include:
• A. caused Low blood sugar
• B. Body or eye movement during the tests.
• C. Lights, especially bright or flashing ones
• D. Drinks containing caffeine, such as coffee and cola
• E. Oily hair or the presence of hair spray

Preparation for EEG:
what a person should do before the test:
• patient will be asked to sign a consent form that gives him/her permission to
do the procedure.
• The patient must wash his/her hair with shampoo, but conditioner must not
be used the night before the test.
• The patient must Tell his/her healthcare provider of all medicines and herbal
supplements that they are taking.
• The patient must Discontinue using medicines that may interfere with the test
if the healthcare provider has directed him/her to do so.
• The patient must Avoid consuming any food or drinks containing caffeine for 8
to 12 hours before the test.
• G- The patient must Avoid fasting the night before or the day of the procedure.
Low blood sugar may influence the results.

During the EEG procedure:
• A standard EEG takes about 1 hour. The patient will be
positioned on a padded bed or table.
• To measure the electrical activity in various parts of the brain,
a nurse or EEG technician will attach 16 to 20 electrodes to the
scalp.
• The brain generates electrical impulses that these electrodes
will pick up.
• To improve the conduction of these impulses to the electrodes,
a gel will be applied to them.
• The electrodes only gather the impulses given off by the brain
and do not transmit any stimulus to the brain.
• The technician may tell the patient to breathe slowly or quickly
and may use visual stimuli such as flashing lights to see what
happens in the brain when the patient sees these things.
• The brain's electrical activity is recorded continuously
throughout the exam on special EEG paper.

After EEG procedure:
• After the test is complete, the technician will remove
the electrodes.
• The patient will be instructed when to resume any
medications.
• The patient generally will be ready to go home
immediately following the test. No recovery time is
required.
• The patient should avoid activities that may harm
them if a seizure occurs, until they have resumed their
seizure medication for an adequate length of time.
• The doctor or technician will tell the patient when and
how they will learn the results of their EEG.

EEG results
• When the EEG is finished, the results are interpreted by a
neurologist
• EEG records the brain waves from various locations in the
brain.
• Each area produces a different brain wave strip for the
neurologist to interpret.
• When examining the recordings, the neurologist looks for
certain patterns that represent problems in a particular area
of the brain.
• For example, certain types of seizures have specific brain
wave patterns that the trained neurologist recognizes.
• Likewise, a normal brain has a specific brain wave pattern
that the trained neurologist recognizes
• The neurologist must look at all recorded tracings, decide
what is normal and what is not, and determine what the
abnormal tracings represent.

Uses of EEG
• Clinical Use
• Distinguish epileptic seizures from non-
epileptic seizures
• To serve as an adjunct test of brain death
• Monitor human and animal brain development
• Test epilepsy drug effects
• Investigate sleep disorder and physiology
• Control anesthesia depth

ELECTROENCEPHALOGRAPH (EEG)
• Electroencephalography:
• This is the technique by which the electrical
activities of the brain are studied.
• Electroencephalograph:
• This is the instrument by which the electrical
activities of the brain are recorded.
• Electroencephalogram:
• This is the record or graphical registration of
electrical activities of the brain.

EEG
• EEG is a medical device for analyzing the electrical
• activity of the brain.
• it can detect Epilepsy or Alzheimer’s diseases. .
• used in humans by Hans Berger in 1924
• the general mechanism is picking up the charge of
electrical potentials
• the neurons are negative (-) when they are at rest and
become positive (+) when they synapse

EEG Waves
• It records the signals as wavy lines on to a
computer screen or paper in order of
microvolt.
• EEG waves:
• frequency range = 0.1 to 100
• amplitude = 2 to 200 micro volt.

Significance of EEG:
• EEG is useful in the diagnosis of neurological
disorders and sleep disorders.
• Helps to detect and localize cerebral brain
lesions.
• Brain lesions a region in an organ or tissue which
has suffered damage through injury or disease,
such as a wound, ulcer, abscess, or tumors.
• An EEG can identify areas of the brain that are
not working properly
• EEGs are also used to determine the level of brain
function in people who are in a coma

Types of EEG
• Routine EEG:
Around 20 electrodes are stuck to the scalp using a
special paste and EEG signals are recorded
• Sleep EEG:
The EEG tracing will be recorded along with the
heart rate,airflow, respiration, oxygen saturation and
limb movement
• Ambulatory EEG:
It involves recording the brain activity throughout
the day and night
A small portable EEG recorder is clipped on to the
clothing

EEG Electrodes
• Transform ionic currents from cerebral tissue into electrical
current used in EEG preamplifier.
• 5 types of electrodes are used.
1. Scalp: silver pads, discs or cups, stainless steel rods,
chlorided silver wires.
2. Sphenoidal: Alternating insulated silver and bare wire and
chlorided tip inserted through muscle tissue by a needle.
3. Nasopharyngeal: Silver rod with silver ball at the tip
inserted through the nostril.
4. Electrocorticographic: Cotton threads soaked in saline
solution that rests on brain surface.
5. Intracerebral: sheaves of Teflon coated gold or platinum
wires used to stimulate the brain.

TYPES OF LOBES
(i) Frontal lobes:
(ii) Parietal lobes:
(iii)Temporal lobes:
(iv)Occipital lobes:

Frontal Lobe
• Most anterior portion of the cerebrum(under
forehead)
• Controls motor function, personality and
speech
• Like center of reasoning, planning,
emotions,problem soving
• Also called as motor cortex

Parietal Lobes
• Most superior portion of the cerebrum(top of
head)
• Receives and interprets nerve impulses from
sensory receptors and interprets language
• Receives sensory input from the skin(touch,
temperature, pain)
• Also called as sensory cortex

Occipital Lobe
• Most posterior portion of the cerebrum(back
of the head)
• Receives input from the eyes and control
vision
• Also called as visual cortex

Temporal lobe
• Left and right lateral portion of the
cerebrum(on the sides of your head above
ears)
• Controls hearing and smell
• Also called as auditory cortex

EEG Waves
• Electrical recordings from the surface of the brain
• Recordings from the outer surface of the head
demonstrate continuous electrical activity in the brain.
• Intensities of the brain waves on the surface of the
scalp range from 0-300uV and their frequencies range
from 0.5 Hz to 100Hz.
• Brain waves are irregular and no general pattern can be
discerned in the EEG.
• However at other times distinct patterns will appear.
• In normal persons it is classified as alpha, beta, theta
and delta waves.

Alpha Waves
• Alpha waves are found in normal persons
(resting state), when they are awake .
• They occur in occipital region .
• Frequency : 8 - 13 HZ.
• Alpha waves are present, they indicate a calm
& relaxed state
• During sleep they disappear.

Beta Waves
• Beta waves are recorded from parietal &
frontal region of scalp.
• Divided into 2 types:
• Beta - 1 which is inhibited by cerebral activity.
• Beta - 2 excited by mental activity like tension.
• Frequency: 13 to 30 Hz

Theta waves
• Theta waves are recorded from temporal
region of scalp from children.
• They occur stress & frustration.
• Frequency : 4 - 8 HZ.
• Theta waves observed when you are day
dreaming & drowsiness.

Delta waves
• Delta waves are recorded from cortex region.
• They occur deep sleep in premature babies &
incases of brain diseases.
• Frequency: 0.5 - 4 HZ.
• Delta waves observed when you are day
dreaming & drowsiness.
• occur only once in every 2 or 3 seconds
during deep sleep

Placement of Electrodes
• In EEG, electrodes are placed in standard
positions on the skull
• Arrangement 10-20 system - placement
scheme devised by the International
Federation of societies of EEG.

• Draw a line on the skull from
the nasion,the root of the
nose, to the inion
• Draw a similar line from the
left preauricular (ear) point
to the right preauricular
point.
• Mark the intersection of
these two lines as Cz which
is the mid point of the
distance between the nasion
and inion

• Points Fpz,Fz, Cz, Pz and Oz:
Mark points at 10, 20, 20,
20, 20 and 10% of the total
nasion - inion distance.
• T3, C3, Cz, C4 and T4 : Mark
points at 10, 20, 20, 20, 20
and 10% of the total
distance between the
preauricular points
• In these odd numbered
points T3 and C3 are on the
left
• Even numbered points C4
and T4 are on the right.

• Measure the distance
between Fpz and Oz along
the great circle passing
through T3 and mark points
at 5, 10, 10, 10, 10 and 5%
of this distance. These are
the positions of Fp1, F7, T3,
T5 and O1.
• Repeat this procedure on
the right side and mark the
positions of Fp2, F8, T4, T6
and O2.

• Measure the distance
between Fp1 and O1 along
the circle passing through
C3 and mark points at 25
and 20%intervals. These
points give the positions of
F3, C3 and P3.
• Repeat this procedure on
the right side and mark the
positions of F 4, C4 and P4.

• Check that F7, F3, Fz, F4
and F8 are equidistant
along the transverse
circle passing through F7,
Fz and F8 and check that
T5, P3, Pz, P4 and T6 are
equidistant along the
transverse circle passing
through T5, Pz and T6.

• Before placing the electrodes, the scalp is
cleaned, lightly abraded and electrode paste is
applied between the electrode and the skin.
• By means of this application of electrode
paste, the contact impedance is less than
10KΩ.
• Generally disc like surface electrodes are used.
In some cases, needle electrodes are inserted
in the scalp to pick up EEG.

Biological Amplifiers
• Generally, biological/bioelectric signals have low
amplitude and low frequency.
• To increase the amplitude level of biosignals amplifiers
are designed.
• The outputs from these amplifiers are used for further
analysis and they appear as ECG, EMG, or any
bioelectric waveforms..
• Need to amplify biopotential which are generated
in the body
• Bioamplifiers are required to increase the signal
strength while maintaining fidelity

Basic Requirements for Biological
Amplifiers
• The biological amplifier should have a high input impedance value.
The range of value lies between 2 MΩ and 10 MΩ depending on the
applications. Higher impedance value reduces distortion of the
signal.
• When electrodes pick up biopotential from the human body, the
input circuit should be protected. Every bio-amplifier should consist
of isolation and protection circuits, to prevent the patients from
electrical shocks.
• Since the output of a bioelectric signal is in mill volts or microvolt
range, the voltage gain value of the amplifier should be higher than
100dB.
• Throughout the entire bandwidth range, a constant gain should be
maintained.
• A bio-amplifier should have a small output impedance.
• A good bio-amplifier should be free from noise.

ECG - Electrocardiograph
• Recording of electrical activity of heart on a
graph
• Graphical representation of electrical activity
of heart
• Machine which is used to record the electrical
activity of heart:
• ECG machine and power lab
• Graph on which electrical activity recorded -
Electrocardiogram

Significance of ECG
• Gives rate and rhythm of heart
• Does not provide any information about the
mechanical activity
• Its diagnostic tool for measuring various heart
conditions
• It is a tool to determine the presence and
severity of acute myocardial issues(heart
attack)

ECG Wave
• Electrocardiogram is a graphical
representation of the bio-electrical currents
generated by myocardial cells.
• By placing a pair of electrodes on the body, an
ECG can be measured and recorded.
• Graphical representation (ECG) can be either
printed on a paper or displayed on a monitor.
• The device which displays the ECG on a
screen is called monitor or cardiac monitor.

Depolarization and Repolarization
• Depolarization - is the loss of resting membrane
potential due to the alteration of the polarization of
cell membrane
• Repolarization - is the restoration of the resting
membrane potential after each depolarization event.
• Depolarization in the right and left atria, causing
contraction, which corresponds to the P wave on an
electrocardiogram.
• It is the restoring of the resting state. In the
ECG, repolarization includes the J point, ST segment,
and T and U waves

Chambers of Heart
• The heart has four chambers:
two atria and two ventricles.
• Right atrium receives oxygen-
poor blood from the body and
pumps it to the right ventricle.
• Right ventricle pumps the
oxygen-poor blood to the
lungs.
• Left atrium receives oxygen-
rich blood from the lungs and
pumps it to the left ventricle.

Einthoven - ECG
• Inventor of the first device capable of printing the ECG
on paper.
• Einthoven named the waves using five capital letters
from the alphabet: P, Q, R, S, and T.
• width of a wave on the horizontal axis represents a
measure of time.
• Height and depth of a wave represent a measure of
voltage
• upward deflection of a wave - positive deflection
• Downward deflection of a wave - negative deflection.

ECG Waveforms
• Description of 5 ECG waveforms :
• P wave : represents the depolarization impulse across
the atria
• Q, R and S waves : all these three waves represent the
ventricular depolarization
• the electrical impulse spreads through
the ventricles and indicates ventricular depolarization
• (the downward stroke followed by and upward stroke
is called Q wave, the upward stroke is called R wave
and any downward stroke preceded by an upward
stroke is called S wave)

• T wave : represents the repolarization of the
ventricles (resting state)
• J-point elevation represents a family of ECG
findings.
• The U wave is a small (0.5 mm) deflection
immediately following the T wave. U wave is
usually in the same direction as the T wave
• Time from beginning of P wave to the beginning
of QRS wave – PR interval(0.12 to 0.20 secs)
• QRS complex includes the Q wave, R wave, and S
wave.

Waveform specifications of ECG

ECG lead Configuration
• Leads are electrodes which record the electric potential of heart.
• The 12 standard ECG leads are divided in to
i) Bipolar limb leads or Standard leads or Einthoven lead system
• Lead I
• Lead II
• Lead III
ii) Augmented unipolar limb leador Wilson Lead System
• aVR
• aVL
• aVF
iii) Unipolar chest leads

Bipolar Leads
• Measure the difference in electric potential
between 2 different points in a body
• In this lead system, the potentials are tapped
from four locations of our body. They are
• i) Right arm ii) Left arm iii) Right Leg iv) Left
Leg
• The Right Leg (RL) electrode acts as the
reference electrode.

Lead I
• It is a lead obtained
between a negative
electrode placed on the
right arm and a positive
left arm.
• It gives voltage VI, the
voltage drop from the
left arm(LA) to the right
arm(RA).

Lead II
between a negative
right arm and a positive
left foot.
• It gives voltage VII, the
left leg (LL) to the right
arm (RA).

Lead III
between a negative
left arm and a positive
left foot.
• It gives voltage VIII, the
left leg (LL) to the left
arm (LA).

Einthoven Triangle
• The closed path RA to LA
to LL and back to RA is
called as Einthoven
Triangle.
• The vector sum of the
projections on all the three
sides is equal to zero.
• Applying KVL, the R wave
amplitude of lead II is
equal to the sum of the R
wave amplitudes of Lead I
and Lead III.

Augmented Unipolar Limb Leads
• Introduced by Wilson.
• The electrocardiogram is recorded between a single electrode
and the central terminal which has a potential corresponding
to the
• center of the body.
• Two equal and large resistors are connected to a pair of limb
electrodes and the center of this resistive network.
• Remaining limb electrode acts as exploratory single electrode.
• By means of augmented ECG lead connections, a small
increase in the ECG voltage can be realized. The
• augmented voltage Right arm (aVR)
• augmented voltage Left arm (aVL)
• augmented voltage Foot arm (aVF)

Contd…..
• aVR: lead obtained between the average signal
obtained from three negative electrodes (left
arm, left leg and right foot) and the signal
obtained from a positive electrode placed on the
right arm
• aVL: (right arm, left foot and right foot) and the
signal obtained from a positive electrode placed
on the left arm
• aVF: left arm, right arm and and right foot) and
the signal obtained from a positive electrode
placed on the left foot

unipolar chest leads
• In unipolar chest leads, the exploratory electrode
is obtained from one of the chest electrodes.
• Exploratory electrode - an electrode placed on or
near an excitable tissue
• The chest electrodes are placed at six different
points on the chest close to the heart.
• By connecting 3 equal large resistors to RA, RL, LL,
a central terminal is obtained.
• This lead system is known as Wilson lead system.

Lead positions
• V1: is a lead obtained between the reference negative electrode and a
positive electrode placed on the chest in the V1 position
positive electrode placed on the chest in the V6 position.

ECG Recorder
(i)Electrode system:
• Metal plate electrodes made of Ag/AgCl are placed at desired limb
positions.
• Good contact between electrodes & skin is ensured with the help of
gel and belts.
(ii)Lead fault detect:
• To detect the improper connection of the electrodes on to the skin
by continually measuring the contact resistance
• warn the operator of this by using an audible tone or a visual
indication.
(iii)Amplifier protection circuitry:
• To protect the remaining part of the circuit from large electrical
discharges resulting from defibrillation process.

ECG Recorder
(iv)Lead selector:
• Function of this block is to select a desired lead system from 12
possible lead systems.
• carried out either manually by an operator or
• automatically by microprocessor or microcontroller or
microcomputer.
(v)Preamplifier:
• Function of this block is to eliminate noise
• Noises - biopotential and various electromagnetic interferences
resulting from nearby communication links etc.
• Differential amplifier with high input impedance and CMRR is used for
this purpose.
(vi)Calibration signal:
• Function of this block is to calibrate the display or the recorder for
predetermined amplitude.
• A sine wave of 1 mV is generally used for this purpose.

ECG Recorder
Baseline restoration:
• Function of this block is to restore any baseline shift
resulting from the low operating frequency of the amplifier.
Right leg driven system:
• The function of this block is to provide a reference point on
the patient generally at ground potential.
Isolation circuitry:
• The function of this block is to provide electrical isolation
between the high power section that is generally driven by 230 V
50 Hz ac mains and the low power patient section that is generally
driven by a low power battery.
• This is required to protect the patient from any electrical hazards
resulting from leakage currents

ECG Recorder
Driver amplifier:
• To amplify the ECG signal sufficiently to level required for the display or the
recorder.
ADC & memory:
• The ECG signal can be digitized and stored for future analysis.
Microcomputer:
• A microcomputer along with a user-friendly software package developed on
a high-level language such as VC++ can be used
(i) to control the entire process of acquiring the ECG and
(ii) to analyze it automatically for various parameters such as heart rate, PR
interval,
QRS interval etc using sophisticated digital signal processing techniques.
Recorder-printer/display:
• A heat sensitive paper can be used to get a hard copy of the ECG signal
obtained
• CRO can be used to display the ECG signal obtained for visual

Holter ECG
• Continuous recording of ECG at a stretch up to
24 hour and playing it in as minimum as 12
minutes
• used to diagnose certain arrhythmias
• Arrhythmia – irregular heart beats may be too
fast or too slow or rhythm may be irregular
• which occur under certain physiological
conditions such as emotional stress.

Problems frequently encountered
during ECG recording
(1) Frequency distortion:
• High frequency distortion rounds off sharp corners of ECG
waveforms and reduces the amplitude of QRS complex.
• Low frequency distortion shifts the base line causing monophasic
waves in ECG to be biphasic.
(2) Saturation or cutoff distortion:
• High offset voltages at the electrodes or amplifier produce
saturation or cutoff distortion – peaks of QRS complex are cut off
due to this.
(3) Ground loop:
• When two or more equipments are grounded via different outlets,
there may exist a potential difference among these grounds.
• This leads to a current from one ground through the patient to
another ground as shown below.

Open lead wires:
• Due to rough handling or bad wiring, one or
more lead wire may become disconnected from
the electrodes.
• This leads to invalid signals.
Artifacts:
• Due to large electrical discharges or patient’s
large movements, serious artifacts are produced
in the recorded ECG signal.
• Example: Internal electrical artifacts can
be caused by muscle shivering, hiccups
• personal adornment such as buttons, jewelry and
clothing.

Bio Electrodes:
Bio electrodes function as an interface between biological structures and
electronic systems.
Electrical activity within the biological structure is either sensed or
stimulated.
The electrical systems are either passively sensing (measuring) or
actively stimulating (inducing) electrical potentials within the biological
structure or unit.
Bioelectric potentials generated in our body are ionic potentials and it is
necessary to convert these ionic potentials into electronic potentials
before they can be measured by conventional methods.
Devices that convert ionic potential into electronic potential are called
electrodes.
A transducer that converts the body ionic current in the body into the
traditional electronic current flowing in the electrode is a Bio Electrode.
16

Bio Electrodes:
Able to conduct small current across the interface between the body and
the electronic measuring circuit.
Oxidation is dominant when the current flow is from electrode to
electrolyte, and reduction dominate when the current flow is in the
opposite.
The net current that crosses the interface, passing from the electrode to
electrolyte consist of
 Electrons moving in a direction opposite to that of current in the
electrode.
 Cations moving in the same direction.
 Anions moving in direction opposite to that of current in electrolyte.
17

Electrode – Electrolyte Interface
•A conductor through which electricity enters or leaves an object,
substance, or region
•use of electrodes : to generate electrical current and pass it through
nonmetal objects to basically alter them in several ways.
•Electrodes are also used to measure conductivity
•Electrolyte :
• A substance that dissociates into ions in solution and acquires the
capacity to conduct electricity.
•Examples: Sodium, potassium, chloride, calcium, and phosphate
•An electrolyte is a chemical compound that dissociates into ions and
hence is capable of transporting electric charge - i.e.
•an electrolyte is an electric conductor; unlike metals the flow of charge is
not a flow of electrons, but is a movement of ions

•When a metal is partly immersed in an electrolyte, a potential is
set up across the two phases, i.e., at the electrode/electrolyte
interface.
•Phases- solids,liquids,gases
•potential difference being set up across the interface of two
phases
•charge transfer occurs across the interface.
•During this process, a charge separation will occur because of
electron transfer across the interface

OXIDATION AND REDUCTION
•Oxidation is loss of electrons
•That means that an oxidizing agent
takes electrons from that other
substance.
•So an oxidizing agent must gain
electrons.
•oxidizing agents are oxygen,
hydrogen peroxide
•Reduction is gain of electrons
•A reducing agent is a substance that
causes another substance to reduce
•Ex:earth metals, formic acid, oxalic
acid

Properties of Bio Electrodes:
Good conductors.
Low impedance.
Should not polarize(restrict) when a current flows through them.
Should establish a good contact with the body and not cause motion.
Should not cause itching swelling or discomfort to the patient.
Metal should not be toxic.
Mechanically rugged – capable of withstanding rough handling
Easy to clean.
19

Electrode – Skin/Tissue Interface
Interface between body and electronic measuring device.
Conducts current across the interface.
Ions carry current in the body.
Electrodes are capable of changing ionic current into electronic current.
Termed as Electrode – Electrolyte or Electrode – Tissue Interface.
20

Electrode – Skin/Tissue Interface 21

HALF CELL POTENTIAL
• Each half-cell is represented by an electrode
in a solution (electrolyte) and both half-cells
are connected together.
•Since one of the electrodes has a higher
tendency to corrode compared to the other,
that electrode (anode) will oxidize and in turn
will donate electrons.
•To keep in equilibrium and balance the
charges in the electrolytes, there will be an
exchange of ions through the salt bridge.
• voltmeter will measure the potential
difference (voltage) between both electrodes,
which indicates the rate of dissolution of the
anode.

Half Cell Potential
Half-Cell potential is determined by
 Metal involved
 Concentration of its ion in solution
 Temperature
22

Nernst Equation in context of Half Cell Potential
Nernst Equation governs the half-cell potential.
When two ionic solutions of different concentration are separated by
semipermeable membrane, an electric potential exists across the
membrane.
a1 and a2 are the ionic activity of the ions on each side of the membrane.(redox)
•R is the ideal gas constant (joules per kelvin per mole), T is the temperature in
kelvins, F is Faraday's constant (coulombs per mole).
•N- number of electrons involved in the process
Ionic activity is the availability of an ionic species in solution to enter into a
reaction.
23

Polarization
Normally Standard Half Cell Potential (E0) is an equilibrium value and
assumes zero-current across the interface.
When current flows, the half-cell potential, E 0 ,changes.
Overpotential ( V p ): Difference between non-zero current and zero-current
half-cell potentials; also called the polarization potential.
Ohmic Overpotential ( V r ) : Due to the resistance of the electrolyte (voltage
drop along the path of ionic flow).
Concentration Overpotential ( V c ): Due to a redistribution of the ions in the
vicinity of the electrode-electrolyte interface (concentration changes).
Activation Overpotential ( V a ): Due to metal ions going into solution (must
overcome an energy barrier, the activation energy) or due to metal plating out
of solution onto the electrode (a second activation energy).
24

Mechanism Contributed to Overpotential
Ohmic overpotential: Voltage drop along the path of the current, and
current changes resistance of electrolyte and thus, a voltage drop does
not follow ohm’s law.
Concentration overpotential: Current changes the distribution of ions at
the electrode-electrolyte interface
Activation overpotential: Current changes the rate of oxidation and
reduction. Since the activation energy barriers for oxidation and
reduction are different, the net activation energy depends on the
direction of current and this difference appear as voltage.
25

Polarizable Electrodes
Perfectly Polarizable Electrodes
Electrodes in which no actual charge crosses the electrode-electrolyte interface
when a current is applied.
The current across the interface is a displacement current and the electrode
behaves like a capacitor.
Overpotential is due concentration.
Example: Platinum electrode
Perfectly Non-Polarizable Electrode
Electrodes in which current passes freely across the electrode-electrolyte
interface, requiring no energy to make the transition.
These electrodes see no overpotentials.
Example: Ag/AgCl Electrode
26

Motion Artefact:
Blurring of a radiographic image, produced by respiratory, muscular, or other
movement of the patient.
When polarizable electrode is in contact with an electrolyte, a double layer of
charge forms at the interface.
Movement of the electrode will disturb the distribution of the charge and results
in a momentary change in the half cell potential until equilibrium is reached
again.
Motion artifact is less minimum for non-polarizableelectrodes.
Signal due to motion has low frequency so it can be filtered out when
measuring a biological signal of high frequency component such as EMG or
axon action potential.
However, for ECG, EEG and EOG whose frequencies are low it is
recommended to use non-polarizable electrode to avoid signals due to motion
artifact.
27

Electrode Types
Surface Electrode
o Metal Plate
o Floating Electrodes
o Flexible Electrodes
Microelectrodes:
Internal Electrode:
Needle Electrode
28

Electrode Types - Surface
Primarily used in ECG, EEG and EMG
With conductive path between metal and skin being electrolyte paste or
jelly.
Sub types are
 Metal
 Suction
 Floating
 Flexible
29

Electrode Types – Surface - Metal
Metal-plate electrode used for application to limbs.
Metal-disk electrode applied with surgical tape.
E lectrodes were separated from subject’s skin by
cotton pads socked in a strong saline solution.
• Have generally smaller contact area and they do
not totally seal on the patient.
• Electrode slippage and displacement of plates
were the major difficulties faced by these type of
electrodes because they have a tendency to lose
their adhesive ability as a result of contact with
fluids on or near the patient.
• Very sensitive, it led to measurement errors
30

Electrode Types – Surface - Suction
•To measure ECG from various
positions on the chest, Suction cup
electrodes are used.
•It suits well to attach electrodes
on flat surface of the body and on
soft tissue regions..
•No need for strap or adhesive and
can be used frequently.
• Higher source impedance since
the contact area is small.
31

Electrode Types – Surface - Floating
Eliminate the movement errors
(called artifacts) which is a main
problem with plate electrodes.
• This is done by avoiding any direct
contact of the metal with the skin.
• Advantage of floating electrodes is
mechanical reliability.
• conductive path between the
metal and the skin is the electrolyte
paste or jelly.
32

Electrode Types – Surface - Flexible
• A flexible electrode is fabricated depositing a
metallic film, for example, Ag–AgCl, on
a flexible substrate (polymeric);
• usually the substrate side receives a layer
of conductive adhesive to fix the electrode to
the skin, and the metallic face, which is
connected to a lead wire, receives a layer of
insulating material.
•Electrode must be thin, flexible, and easy to
adjust
• used primarily for monitoring children and
premature newborns because they are light,
thin, and adapt to body contours, avoiding
motion artifact.
33

Microelectrodes
•Microelectrodes are biopotential electrodes with an ultrafine tapered tip
that can be inserted into individual biological cells.
•Important role in recording action potentials from single cells and are
commonly used in neurophysiological studies.
•The tip of these electrodes must be small - to avoid cell damage and at
the same time sufficiently strong to penetrate the cell wall.

Microelectrodes:
It is an electrode of very small size, used in electrophysiology for either
recording of neural signals or electrical stimulation of nervous tissue.
MEAs are Circuit less chips
Sufficiently small to be placed into cell.
Sufficiently strong to penetrate cell membranes.
Tip diameter: 0.05 – 10 microns.
Useful to access the behavior of electrogenic cells.
34

Glass micropipettes, metal microelectrodes, solid-state
microprobes.

Needle Electrodes
It penetrates the skin to record the
potentials.
It reduces interface impedance.
Single wire inside the
needle which acts as the
unipolar electrode measuring
potential at the point of contact.
Types are concentric, bipolar and
monopolar.
37

Applications of Bio Electrodes
Cardiac Monitoring
Infant Cardiopulmonary Monitoring
Sleep Encephalography
Diagnostic MuscleActivity
Cardiac Electrogram
Implanted Telemetry of Biopotentials
Eye Movement
38

Action Potential:
change in electrical potential that occurs between inside and outside of a
nerve or muscle fiber when it is stimulated , resulting to transmit nerve
signals.
The maximum value of action potential is generally 30 mV.
The action potential is an explosion of electrical activity that is created
by a depolarizing current.
This means that some event (a stimulus) causes the resting potential to
move toward 0 mV.
When the depolarization reaches about -55 mV a neuron will fire an
action potential. This is the threshold.
6

Propagation of Action Potential:
The action potential generated at the axon hillock propagates as a wave
along the axon.
The currents flowing inwards at a point on the axon during an action
potential spread out along the axon, and depolarize the adjacent sections
of its membrane.
The currents flowing inwards at a point on the axon during an action
potential spread out along the axon, and depolarize the adjacent sections
of its membrane.
The absolute refractory period keeps the direction of propagation
unidirectional.
8

Neurons send messages electrochemically. This means that chemicals
produce electrical signals.
The important ions in the nervous system are sodium and potassium
(both have 1 positive charge, +), calcium (has 2 positive charges, ++)
and chloride (has a negative charge, -).
As sodium ions are more on the outside, and the inside of the neuron is
negative relative to the outside, sodium ions rush into the neuron.
When the potassium channel opens, potassium rushes out of the cell,
reversing the depolarization.
9

A cell in the resting state is called polarized.
The process of changing from the resting state to the action potential is called
depolarization and the process of returning back to the resting state is called
repolarization.
During the process of repolarization, sodium pump pushes three sodium ions
quickly out of the cell for every two potassium ions it puts in.
Following the generation of action potential, there is a small gap within which
the cell cannot respond to any new stimulus and this period is called the
absolute refractory period which lasts for about 1ms.
Beyond this point is the relative refractory period when cells do respond but the
stimulus needed is much stronger and this may last for several milliseconds.
Measurement methods which are based on bio potential are ECG, EEG, EMG,
EOG, VCG and several others.
10

Propagation of Action Potential: 11

Absolute refractory period: During a short period after the
generation of an action potential, the cell does not respond to
any stimulus at all..
Relative refractory period: It is the time period between the
instant when the membrane potential becomes negative again
and the instant when the membrane potential
returns to RMP.
During this period, the cell responds to a stimulus but less
strongly than usual

Different action potential graph of many cell

Phases of action potential
• :
A. Depolarization phase.
B. Repolarization phase.
• Another phase present in cardiac muscle and others have additional
phase called the Plateau phase.
• Also we can divide an action potential into 5 phases:
1. The resting potential.
2. Threshold.
3. The rising phase.
4. The falling phase.
5. The recovery phase.

Depolarization Phase
• Definition: Is change from negative direction toward the positive
direction.
• Causes: Na enter inside the cell = (+).
• From: example – 70 mV upto + 35 mV.
• The graph will be:

Repolarization phase
• Definition: Is change from positive direction toward the negative
direction.
• Causes: more K exist outside the cell = (-).
• From: example +35 mV down to – 70 mV.

Plateau Phase
• Definition: Is maintaining the positive charge inside the cell.
• Causes: more Ca enter inside the cell = (+).
• From: example maintaining toward +35 mV.

BIOELECTRIC POTENTIALS
• An electric potential that is measured
between points in living cells, tissues and
organisms and which accompanies all
biochemical processes.
• Describes the transfer of information between
and within cells

Origin of Bioelectric Potentials
• Bioelectric potentials - ionic voltages
produced as a result of electrochemical activity
of certain special types of cells such as nerve
cell or muscle cells
• ECG (Electrocardiogram), EMG
(Electromyogram), EEG (Electroencephalogram),
ENG (Electroneurogram), EOG (Electro-
oculogram), ERG (Electroretinogram), etc. are
some examples of biopotentials

To understand the origin of
biopotentials we need to focus on:
 Bioelectric phenomena at the cellular level
 Volume conductor fields of simple bioelectric
sources
 Volume conductor fields of complex bioelectric
sources
need to focus on:
 Bioelectric phenomena at the cellular level
 Volume conductor fields of simple bioelectric sources

Introduction- Cell
• Cell consists of a plasma membrane, a nucleus and cytoplasm.
• Plasma membrane: It is selectively permeable to (various ions such as)
Na+, K+ and intracellular anions.
• The fluid inside the plasma membrane called the intracellular fluid
• The fluid outside the plasma membrane is called the extracellular fluid
• The plasma membrane separates the cell’s contents from its surroundings.
• Nucleus: It is the largest single organized cellular component. It is a
distinct spherical or oval structure located near the center of the cell. It is
covered by a double-layered membranous structure.
• Cytoplasm: It is a gel-like mass with membrane-bound structures
suspending in it.

Introduction:
• All excitable tissue mainly (nerve & muscle tissue)
have membrane potential.
• Membrane potential: cell that exhibit the polar
(charged) electricity.
• Resting membrane potential or polarized cell: is a
excitable cell that have positive charge outside and
negative charge inside during resting state.
• Two factors determine the polarity of the
cell:
1. Ion Channels.
2. Electrolyte (Na, K, Ca) movement in/out of the
cell.

Electrical activity of excitable cells
• Cells like nerve and muscle cells in the body are encased in
semi- permeable membrane that permits some substance to pass
through the membrane while others are kept out.
• Cells are surrounded by fluid.
• Fluid contains ions such as sodium, potassium, chloride etc.
• Fluid outside the cell membrane - Extracellular fluid (ECF)
and it is rich in Na+
, Cl−
• Fluid inside the cell membrane - Intracellular fluid (ICF) and it
is rich in K+
, Mg++
, phosphates.

Contd…
• Normal condition when the semi-permeable membranes
are in polarized state - Sodium (Na+) ions will be outside the
membrane.
• Since the size of Na+ ions is more than the size of holes in
semi-permeable membrane, they cannot enter inside
• Other ions like potassium (K+) and Chloride (Cl−) can
enter the membranes and exhibits resting potential.
• The sodium ions can enter the membrane when the holes of
it are increased by stimulation (excitation)
• Depolarization - After stimulation of membrane, all sodium
ions can enter inside by its increased diameter of pores or
holes. It constitutes depolarisation and gives action
potential.

RESTING POTENTIAL
• Electric potential of a neuron or other excitable
cell relative to its surroundings when not
stimulated(at rest)
• RMP – product of distribution of charged
particles
• RMP of a neuron is -70mv.which means potential
inside neuron is 70mv less than outside.
• At rest – more sodium ions outside neuron and
more potassium ions inside the neuron

• Fluids surrounding the cells of the body are
conducting and these conductive solutions
contain atoms known as ions.
• Principal ions present are: Sodium-Na+,
Potassium-K+ and Chlorlde-Cl−.
• Membrane of excitable cells - permits entry of
K+ and Cl-, but effectively blocks Na+ Ions.
• According to concentration and electric charge,
various Ions seek a balance between inside and
outside of cell.

Inability of Na+ ions
• Due to inability of Na+, to penetrate the
membrane results two conditions:
• The Na+ ions inside the cell become much lower
than in the Extracellular fluid outside. (Sodium
ions are +ve. It tends to make outside of cell
more +ve than inside).
• In an attempt to balance the electric charge,
additional potassium ions, which are also +ve,
enter the cell causing a higher concentration of
potassium on the inside than on the outside.

Contd..
• Equilibrium is reached with a potential difference across the
membrane, -ve on inside and +ve on the outside.
• And this membrane potential is known as resting potential of
cell.
• This potential is maintained until some disturbance upsets the
equilibrium.
• The membrane potential is made from inside the cell with
respect to the body fluids - resting potential is -ve

SIGNAL
 function of one or several variables that carries
useful information
 biological Signal - if it is recorded from a living
system and conveys information about the state or
behavior of that system.
 Examples: the temperature record of a patient, the
voltage record by an electrode placed on the scalp,
and the spatial pattern of X-ray absorption
obtained from a CT scan are biological signals.

BIOMEDICAL SIGNAL
 Observations of physiological activities of
organisms, ranging from gene and protein
sequences, to neural and cardiac rhythms, to
tissue and organ images.
 Biomedical signal processing aims at
extracting significant information
from biomedical signals.

Origin of Biomedical Signals
 Human body is made up of a number of
systems e.g- respiratory, cardiovascular,
nervous system, etc.
 Each of these systems is made up of several
subsystems that carry on many physiological
processes.
 Each physiological process is associated with
certain types of signals referred as Biomedical
signals that reflect their nature and activities.

SOURCES OF BIOMEDICAL SIGNALS
 A number of signal sources may result into
a biomedical signal.
 bioelectric Signals,
 bioimpedance signals,
 bioacoustic signals,
 biomagnetic signals,
 biochemical signals and
 bio-optical signals.

BIOELECTRIC SIGNALS
 Bioelectric signals are specific types of
biomedical signals which are obtained by
electrodes that record the variations in
electrical potential generated by physiological
processes.
 Examples of bioelectric signals are:
 Electrocardiogram (ECG)
 Electroencephalogram (EEG)
 Electromyogram (EMG)

BIOELECTRIC SIGNALS
 generated by nerve cells and muscle cells.
 Their basic source is the cell membrane
potential
 . The electric field generated by the action of
many cell constitutes the bio-electric signal.
 The most common examples of bioelectric
signals are the ECG (electrocardiographic) and
EEG (electroencephalographic)signals

BIOACOUSTIC SIGNALS
 The measurement of acoustic signals created
by many biomedical phenomena provides
information about the underlying phenomena.
 The examples of such signals are; flow of blood
in the heart, through the heart's valves and
flow of air through the upper and lower airways
and in the lungs which generate typical
acoustic signal

BIOMECHANICAL SIGNALS
 These signals originate from some mechanical
function of the biological system.
 They include all types of motion and
displacement signals, pressure and flow
signals.
 Example:The movement of the chest wall in
accordance with the respiratory activity

BIOCHEMICAL SIGNALS
 The signals which are obtained as a result of
chemical measurements from the living tissues
or from samples analyzed in the laboratory.
 Examples :measurement of partial pressure of
carbon dioxide (pCO2 ), partial pressure of
oxygen (pO2 ) and concentration of various ions
in the blood.

BIOMAGNETIC SIGNALS
 Extremely weak magnetic fields are produced
by various organs such as the brain, heart and
lungs.
 The measurement of these signals provides
information which is not available in other types
of bio signals such bioelectric signals.
 A typical example is that of magneto
encephalograph MEG signals from the brain

BIO OPTIC SIGNALS
 These signals are generated as result of optical
functions of the biological systems, occurring
either naturally or induced by the measurement
process.
 For example, blood oxygenation may be
estimated by measuring the transmitted/back
scattered light from a tissue at different
wavelengths.

BIO IMPEDANCE SIGNALS
 The impedance of the tissue is a source of
important information concerning its
composition, blood distribution and volume.
 Example: measurement of galvanic skin
resistance GSR
 The bio-impedance signal is also obtained by
injecting sinusoidal current in the tissue and
measuring the voltage drop generated by the
tissue impedance.

COMMONLY USED BIOMEDICAL SIGNALS
 The electromyogram (EMG): It is the electrical activity of the muscle
cells.
 The electrocardiogram (ECG): It is the electrical activity of the heart
/cardiac cells.
 The electroencephalogram (EEG): It is the electrical activity of the
brain.
 The electrogastogram (EGG): It is the electrical activity of the
stomach.
 The phonocardiogram (PCG): It is the audio recording of the heart’s
mechanical activity.
 The carotid pulse (CP): It is the pressure of the carotid artery.
 The electoretinogram (ERG): It is the electrical activity of the retinal
cells.
 The electrooculogram (EOG): It is the electrical activity of the eye
muscles.

IMPORTANCE OF BIOSIGNALS
 Diagnosis
 Patient Monitoring
 Biomedical research

SIGNAL CONDITIONING
 Signal conditioning is the manipulation of
a signal in a way that prepares it for the next
stage of processing.
 Many applications involve environmental or
structural measurement, such as temperature
and vibration, from sensors

STAGES OF BIOSIGNAL PROCESSING
 Transformation and reduction of signals
 Computation of signal parameters
 Interpretation or classification of signals
 Signal acquisition and reconstruction,
 Quality improvement including filtering, smoothing
and digitization,
 Feature extraction,
 Signal compression,
 Prediction.

What is Blood Pressure
 Blood pressure is the
force that blood
exerts against blood
vessel walls.
 The pumping action
of the heart generates
the flow
 Pressure occurs
when the flow is met
by resistance from
blood vessel walls

Blood Pressure
 Heart supplies the organs and tissues of
the body with blood.
 With every beat, it pumps blood into the
large blood vessels of the circulatory
system.
 As the blood moves around the body, it
puts pressure on the walls of the vessels.

Laminar Flow
 Blood flows faster in
the center of a blood
vessel, because the
blood near the sides
are hitting the walls of
the vessels.
 Is caused by the
friction (resistance)
between the blood
and the vessel walls.

Blood Pressure Graph
 By taking your pulse, you can feel that blood
pressure fluctuation with each heartbeat.
 The pulse which you feel is actually a pressure
wave that travels from your heart though your
arteries
Systolic
Dicrotic
Notch Diastolic Average
Pressure
Pulse Pressure

Blood pressure
 Measured on a number of different days
and when you are at rest.
 If several of these measurements are too
high - high blood
 Medical term for high blood pressure is
hypertension.
 In adults- normal blood pressure is under
a systolic value of 140 mmHg
 under a diastolic value of 90 mmHg.

Systolic Pressure
 Systolic pressure is the maximum pressure
exerted by the blood against the artery walls.
 It results when the ventricles contract.
 Normally, it measures 120 mm Hg.
Systolic

Dicrotic Notch
 The Dicrotic Notch represents the interruption of
blood flow due to the brief backflow of blood that
closes the aortic semilunar valve when the
ventricles relax.
Dicrotic
Notch

Diastolic Pressure
 Diastolic Pressure is the lowest pressure in the
artery.
 It result when the ventricles are relaxed and is
usually around 80 mm Hg.
Diastolic

Blood Pressure Sounds
 A cuff is inflated to constrict an artery so
that no blood flows.
 As the cuff pressure is slowly released, but
the artery is still partially constricted, blood
flow begins again.
 Sounds can be heard because the blood
flows violently causing audible sounds.
 When the artery is fully open, the blood
flows freely and the sounds disappear.

Checking Blood Pressure
 The first sounds that are heard indicate
systolic pressure (top number).
 When the sounds stop, diastolic pressure
has been reached (bottom number).
 Average blood pressure is:
120
80

How is blood pressure measured?
 change due to things like physical exertion,
stress, pain, or extreme heat or cold.
 Reliable readings- while you are at rest
 sitting down and relaxing on a chair, and waiting
about three minutes before taking a
measurement
 circulatory system comes to rest
 During process - upper arm that is being used
for the measurement should rest on a table, at
about the same height as the heart

Digital blood pressure monitors
 used on the wrist, also be placed on the finger or
upper arm and activated simply by pressing a
button.
 Read the blood pressure automatically based on
variations in the volume of blood in the arteries
 Digital meters can sometimes be inaccurate and
produce unreliable readings anyway
 people with certain heart rhythm problems or
arteries

Blood pressure Measurement
 Accurate measurement of the BP is
important in:
 Assessment and management of
hypotension(low blood pressure)
 The diagnosis and management of
hypertension(high blood pressure)

POSTURAL HYPOTENSION
 Fall in blood pressure that occurs when
changing position from lying to sitting
 or from sitting to standing.
 A fall of >20mmHg in systolic pressure on
standing – postural hypotension
 It is also known as orthostatic
hypotension.

HYPOTENSION
 person’s blood pressure is much lower
than usual.
 When the blood pressure is too low, there
is inadequate blood flow to the heart, brain
and other vital organs.
 Important factor is how the BP changes
from the baseline and how that change
affects the person

 The majority of patients have Primary
(Essential)
 Hypertension, in other words there is
no
 identifiable underlying cause.
 The remainder suffer from Secondary
 Hypertension whereby the raised blood
pressure
 arises from an identifiable disease.

HYPERTENSION
 Bp increases when large blood vessels
begin to lose their elasticity
 smaller vessels start to constrict, causing
the heart to try to pump the same volume
of blood through vessels with a smaller
internal diameter.
 Hypertensive - if blood pressure is equal
to or greater than 140mmHg systolic, or
over 85mmHg diastolic.

CAUSES OF HYPERTENSION
 Majority of patients have Primary
 Hypertension, in other words there is
no identifiable underlying cause.
 Secondary Hypertension - raised blood
pressure arises from an identifiable
disease.
 Hypertension is usually asymptomatic.

Indirect Blood Pressure Measurement -
Sphygmomanometer
 Pressure cuff on the upper arm is first increased to
a pressure well above the systolic pressure.
 •At this point no sound can be heard through the
stethoscope, which is placed over the brachial
artery
 Brachial Artery - major blood vessel located in the
upper arm and is the main supplier of blood to the
arm and hand
 The pressure in the cuff is then gradually reduced.

Ultrasonic Based Blood Pressure
Measurement
 Employs a Doppler sensor that detects the
motion of the blood-vessel walls in the
various states of occlusion.
 Occlusion - blockage or closing of a blood
vessel
 Doppler ultrasonic transducer is focused
on the vessel wall and the blood.
 The reflected signal (shifted in frequency)
is detected by the receiving crystal and
decoded.

What is pH?
• pH is a method of measurement of hydrogen ion concentration.
• The lower-case alphabet “p” in pH denotes negative common (base ten)
logarithm, while the upper-case alphabet “H” denotes the element
hydrogen.
• pH is a negative logarithmic measurement of the number of moles of
hydrogen ions (H+) per litre of solution.
• In purewater, hydrogen ion concentration is 10-7 moles
per litre under standard conditions (25°C), giving it a pH of 7.
• pH scale ranges - 0 and 14.
3

PH
Acidity describes higher concentration of hydrogen ions and
reads below pH 7.
•Alkali or base fully ionized in water reads pH 14.0.
• Alkalinity ranges between 7 and 14, excluding 7 as it depicts
pure water as a neutral solution.
• As pH is measured on a logarithmic scale, one unit increase
of pH corresponds to a decrease in concentration by a factor
of 10.
• For example, the concentration of hydrogen ions for pH 3 is
10 times greater than that of pH 4.

pH..
The pH value is expressed as
𝑝𝐻
=
1
𝑙𝑜𝑔10𝐶
,𝐶 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓𝐻+𝑖𝑜𝑛𝑠
Solution's pH is measured on its net concentration of hydrogen ions [H+] compared to
concentration of hydroxide ions [OH-].
Acids dissociates to produce H+ ions whereas bases dissociates to produceOH- ions.
The product of these two concentrations of H+ and OH – gives the dissociation constant
pH + pOH = pKW. At 25°C under standard conditions, pH + pOH = 14, which is why
the scale for pH usually ranges from 0 to14.
4

Nernst’s Equation
This equation enables the determination of cell potential under non – standard
conditions.
It establish the relationship between measured cell potential and reaction quotient
allowing accurate determination of equilibrium constants. This equation relates
potential of electrodes to ion concentration at equilibrium.
The equation is expressedas
𝑅𝑇
𝐸 = 𝐸0 +
𝑛𝐹
ln(𝑎𝐶)
E = e.m.f of the half-cell; E0 = e.m.f of the half-cell under standard conditions;
R = Gas constant (8.314 J/°C); T = Absolute temperature (K); n = valence ofion;
F = Faraday constant (96493 C); a = activity coefficient (0 ≤ a ≤ 1)
C = molar concentration of ions.
The pH of any solution is measured on the basis of this equation where the potential
developed across membranes reflects the concentration of H+ ions.
5

Measurement of pH
When very precise and accurate pH measurement is not required, litmus papers that
change colour on coming in contact with solutions of certain pH values are used.
• But continuous and process measurement demands more sophisticated measurement
techniques.
Essentially two electrodes, measuring and reference electrodes are employed for its
measurement.
Both these electrodes are dipped in the solution whose pH is to bemeasured.
These two electrodes form two half-cells and the total potential generated is the
difference between these two electrodes separately produced in each one ofthem.
The generation of this potential is dependent on the H+ ion concentration governed by
Nernst equation. This potential is sensitive to the H+ ion concentration, having a
sensitivity of 59.2 mv/pH at 25°C.
𝐸 = 𝐸0 −
2.303𝑅𝑇.
𝑝𝐻
𝐹
6

Measuring Electrodes
Measuring electrodes of specific make (thin ion selective glass) having buffer solution
of constant H+ ion concentration and silver wire inside the glass bulb is dipped in the
unknown solution where a potential is generated across the glassbulb.
It forms one of the two half-cells. The measurement electrode’s purpose is to generate
the voltage used to measure the solution’s pH.
7

Reference Electrodes
provides continuity to the electric circuit as one half-cell is unable to measure the
potential generated.
Commonly of two types :
i) Calomel (Mercury – Mercurous Chloride) and
ii) Silver – Silver Chloride electrode.
The two sets of silver wires coming out from both the measuring electrode and
reference electrode complete the circuit measuring the potentialgenerated.
RE consists of a chemical solution of neutral (7) pH buffer solution (usually potassium
chloride) allowing exchange of ions with the process solution through a porous
separator.
purpose is to provide the stable, zero-voltage connection to the liquid solution so that a
complete circuit can be made to measure the glass electrode’s voltage and the electrical
connection is maintained through a salt bridge.
8

Electrodes
The resistance offered by the measuring electrode and reference electrode is
substantially high while the voltage produced per scale of pH is of the order of
millivolts.
The common solution to this problem is to use an amplified meter with an extremely
high internal resistance to measure the electrode voltage, so as to draw as little current
through the circuit as possible.
The other method is to use a potentiometric “null-balance” voltage measurement setup
to measure this voltage without drawing any current from the testcircuit.
9

Electrodes
A new method for pH value measurement is the use of an ion selective field effect
transistor (ISFET).
The ISFET is a transistor with power source and drain, divided by an isolator. This
isolator (gate) is made of a metal oxide where hydrogen ions accumulate in the same
way as an electrode.
The positive charge that accumulates outside the gate is 'mirrored' inside the gate by an
equal negative charge generates.
Once this happens the gate begins to conduct electricity. The lower the pH value the
more hydrogen ions accumulate and the more power can flow between source and
drain.
The ISFET sensors act according to the Nernstequation.
The advantage of an ISFET is that they are very small. The disadvantage of using an
ISFET for pH measurements is that they have comparatively short durability and low
long-term stability.
10

Electrode Types
Electrodes are designed to allow H+ ions in the solution to migrate through a selective
barrier, producing a measurable potential (voltage) difference proportional to the solution’s
pH.
The glass is chemically doped with lithium ions, which is what makes it react
electrochemically to hydrogen ions. All pH electrodes have a finite life depending greatly
on the type and severity of service.
The pH electrodes come in different types based on different kind of application and use.
They are as given below
I. Glass Electrode
II. Hydrogen Electrode
III. Quinhydrone Electrode
Saturated Calomel Electrode and Silver Chloride
IV. Reference Electrodes -
Electrode
V. Combination Electrode
VI. Ion-Selective Electrodes
11

Glass Electrode
It is one type of ion-selective electrode made of doped glass membrane and sensitive to
a specific ion.
The response may be to H+ ion or it may be to the other cations based on the glass
composition.
Construction wise, the glass electrode consists of a thin walled bulb of pH sensitive
glass sealed to a stem of non pH sensitive high resistanceglass.
The pH response is limited to the special glass membrane making it independent of the
depth of immersion.
On the inside of the membrane is a system of effectively constant pH. It is composed
of Ag – AgCl or calomel electrode dippedin HCl acid.
Changes in the electrical potential of the outer membrane surface are measured by
means of an external reference electrode and its associated saltbridge.
12

Glass Electrode
The complete pH cell is represented as follows:
Glass Electrodes have two disadvantages and they are
a) Measuring solutions containing particulates can damage the glassmembrane
b) The glass membrane is vulnerable towards breaking
13
Internal
Reference
Electrode
Internal
Electrolyte
Glass
Membrane
Test Solution External Reference
Electrode

Hydrogen Electrode
The hydrogen electrode is the primary electrode to which all electrochemical
measurements are referred.
The performance of all other electrodes is always evaluated in terms of the hydrogen
electrode.
The hydrogen electrode consists of an inert but catalytically active metal surface, most
frequently platinum, over which hydrogen is bubbled to achieve electrochemical
equilibrium with the hydrogen ions in the solution.
14

Hydrogen Electrode
The choice of platinum for the hydrogen electrode is due to severalfactors:
• Inertness of platinum (it does not corrode)
• The capability of platinum to catalyse the reaction of proton reduction
• A high intrinsic exchange current density for proton reduction onplatinum
• Excellent reproducibility of the potential (bias of less than 10 μV when two well-
made hydrogen electrodes are compared with one another)
The surface of platinum is platinized (i.e. covered with platinum black)to:
•Increase total surface area. This improves reaction kinetics and maximum possible
current
•Use a surface material that absorbs hydrogen well at its interface. This also
improves reaction kinetics.
15

Quinhydrone Electrode
The quinhydrone electrode is a redox electrode which is used to measure the hydrogen ion
concentration (pH) of a solution in a chemical experiment.
• It acts as an alternative to the common glass electrode in a pH meter.
The electrode consists of an inert metal electrode (usually a platinum wire) in contact with
quinhydrone crystals and a water-based solution.
• Quinhydrone is slightly soluble in water, dissolving to form a mixture of two substances,
quinone and hydroquinone, with the two substances present at equal concentration.
• Each one of the two substances can easily be oxidised or reduced to the other.
The potential at the inert electrode depends on the ratio of the activity of two substances
(quinone-hydroquinone), and also the hydrogen ion concentration.
The quinhydrone electrode is not reliable above pH 8.
• unreliable in the presence of strong oxidising or reducing agents, which would disturb the
equilibrium between hydroquinone and quinone.
• subject to errors in solutions containing proteins or high concentrations of salts.
17

Reference Electrode - Calomel
Reference electrode provides a stable, reproducible voltage to which the working
electrode potential may be referenced.
A reference electrode acts as a battery whose voltage is dependent on the reaction
taking place between a solid conductor (metal salt) and the electrolyticsolution.
It is necessary that the pH cell be completed by means of a stable reference electrode,
whose potential remains unchanged by changes in the composition of the cellsolution.
The Saturated calomel electrode (SCE) is a reference electrode based on the reaction
between elemental mercury and mercury (I) chloride.
The aqueous phase in contact with the mercury and the mercury (I) chloride (Hg2Cl2,
"calomel") is a saturated solution of potassium chloride in water.
The electrode is normally linked via a porous membrane (the salt bridge) to the
solution in which the other electrode is immersed.
18

Reference Electrode - Calomel
SCE consists of a metallic internal element, typically of mercury – mercurous chloride
(calomel) or silver – silver chloride, immersed in an electrolyte, which is usually a
saturated solution of potassium chloride.
One drawback of calomel electrode is its mercury content which sometimes may create
health hazard.
Also one cause of malfunction is due to the trapped airbubbles.
19

Reference Electrode – Silver Chloride
This type is used usually in electrochemical measurements.
The electrode functions as redox electrode where reaction takes place between silver
(Ag) and Silver Chloride salt.
This type is constructed with glass having a porous ceramic membrane at the interface.
Sodium Chloride is used as the filling solution which is in a semi – solidstate.
A porous reference junction separates the filling solution in the electrode from the
solution whose pH is to be measured.
The filling solution’s constant chloride ion concentration generates potential at a pure
silver wire with silver chloride on it.
The silver wire passes a signal from the solution being measured to the electrode’s
cable.
This configuration of the electrode is called “Single JunctionReference”.
20

Reference Electrode – Silver Chloride
The potential developed is dependent on the effective concentration of Cl- ions as
established by Nernst’s equation.
The salient features of this type are simple construction, stable potential, inexpensive
and non – toxic components.
But the issues of concern are it is very sensitive to bromide ion traces and the
electrodes get easily damaged by drying.
21

Combination Electrode
As stated before, here the measuring and reference electrodes are builttogether.
Here the internal reference electrode and external reference electrode are identical both
being Ag/AgCl type.
Inner solution in both the electrodes are also same held at same temperature and
protection against light is provided by ruby red glasses.
The potential of a combination pH electrode is due to the difference in activities of H+
ions between the test solution and reference solution sides of the glassmembrane.
The potential of the combination electrode is proportional to the pH of the testsolution.
22

Ion – Selective Electrode (ISE)
As the name implies, these electrodes are sensitive to the activity of a particular ion in
solution and quite insensitive to the other ions present necessitating different electrodes for
different measurement.
ISEs work on the basic principle of the galvanic cell.
By measuring the electric potential generated across a membrane by selected ions and
comparing it to a reference electrode, a net charge is determined.
The strength of this charge is directly proportional to the concentration of the selected ion.
An ion selective membrane is fixed at one end so that the external solution can only come
into contact with the outer surface and the other end is fitted with a noise cable or gold
plated pin for connection to the millivolt measuring device.
Most commonly used ISE:
Cations: Ammonium (NH+), Barium (Ba++), Calcium (Ca++), Cadmium (Cd++), Copper
(Cu++), Lead (Pb++), Mercury (Hg++), Potassium (K+), Sodium (Na+), Silver (Ag+).
Anions: Bromide (Br-), Carbonate (CO3-), Chloride (Cl-), Cyanide (CN-), Fluoride (F-),
Iodide (I-), Nitrate (NO3-), Nitrite (NO2-), Perchlorate (ClO4-), Sulphide (S-), Thiocyanate
(SCN-).
23

Ion – Selective Electrode (ISE)
Advantages of ISE:
a) Relatively inexpensive and simple to use.
b) Robust and durable
c) Rapid operation even in relative dilute aqueous solution viz. lakes or rivers
d) Able for continuous monitoring
e) Measure activity of ions directly rather than measuring the concentration
f) Higher accuracy and precision
g) Can measure both positive and negative ions
h) Unaffected by colour or turbidity
i) Has got a wide temperature range
Limitations of ISE:
a) Effect of interference with other ions in solution
b) Effect of ionic strength of the solution, reducing activity
c) Drift in electrode potential during a sequence of measurement
d) Vulnerable towards contamination by organic molecules
24

Buffer Solution
A buffer (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution
comprising a mixture of a weak acid and its conjugate base.
Adding a small amount of strong acid or base changes its pH value very slightly,
therefore it is used to prevent changes in pH of solution, keeping the pH value at a
nearly constant value.
The consistency of buffer pH value is maintained as it maintains the equilibrium
between acid HAand its conjugate baseA-. 𝐻𝐴 ⇋ 𝐻+ + 𝐴−
Adding strong acid to an equilibrium mixture of weak acid and its conjugate base, the
equilibrium is shifted to the left. Increase in H+ ion concentration is lesser than
expected for the quantity of strong acid added.
Likewise happens in case of addition of strongalkali.
A buffering agent is a weak acid or base applied to maintain the acidity of a solution
near a chosen value even after addition of another acid or base, thus preventing rapid
changes in pH value.
25

Buffer Solution
Buffer capacity, β, is a quantitative measure of the resistance of a buffer solution to pH
change on addition of hydroxide ions. It can be expressedas
𝛽 =
𝑑
𝑛
𝑑(𝑝[𝐻
+])
26
.
d n = infinitesimal amount of added base;
d (p[H+]) = resulting infinitesimal change in the co logarithm of
the hydrogen ion concentration
Buffer capacity for a 0.1 M solution of an acid with p Ka of 7

There are three regions of high buffercapacity.
• At very low p [H+] the first term
predominates and β increases in proportion
to the hydrogen ion concentration. This is
independent of the presence or absence of
buffering agents and applies to all solvents.
• In the region p [H+] = p Ka ± 2 the second
term becomes important. Buffer capacity is
proportional to the concentration of the
buffering agent, CA, so dilute solutions have
little buffer capacity.
• At very high p [H+] the third term
predominates and β increases in proportion
to the hydroxide ion concentration. This is
due to the self-ionization of water and is
independent of the presence or absence of
buffering agents.

Buffer Calibration
Buffers are standard solutions formulated to preserve a known pH in spite of small
amounts of impurity.
Buffer calibrations use two buffer solutions, separated by 3 pH units allowing the pH
analyser to evaluate a new slope and zero value to be used for deriving pH from the
millivolt and temperature signals.
The slope and zero value resulting from a buffer calibration provide an indication of the
state of the glass electrode from the scale of its slope, while the zero value indicates
reference poisoning or asymmetry potential.
Buffer calibration demonstrates how well the pH sensor responds topH.
27

Process Effects on The Glass pH Electrode
Temperature Effects: Fluctuating and increasing temperature accelerate the aging of
electrodes. Elevated temperature affects the interior and exterior of the electrodes
resulting in the shift of zero point.
Sodium Error: Also termed as alkali error occurs in high pH where Na+ concentration
is more than H+ concentration. Under this condition, electrodes start responding to
Na+ ions resulting lower reading than actual. Li+ ion effects are even more prominent
than Na+ whereas K+ ions effect is negligible.
Components attacking pH Electrodes: High concentrations of hydroxyl ions shorten
the life of pH electrodes. Solutions that reach a pH in excess of 14 pH (equivalent to
4% caustic soda) can destroy a pH electrode within hours.
Hydrofluoric Acid: This can even dissolve pH glass decreasing the life of electrodes.
HF acids attack glass but not the fluoride ion (F-).
Alkaline Error: This error can result when cations other than H+ are present in solution.
These cations can exchange for H+ in the gellayer.
28

Process Effects on Reference Electrodes
Reference Poisoning: If instead of Ag-AgCl, other silver compounds viz. bromide,
iodide and sulphide ions are used, they may cause this effect as the salts produced are
less soluble than AgCl leaving behind insoluble particulates in the fill solution.
• To counter this effect, multiple reference electrodes are used which slows down the
effect of poisoning
Plugging of the Liquid Junction: Large concentrations of an ion that forms an
insoluble precipitate with silver ion (most notably sulphide ion) will precipitate within
the liquid junction and plug it.
• Metal ions that form insoluble salts with chloride ion will also precipitate in the liquid
junction.
• Multiple junction reference electrodes with the outermost fill solution containing
potassium nitrate, rather than potassium chloride are used
Liquid Junction Potential: Potassium chloride is chosen for the fill solution because
of its ability to solubilize silver ion
29

Merits of pH Meters
• Advantages are that they produce reasonably good and
reproducible measurements.
• Recent advances do provide measurements up to the 4th place
of decimal results with digital outputs.
30

Demerits of pH
• They are slow to register devices with little drifts in final
values.
•Temperature influences the output results greatly so definite
temperature compensation is required.
•Though glass electrodes are selective for H+ ions but not
uniquely responsive to them only leading to response to other
ions as well.
•Depositions on glass electrodes affect results.
•Carbon dioxide absorption influences the output and
measurement becomes seemingly tough with solutions having
varying pH

Application of pH Meters
The measurement of pH reflects the effective concentration and
activity of H+ and other ions present in solution.
For chemical reactors and scrubbers, they provide indications of
the solution used being acidic or basic qualitatively.
These meters find major application to correct the hypochlorite
concentration for an Oxidation – Reduction Potential (ORP)
measurement.
Water treatment plants, micro-electronics laboratories and
pharmaceutical laboratories are in constant need of pH level
monitoring and control for their very accurate and precise
applications.
31

 The methods used to detect volume
(pulse) change due to blood flow are,
 1. Electrical Impedance changes
 2. Strain Gauge or microphone
(mechanical)
 3. Optical change (Changes in density

Electrical Impedance changes
 Measures the impedance change between 2
electrodes caused by the change in blood
volume between them.
 change in the impedance (0.1 ohm) may be as
small as compared to the total impedance
(Several hundred ohms).
 Impedance is measured by applying an
alternating current between electrodes attached
to the body.
 An alternating current (10 – 100 KHz) is used.

Strain Gauge or microphone
 Mechanical method involves the use of
strain gauge connected to a rubber band
placed around the limb or finger.
 Expansion in the band due to change in
blood volume causes a change in
resistance of the strain gauge.
 A sensitive crystal microphone is placed
on the skin surface to pick up the
 pulsation.

Optical change (Changes in density)
 Commonly used method to measure blood
volume change is photo electric method.
 In this method we have 2 types of method
 1. Transmittance method
 2. Reflectance method

Transmittance method
 Light emitting diode (LED) and photoresistor
are mounted in an enclosure that fits over the
tip of the patient’s finger.
 The light is transmitted through the finger tip
 Resistance of photoresistor is determined by
the amount of light reaching it
 With each contraction of the heart, blood is
forced to the extremities and the amount of
blood in the finger increases.

Transmittance Method
 It alters the optical density resulting in light
transmission through the finger reduces
 Resistance of the photoresistor increases
 The photoresistor is connected as part of a
voltage divider circuit
 Produces a voltage that varies with the amount
of blood in the finger.
 Voltage follows the pressure pulse and its
waveshape can be displayed on an oscilloscope

Reflectance method
 Photo resistor is placed adjacent to the exciter
lamp.
 Part of the light rays emitted by the LED is
reflected and scattered from the skin and the
tissues and falls on the photoresistor.
 Quantity of light reflected depends upon the
amount of blood filling the capillaries
 voltage drop across the photoresistor,
connected as a voltage divider, will vary in
proportion to the volume changes of the blood
vessels.

Plethysmographic
signal
 A plethysmograph is an instrument for
measuring changes in volume within
an organ or whole body
 resulting from fluctuations in the amount of
blood or air it contains
 used to measure the functional residual
capacity (FRC) of the lungs

plethysmographic pulse rate
 circuit consists of two parts, a LED oscillator
and driver
 produce 300 Hz, 50 µS infrared light pulses
to the finger probe attached to the patient
 phototransistor that picks up the attenuated
light
 The electrical signal obtained from
 the phototransistor is amplified and its peak
value is sampled and filtered.

 An automatic gain control circuit adjusts the
amplifier gain to yield a constant average
pulse height at the output
 The ac component with a frequency in the
heart rate range (0.8-5 Hz) is further
amplified to output the plethysmographic
pulse rate form.
 This signal is transmitted across the isolation
barrier, demodulated, low-pass filtered and
transmitted to the analog multiplexer
resident on the CPU board

MEDICAL ELECTRONICS.pdf

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