• Unit 2- Biopotential Measurements

Biosignal characteristics- frequency and amplitude range. ECG- Einthoven’s triangle, standard 12 lead system, Principle of vector cardiography, EEG- 10-20 electrode system, Unipolar, Bipolar and average mode. EMG- unipolar and bipolar mode. Recording of ERG, EOG and EGG

Biosignals

• A biosignal is any signal in living beings that can be continually measured and monitored.
• How biosignals are measured?

Measured using invasive and non-invasive sensors.

These type of sensors are wearable devices for health monitoring

• Importance of biosignal:

Diagnosis

Patient monitoring

Biomedical research

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• Types of Bioelectric signals:

ECG- heart activity

EEG- brain signals

EMG- Muscle movement

EOG- eye movement

GSR- galvanic skin response

The biosignals are measured with the help of transducer/ sensor

Primary characteristics of Biosignal

Heart

• The heart is a muscular organ about the size of a fist, located just behind and slightly left of the breastbone.
• The heart pumps blood through the network of arteries and veins called the cardiovascular system.�The four main functions of the heart are:
• Pumping oxygenated blood to the other body parts.
• Pumping hormones and other vital substances to different parts of the body.
• Receiving deoxygenated blood and carrying metabolic waste products from the body and pumping it to the lungs for oxygenation.
• Maintaining blood pressure.

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ECG

• ECG is defined as recording of electrical activity of heart on a graph paper
• The machine which is used to record the electrical activity of heart is Electrocardiography

ECG machine

Power lab

• The graph on which the electrical activity is recorded is called Electrocardiogram

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Significance of ECG

• ECG gives information about rate and rhythm of the heart
• The physical orientation of heart is axis
• It’s a diagnostic tool for various heart conditions like hypertrophies, ischemia, infarction, arrhythmias conduction problems and heart pace maker activity
• ECG does not provide information about mechanical activity

ECG paper

• This is a long paper, composed of small squares
• One square is 1mm wide and 1mm high
• On ECG paper there are thick lines between two thick lines there are 5 small squares
• The speed of ECG machine is 25 mm per second

• The time interval of each small square can be calculated as

25 small sqrs are equal to 1 second

1 small sqr is equal to 1/25 second

• Vertically the small square represent the amount of electrical potential
• One small sqr represent the potential of 0.1mv
• 10 small sqrs represent the potential of 1 mv

ECG leads

• Leads are electrodes which record the electrical potential of heart at different sites.
• There are 12 ECG leads

3 bipolar limb leads

3 augmented limb leads (unipolar)

6 chest leads

Bipolar limb leads

• Lead 1= left arm +ve, right arm –ve
• Lead 2= right arm –ve, left leg +ve
• Lead 3= left arm –ve, left leg +ve

Standard limb leads

Precordial leads

Precordial Lead Placement

• V1 is placed to the right of the sternal border, and V2 is placed at the left of the sternal border.
• Next, V4 should be placed before V3.
• V4 should be placed in the fifth intercostal space in the midclavicular line

What is the purpose of precordial leads

• The precordial leads, or V leads, represent the heart's orientation on a transverse plane, providing a three- dimensional view
• They are placed anatomically over areas of the left ventricle.

Augmented limb leads

• AVR- attached to right arm
• AVL- attached to left arm
• AVF- attached to left foot

Einthoven’s Triangle

• It is a diagrammatic way of illustrating that the two arms and left leg form the apices of triangle surrounding the heart.
• It is an equilateral triangle
• Einthoven’s rule: If the electrical potentials of two leads are known at any instant, the third can be determine by simply summing the first two.

Waveforms in ECG

• Positive electrode is always a reference electrode
• Depolarization wave moves towards the positive electrode gives positive deflection
• Depolarization wave moves away from positive electrode gives the negative deflection

• P wave:

Atrial depolarization

• QRS complex:

Ventricular depolarization

• T wave:

Ventricular repolarization

Basic terminologies

• Base line: flat, straight and isoelectric lines
• Wave form: deviation or movement away from base line may be upward or downward
• Segment: a line between two waves
• Interval: a wave form plus a segment this shows time duration
• Complex: combination of several wave form without segment

P Wave

• P wave shows atrial depolarization
• Duration: 0.1 sec (2 and half small sqr)

height : 2.5mv (2 and half small sqr)

• It shows the sinus rhythm
• Abnormalities of P wave

P. pulmonale

P. mitrale

• P. pulmonale: This is tall and peaked P wave in lead 1 and lead 2 and 3 in right atrial hypertropy
• P. mitrale: it is biphasic seen in left atrial hypertropy
• Seen in lead 2
• Inverted P wave may be present in

normally in lead V1and V2

In dextro cardia

Incorrectly placed electrodes

QRS complex

• It represents the ventricular depolarization
• Duration: 0.08 sec (less than 2 small sqr)

height: 5 to 20 small sqr

• Wide complex because it mask the atrial repolarization
• Q wave is first wave of this complex but often absent
• Q wave present the interventricular septal depolarization

• First wave in ECG with negative deflection
• Q wave greater than 1/3 the height of the R wave greater than 0.04 sec are abnormal and may represent the old infarction

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• Low voltage QRS complex:

When the height of R or S wave is not more than 5mm

Hypothyroidism

Pericardial effusion

Thick chest wall

Problem in ECG machine

High voltage QRS complex

• Present in ventricular hypertrophies
• Maximum voltage of QRS complex may be 35mv
• V1 and V2 show high voltage QRS complex in right ventricular hypertrophy (s wave)
• V5 and V6 show such QRS complex in left ventricular hypertrophy (r wave)

T wave

• It represent the ventricular repolarization
• It shows the upward deflection because the part depolarized in the last is first to be repolarized, that is base of heart depolarized in the last but is first to be repolarized
• Should not be more than one third of R wave
• T wave inversion represent ischemia of heart
• Tall and peaked R wave is present in hyperkalemia
• Flattened R wave in pericarditis and myocarditis

PR interval

• This is from beginning of P wave to the beginning of Q wave
• Duration: 0.12 to 0.2 sec
• It represent the conduction time of impulse from SA node to the ventricles and AV delay
• Prolong PR interval shows delayed conduction from SA to AV node

In first degree, second degree and complete heart block

Digitalis therapy

Hyperkalemia

• A-V block, First degree

Interval is about 0.2 to 0.3 sec

• A-V block, Second degree

Sudden dropped QRS complex

PR interval is about 0.4 sec

• 3^rd degree block

When there is AV bloc, atria continue to beat as normal rhythem while new pace maker develops in purkinje system

With a sudden block in purkinje system cannot takes over pace maker activity immediately, during which ventricles fail to contract and person faint

This delayed pick up of heartbeat is called stokes adams syndrome

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QT interval

• Measured from beginning of Q to the end of the T wave
• Duration; 10 small sqrs
• Indicates total systolic time of ventricles

ST segment

• Present between S wave and T wave
• Represent the plateau phase
• Duration- 0.04 sec

J Point

• The point at which all parts of ventricles are depolarized

( just end of QRS complex and at the beginning of ST segment)

• At this point no current is flowing through the heart with potential of ECG value as zero

ECG setup

• Defibrillator Protection Circuit

The one end of the electrode leads are connected along RA, LA, chest and LL of the patient. The other end of electrode passes through defibrillator protection circuit.

• Lead Selection Logic

This block helps to select the type of electrode lead system. We can choose either bipolar or augmented electrode system.

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• Calibration circuit

Calibration is a process that helps to eliminate errors in the system. Here, any changes in the lead selection circuitresults in artefacts in the ECG output.

• Pre Amplifier

An instrumentation amplifier and a differential amplifier with high gain and high CMRR is used as pre amplifier.

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• Power Amplifier

The output unit is driven with power amplifier. After the signal is amplified using pre-amplifier it goes to the power amplifier.

• Feedback Network

Feedback network is used to provide damping to the pen motor.

• Output display unit

Either a CRO or a pen chart recorder acts as the output device.

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• Vectorcardiography: In ECG only the generated by the electrical activity of the heart is recorded
• But in vectorcardiography the cardiac vector is displayed along with its magitude and spatial orientation
• Phonocardiography: The graphic record of the heart’s sound is called phonogram
• The basic aim of the phonogram is to pick the different heart sounds filter out the heart sounds and to display them and record them.

Heart sounds

• Valve closure sound
• Ventricular filling sound
• Valve opening sound
• Extra cardiac sound
• Heart murmur

EEG

• It is a medical device used for analysing the activity of brain
• It can detect Epilepsy or Alzheimer’s disease
• The neurons are –ve when they are at rest and becomes +ve when they synapse
• The EEG measure this change using electrodes
• The signals are transferred from the electrodes to the amplifiers because the power of brain signal is very small

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Action potential of brain

• The human central nervous system consists of 100 billion neurons
• Gilal cells are recognized for their role to communicate within CNS in partnership with neurons
• Progressive transient disturbance of the resting potential along a nerve fiber is used to transmit information from one end to another

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• This action potential is caused by a very rapid change of membrane permeability to sodium ions
• When the propagated action potential reached the cell the cell fires and produce spike wave
• If the transmitter substance is inhibitory the membrane potential of the receptor neuron increases in a negative direction. This induced potential is called an Inhibitory Post Synaptic Potential (IPSP)

• If the transmitter substance is excitatory, the receptor membrane potential increases in a positive direction. This induced change is called an Excitatory Post Synaptic Potential (EPSP)
• Evoked Potential: Potential developed in the brain as the responses to external stimuli like light, sound etc
• The external stimuli are detected by the sense organs which causes changes in the electrical activity of the brain.

Brain waves

• Electrical recordings from the surface of the brain or from the outer surface of the head gives continuous electrical activity of the brain
• The intensity of the brain waves on the surface of the scalp range from 0-300μV

Frequency- few seconds to 50 or more per second

Brain waves in normal persons

• Classified into alpha, beta, theta and delta waves
• Alpha Waves

Fre- 8-13Hz

Occurrence- In normal persons when they awake in a quiet, resting state

During sleep these disappear

Amplitude- 20-200μV

• Beta Waves

Fre- 13-30 Hz

Occurrence- Recorded from parietal and frontal regions of the scalp

2 types- beta I, beta II

Beta I- Inhibited by the cerebral activity

Beta II- Excited by the mental activity like tension

• Theta Waves

Fre- 4-8 Hz

Occurrence- Recorded from the parietal and temporal regions of the scalp of the children

Also occur during emotional stress, particularly during disappointment and frustration

• Delta Waves

Fre- 0.5-4 Hz

Occurrence- occur only once in every 2 or 3 seconds

Occur in deep sleep, in premature babies and in very serious organic brain diseases.

Occur in cortex

Placement of electrodes

• In EEG electrodes are placed in standard positions on the skull in an arrangement called 10- 20 system
• Steps:

Draw a line on the skull from the nasion to the inion

Draw a similar line from the left preauricular (ear) point to the right point

• Mark the intersection of these two lines as Cz
• Mark points at 10,20,20,20 and 10% of the total nasion - inion distance. These points are Fpz, Fz, Cz, Pz and Oz
• Mark points at 10,20,20,20,20 and 10% of the total distance between preauricular points. These points are T3, C3, Cz, C4 and T4.

Odd numbers are at the left and even numbers at the right

• Measure the distance between Fpz and Oz along the great circle passing through T3 and mark points at 10,20,20,20,20 and 10 % of this distance. These points are Fp1, F7, T3, T5 and O1
• Repeat the procedure on the right side and mark the positions of Fp2, F8, T4, T6 and O2
• Measure the distance between Fp1 and O1 and mark the point at 25% interval. These points give the positions of F3, C3 and P3

• The ground reference electrode is a metal clip on the earlobe
• Repeat this procedure on the right side and mark the positions of F4, C4 and P4
• Before placing the electrode the scalp is cleaned lightly abraded and electrode paste is applied between the electrode and the skin
• Application of electrode paste the contact impedance is less than 10kΩ
• In some cases needle electrodes are used to pick up EEG

• Both unipolar and bipolar electrode systems are used to facilitate the location of foci from which abnormal waves spread.
• In bipolar technique the difference in potential between two adjacent electrodes is measured
• In unipolar technique the potential of each electrode is measured with respect to reference electrode attached to ear lobe
• In average mode or Wilson recording techniques the potential is measured between one of the electrodes and the central terminal which is formed by connecting all electrodes through high, equal resistors to a common point

EEG recording setup

• 21-electrode system is used. Electrodes are connected to 8-channel selector. Output from 8-channel connector goes to the differential amplifier bank
• Differential amplifier is made of preamplifiers that are used to reduce noise.
• The output obtained from differential amplifier is connected with signal processing unit.
• This system helps to record the potentials generated from the sensory parts of the brain.

• It can also measure the time delay between stimulus and response from brain.
• Applications of EEG
• Epilepsy diagnosis.
• Anesthetic level.
• Brain injury.
• Monitoring during surgery.

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Analysis of EEG signal

• Used to diagnose the level of consciousness, sleep disorders, brain death, brain tumours etc

EMG (Electromyography)

• EMG is an experimental technique concerned with the development, recording and analysis of myoelectric signals
• Or it is the study of motor unit activity
• Meanwhile the recording of the peripheral nerve’s action potentials is called electroneurography.
• The electrical activity of the underlying muscle can be measured by placing surface electrodes on the skin

• To record the action potentials of individual motor units the needle electrode is inserted into the muscle.
• The action potentials occur both positive and negative polarities at a given pair of electrodes.
• Thus EMG appears very much like a random noise wave form
• The contraction of a muscle produces action potentials
• In a relaxed muscle there is no action potential

Recording setup

• The surface electrodes are from Ag-AgCl and are in disc shape
• The electrodes in kept in place by means of elastic bands
• Therefore the contact impedances are reduced below 10kΩ
• In bipolar electrode, the potential difference between two surface electrodes resting on the skin is measured
• In case of unipolar electrode, the reference electrode is placed on the skin and the needle electrode which acts as active electrode is inserted into the muscle

• In case of Coaxial electrode, consists of an insulated wire threaded through a hyperdermic needle with an oblique tip for easy penetration.

Surrounding steel jacket- reference electrode

Metallic wire- exploring electrode

• Amplitude of EMG depends upon the type and placement of electrodes
• EMG signals range from 0.1 to 0.5 mV
• Frequency components- 20Hz to 10kHz

• Normal frequency of EMG- 60Hz
• Output can be taken from oscilloscope, tape recorder and speaker or paper
• Speed of paper in continuous recording- 5 to 25 cm/second
• The paper width is about 10cm
• The amplifier should have high CMRR and high input impedance

Determination of conduction velocities in motor nerves

• Used to indicate the location and type of the nerve lesion
• Can be examined with a brief shock having a pulse duration of 0.2 to 0.5 milliseconds and measuring the latencies
• Latency- Elapsed time between the stimulating impulse and the muscles action potential

Recording of ERG and EOG

• ERG- The recording and interpreting the electrical activity of eye is called electroretinography
• Potentials within the eye may be recorded easily because of its exposed position.
• The cornea is about 20mV positive relative to the fundus of the eye
• Fundus is the black of the interior of the eye

• Silver-silver chloride electrode is used
• Ag-AgCl is placed in the lens and a distant electrode on the cheek
• Bipolar recording technique is used
• Exploring electrode is placed in saline filled contact lens
• Common contact lens is not used for recording electric potentials
• Recording techniques
• When light falls on the retina the absorption of photons in the outer segment of the retina’s photoreceptors is taking place

• This causes the breakdown of photopigments which results in the liberation of ions that causes to develop action potential.
• Recording setup is similar to ECG recorder
• A wave- Early receptor potentials
• B wave- delay of 1 to 5 milliseconds due to Later receptor potentials
• C wave- Absent of LRP & ERP

EOG (Electrooculogram)

• Record potentials associated with eye movements
• One pair of disc like skin electrode on either side of the eye for recording horizontal movement of eye
• Another pair of electrodes on the forehead and cheeks for recording vertical movement

• The outputs from these two pairs are given separately to the preamplifier and then are recorded
• Some diseases can be analysed using EOG
• Effects of certain drugs on the eye movement system can be determined
• State of the semicircular canals
• Diagnosis of the neurological disorder may be possible