BM T46 – BIOMEDICAL SENSORS AND TRANSDUCERS

UNIT-4

2 MARKS

1. Mention the different types of electrodes used in biomedical field. (Sep 2020)
There are three main types of electrodes used in biomedical field
 Microelectrodes.
 Depth and Needle electrodes
 Body Surface electrodes

2. What are polarizable and non polarizable electrodes? (Sep 2020)

 A polarizable electrode is an electrode in an electrochemical cell that is
characterized by charge separation at the electrode-electrolyte boundary.
 A non-polarizable electrode is an electrode in an electrochemical cell that can be
characterized by no charge separation at the electrode-electrolyte boundary.
3. What are the reasons for artifacts? (May 2019)
Artifacts include tools, clothing, and decorations made by people. They provide
essential clues for researchers studying ancient cultures material remains of a culture,
such as tools, clothing, or food to expose by digging.

4. Mention the need of an electrode gel. (May 2019, May 2015)

Electrolyte gel is designed to provide carbohydrate for fast energy and
key electrolytes for hydration. GO Energy + Electrolyte gels are particularly effective
when exercising in hot conditions causing sweat rates to be high. Available in Lemon &
Mint, Salted Caramel or Raspberry flavours, for a fresh or fruity taste.

5. Define half cell potential. (Nov 2018, May 2018)

Half-cell potential refers to the potential developed at the electrode of each half
cell in an electrochemical cell. In an electrochemical cell, the overall potential is the
total potential calculated from the potentials of two half cells.

6. What are perfectly polarizable electrodes? Give an example. (Nov 2018, May 2018)
Electrodes in which no actual charges cross the electrode-electrolyte interface
when current is applied.

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 E.g. Platinum electrode

7. What are motion artifacts. (Nov 2017, May 2016)

Motion artifact is a patient-based artifact that occurs with voluntary or
involuntary patient movement during image acquisition. Misregistration artifacts, which
appear as blurring, streaking, or shading, are caused by patient movement during a CT
scan.

8. What are the types of surface electrodes. (Nov 2017, May 2015)
There are five types of surface electrodes
 Metal Plate electrodes
 Suction cup electrodes
 Adhesive tape electrodes
 Multi point electrodes
 Floating electrodes.

9. Define polarization. (May 2017)

Polarization is the change of potential from a stabilized state, e.g. from the open-
circuit electrode potential as the result of the passage of current. Polarization is
sometimes also referred to as "over voltage" or "over potential".

10. List out the biomedical applications of silver chloride electrodes. (May 2017)
Silver chloride electrodes are also used by many applications of biological
electrode systems such as bio monitoring sensors as part of

 Electrocardiography (ECG)
 Electroencephalography (EEG)
 Transcutaneous electrical nerve stimulation (TENS) to deliver current.
11. Write down the method of insertion in needle electrode. (Nov 2016)
The Needle Electrode insertion site is on the inner leg, approximately 3 finger
widths (2”) away from the center of the ankle and one finger width towards the back of
the leg. Place 3 fingers on the center of the ankle bone. Move the top finger one finger
width towards the back of the leg.

12. Define microelectrodes. (Nov 2016)

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 A microelectrode is an electrode used in electrophysiology either for
recording neural signals or for the electrical stimulation of nervous tissue.

13. List the electrodes used in ECG and EEG. (May 2016)
ECG
 Flat paper thin sticker electrode
 Self-adhesive circular pad electrode
EEG
 Traditional Wet Ag/AgCl Electrodes
 Active Dry Single Gold Pin-Based Electrodes
 Hybrid Dry Multiple Spikes-Based Electrodes
 Passive Dry Solid-Gel Based Electrodes

14. Write a note on Einthoven triangle. (Nov 2015)

Einthoven's triangle is an imaginary formation of three limb leads in
a triangle used in electrocardiography, formed by the two shoulders and the pubis. The
shape forms an inverted equilateral triangle with the heart at the center. It is named after
Willem Einthoven, who theorized its existence.

15. Define electrical double layer. (Nov 2015)

A double layer (DL, also called an electrical double layer, EDL) is a structure
that appears on the surface of an object when it is exposed to a fluid. The object might be
a solid particle, a gas bubble, a liquid droplet, or a porous body.

11 MARKS

1. Mention the electrodes used in biomedical application and explain the various
electrodes with neat sketch. (Sep 2020, May 2019, Nov 2018, May 2018, Nov 2017,
Nov 2016, May 2016, May 2015, Nov 2015)
There are three main types of electrodes:

 Microelectrodes.
 Depth and Needle electrodes.
 Surface electrodes.
Microelectrodes:

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A microelectrode is an electrode which is used to measure bio electric potential near or
within the cell.

There are two types of microelectrodes

 Metallic microelectrode
 Non-Metallic microelectrode
Metallic microelectrode:

 Metal Microelectrodes are made up of fine tungsten or stainless steel of 0.5 to 5

microns cross section.
 They are made by electrolytic itching.
 They are coated almost up to the tip by insulating material.
 To reduce impedance, micro tip is chloride.

Non-Metallic microelectrode:

 It consists of glass micropipette of diameter 1 micrometer.

 Micropipette filled with electrolyte solution that is compatible with cellular fluids
is used.
 Stem of Micropipette has a thin flexible wire made out of chloride silver,
stainless steel or tungsten.

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Depth electrode:

 Depth electrodes are placed through a small hole using stereotactic navigation to
target locations identified through high-resolution imaging.
 Common targets include the mesial temporal lobe, inter hemispheric fissure, and
per ventricular gray matter.
Needle electrode:

 A fine wire through which electrical current may flow when attached to a power
source; used to carry high frequency electrical currents that create heat or destroy
diseased tissue (called radiofrequency ablation) or seal blood vessels.

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Surface electrodes:

 Surface electrodes are those which are placed in contact with the skin of the subject
in order to obtain bioelectric potentials from the surface.
 Electrode can be used to sense ECG, EEG, EMG etc.
There are five types of surface electrodes

 Metal Plate electrodes

 Suction cup electrodes
 Adhesive tape electrodes
 Multi point electrodes
 Floating electrodes

1. Metal plate electrode:

 One of the most frequently used forms of biopotential sensing electrodes is
the metal-plate electrode.
 In its basic form, it consists of a metallic conductor in contact with the skin.
 An electrolyte gel is used to establish and maintain the contact.

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 2. Suction cup electrode:
 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.
 They have a good contact surface. Physically they are large but the skin contacts
only the electrode rim.
 It has high contact impedance. They have a plastic syringe barrel, suction tube and
cables.
 Recently, due to infection and cleaning procedures, these electrodes are not used.

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 3. Adhesive tape electrode:
 In the surface electrode, the pressure of surface electrode against the skin squeezes
out the electrode paste.
 To avoid this problem, adhesive electrodes are used. It has a lightweight metallic
screen.
 They have a pad at behind for placing electrode paste. This adhesive backing hold
the electrode on place and tight.
 It also helps to avoid evaporation of electrolyte present in the electrode paste.

4. Multipoint electrode:
 Multipoint electrodes are very practical electrode setup for ECG measurements. It
has more than 1000 active contact points.
 This helps to establish low resistance contact with the human. Under any
environmental condition, doctors can use multipoint electrode.

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 5. Floating electrode:
 In metal plate or limb electrodes, the major disadvantage is the movement errors.
Motion artefact occurs due to the motion at the interface between electrode and
electrolyte.
 The interface gets stabilized using Floating electrodes. The floating electrodes do
not contact the human subject directly.
 They contact the subject via electrolytic paste or jelly. The advantage of this type is
the mechanical stability.

2. Brief on (May 2019)

(a) Electrode tissue interface
(b) Polarization
a) ELECTRODE TISSUE INTERFACE:
 The most commonly used electrodes in patient monitoring and related studies are
surface electrodes.
 The notable examples are when they are used for recording ECG, EEG and
respiratory activity by impedance pneumography.
 In order to avoid movement artefacts and to obtain clearly established contact (low
contact impedance) an electrolyte or electrode paste is usually employed as an
interface between the electrode and the surface of the source of the event.
Metal - Electrolyte interface:
 The metal - Electrolyte interface appears to consist of a voltage source in series
with a parallel combination of a capacitance and reaction resistance. The voltage
developed is called the half-cell potential.

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 Metal Ionic symbol Electrode potential

Aluminium AI+++ –1.66V

Iron Fe++ –0.44V

++
Lead Pb –0.12V
Hydrogen H+ 0
Copper C++ +0.34V
Silver Ag+ +0.80V
Platinum Pt+ +1.2V
Gold Au+ +1.69V

Table: Electrode Potential for some Metals with Respect to Hydrogen

 This table shows that the electrode potentials are appreciable when dissimilar
metals are used.
 They also exist, though of smaller magnitude, even if electrodes of similar
materials are employed.
 The lowest potential has been observed to be in the silver-silver chloride
electrodes.
 The values of the capacitance and the resistance depend upon many factors which
include the current density, temperature, type and concentration of the electrolyte and
the type of metal used.
 The difference in half-cell potentials that exists between two electrodes is also
called ‘offset potential’.
 The differential amplifiers used to measure potentials between two electrodes are
generally designed to cancel the electrode offset potential so that only the signals of
interest are recorded.
 The electrode offset potential produced between electrodes may be unstable and
unpredictable.
 The long-term change in this potential appears as baseline drift and short-term
changes as noise on the recorded trace.
 If electrodes are used with ac-coupled amplifiers, the long term drift may be
partially rejected by the low frequency characteristics of the amplifier.
 But it will depend upon the rate of change of electrode offset potential in relation to
the ac-coupling time constant in the amplifier.
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  For example, if the electrode offset potential drift rate is 1 mV/s, satisfactory
results can only be obtained if the low frequency response of the amplifier is1Hz.

Electrolyte-Skin Interface:
 An approximation of the electrolyte-skin interface can be had by assuming that the
skin acts as a diaphragm arranged between two solutions (electrolyte and body fluids)
of different concentrations containing the same ions, which is bound to give potential
differences.
 The simplest equivalent representation could then be described as a voltage source
in series with a parallel combination of a capacitance and resistance.
 The capacitance represents the charge developed at the phase boundary whereas
the resistance depends upon the conditions associated with ion-migration along the
phase boundaries and inside the diaphragm.
 The above discussion shows that there is a possibility of the presence of voltages of
non-physiological origin. These voltages are called contact potentials.
 The electrical equivalent circuit of the surface electrode suggests that the voltage
presented to the measuring instrument from the electrode consists of two main
components.
 One is the contact potential and the other is the biological signal of interest. The
contact potential depends upon several factors and may produce an interference signal
which exceeds several times the useful signal.
 The contact potential is found to be a function of the type of skin, skin preparation
and composition of the electrolyte.
 When bioelectric events are recorded, interference signals are produced by the
potential differences of metal-electrolyte and the electrolyte-skin interface.
 Normally, these potential differences are connected in opposition during the
recording procedure, and in the case of a truly reversible and uniform electrode pair,
their difference would be nil.
 However, in practice, a difference of potential may be extremely small and is found
to exist between electrodes produced even under conditions of utmost care during
manufacture.

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  Also, some of the elements in the equivalent circuit are time-dependent and are
bound to show slow variations with time.

b) POLARIZATION:
 If a low voltage is applied to two electrodes placed in a solution, the electrical
double layers are disturbed.
 Depending on the metals constituting the electrodes, a steady flow of current may
or may not take place.
 In some metal / liquid interfaces, the electrical double layer gets temporarily
disturbed by the externally applied voltage, and therefore, very small current flows
after the first surge, thus indicating a high resistance.
 This type of electrode will not permit the measurement of steady or slowly varying
potentials in the tissues.
 They are said to, be polarized or non reversible. Thus, the phenomenon of
polarization affects the electro-chemical double layer on the electrode surface and
manifests itself in changing the value of the impedance and voltage source
representing the transition layer.
 Parsons (1964) stated that electrodes in which no net transfer of charge takes place
across the metal-electrolyte interface can be termed as perfectly polarized.
 Those in which unhindered exchange of charge is possible are called non-
polarizable or reversible electrodes.
 The ionic double layer in metals of these electrodes is such that they allow
considerable current to flow when a small voltage is applied, thus offering a low
resistance.
 Non-polarizing electrodes on the other hand, are designed to rapidly dissipate any
charge imbalance induced by powerful electrical discharges such as a defibrillation
procedure.
 Depolarization enables the immediate reappearance of bioelectric signals on the
monitor after defibrillation.
 For this reason, non-polarizing electrodes have become the electrodes of choice for
monitoring in the intensive care units and stress testing procedures.

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  Although polarizable electrodes are becoming less common, they are still in use.
 They usually employ stainless steel and are used for resting ECGs or other
situations where there is small likelihood that the electrodes would be exposed to a
large pulse of energy (such as a defibrillation discharge) in which case they would
retain a residual charge, become polarized, and will no longer transmit the relatively
small bioelectric signals, thus becoming useless.
 The choice of metals for electrodes is not determined only by their susceptibility to
polarization, but other factors such as mechanical properties, skin irritation or skin
staining, etc. have also to be taken into consideration.

3. How metal microelectrodes are formed and explain its electrical equivalent ciruit
with neat diagram. (Sep 2020)

Metal Microelectrodes:

 Metal electrodes with very fine tips used for recording from single cells have the
advantage over glass micropipetes of being relatively robust.
 Steel microelectrodes can be made from ordinary darning needles but preferably
they should be of good stainless steel wire.
 They can be easily made up to 10 m diameter but great care has to be taken for
diameters as small as 1 m.
 These very small tips are not very satisfactory as they are extremely brittle and
have very high input impedance.
 Hubel (1957) described a method to make tungsten microelectrodes with a tip
diameter of 0.4 m.
 He used electropointing technique which consists in etching a metal rod while the
metal rod is slowly withdrawn from the etching solution, thus forming a tapered tip on
the end of the rod.
 The etched metal is then dipped into an insulating solution for placing insulation on
all but the tip.

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  This arrangement offers lower impedance than the microcapillary electrode,
infinite shelf life and reproduciable performance, with ease of cleaning and
maintenance.
 The metal—electrolyte interface is between the metal film and the electrolyte of
the cell.
 Skrzypek and Keller (1975) illustrated a new method of manufacturing tungsten
microelectrode permitting close control of microelectrode para- meters.
 In this technique, the tips are dc electroetched to diameters below 500° A and
completely covered with polymethyl methacrylate.
 An electron beam from a scanning electron microscope is then used to expose a
precise area on the tip for later removal by chemical methods.
 Recording results with these electrodes suggested good desirable recording
characteristics, i.e. ability to isolate and hold single cells.
 Tungsten is preferred for constructing micro-electrodes due to its mechanical
strength and its apparent inertness.
 Although tungsten itself is reactive, a surface layer of tungsten oxide will, in most
situations, protect the metal against corrosion.
 The electrical properties of tungsten microelectrodes made with a taper of the tip of
about 1:10 and insulated with lacquer leaving a tip length of about 10–100 μm were
studied by Zeuthen (1978).
 The resting potential in saline was found to be –0.3 V relative to a silver-silver
chloride reference electrode for input currents less than 10–12 A.
 Metallic electrodes are formed from a fine needle of a suitable metal drawn to a
fine tip.
 On the other hand, glass electrodes are drawn from Pyrex glass of special grade.
 The metal microelectrodes are used in direct contact with the biological tissue and,
therefore, have a lower resistance. However, they polarize with smaller amplifier
input currents.
 The glass microelectrode has a substantial current carrying capacity because of the
large surface contact area between the metal and the electrolyte.

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Equivalent circuit:

 Equivalent circuit model and methods. (A) Equivalent circuit model of a metal
microelectrode in the brain adapted from Robinson (1968). The entire circuit is
comprised of the electrode in the brain and the amplifier with a filter.
 The effective impedance of the electrode (Z e ) is comprised of the resistance of the
electrolyte solution (R s ), the resistance and capacitance at the double layer interface

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 of the electrolyte and the un insulated electrode tip (R e and C e ) and the (negligible)
resistance of the metal electrode (R m ).
 The effective input impedance of the amplifier (Z a ) is comprised of the input
impedance of the head-stage amplifier (Z a ) and the shunt resistance and capacitance
to ground from the tip of the electrode to the input of the amplifier (R sh and C sh ).
 Two aluminium plates were connected and separated from each by non-conducting
plastic supports, shown here from a top and side view.
 The apparatus was immersed in dilute saline with voltage signals applied to the
signal plate with an electrode suspended from above 3 mm away.

4. Draw the equivalent diagram of an electrode tissue interface and explain the
need of electrode jellies. (Nov 2018, May 2018)
 Warburg (1899) in his pioneering studies discovered that a single
electrode/electrolyte interface can be represented by a series capacitance C and
resistance R as shown in Fig.
 However, C and R are unlike real capacitors and resistors because their values are
frequency and current- density dependent. Often, these components are called the
polarization capacitance and resistance.
 The consequence of this relationship is that the phase angle is constant at /4 for
all frequencies.
 However, only a limited number of studies have tested the accuracy of the
Warburg model (Ragheb and Geddes, 1990).
 It has been observed that the Warburg series RC equivalent does not adequately
represent the behaviour of an electrode/electrolyte interface as this equivalent does
not truly account for the very low-frequency behaviour of the interface.
 It is well known that such an interface can pass direct current. Therefore, a
resistance Rf placed in parallel with the Warburg equivalent is more appropriate.
 The below figure shows this equivalent circuit in which Rf represents the Faradic
leakage resistance. The value of Rf is high in the low-frequency region and is
dependent on current density, increasing with a increase in current density.

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  a) Equivalent circuit model of electrode–tissue interface, with interface reactance
represented by a single capacitor CP, RP charge transfer resistance, RS
Tissue/electrolyte resistance.
 b) Equivalent circuit model of electrode–tissue interface, with interface reactance
represented by a constant phase element (ZCPA ).
 c) Illustration of experimental setup. Ag/AgCl reference electrode, Platinum core
of the concentric bipolar electrode, and Stainless steel outer shell of the bipolar
electrode were connected to the potentiostat as the reference, working, and counter
electrode, respectively.
 d) Illustration of the geometry and dimension of the bipolar electrode tip.

ELECTRODE JELLIES:

 Conducting creams and jellies have for long been used to facilitate a more intimate
contact between the subject’s skin and the recording electrodes.

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  The outer horny layer of the skin is responsible for the bulk of the skin contact
impedance, and for this reason careful skin preparation is essential in order to obtain
the best results.
 The recording site should first be cleaned with an ether-meth mixture. In addition
to having good electrical conductivity, the electrode jelly must have a particular
chloride ion concentration (about 1%) close to the physiological chloride
concentration.
 This is primarily important for long-term monitoring because it should not produce
a harmful diffusion between the jelly and the body.
 It is to be particularly ensured that the jelly chosen is of a bland nature and does not
contain soap or phenol which can produce a marked irritation of the skin after a few
hours.
 The electrical conductivity of different makes of electrode cream can be measured
(Hill and Khandpur, 1969) by means of the Schering ac bridge circuit.
 The cream is placed in a perspex conductivity cell of known dimensions and the
resistive component of the cell impedance is measured at 10 Hz, the conductivity
being calculated from the cell dimensions.
 The contact impedance of the skin depends upon the type of electrolyte used and
the time (Trimby, 1976)
 A low concentration sodium chloride electrolyte has 0.5% NaCI and a high
concentration electrolyte has a concentration in the range of 5 to 10% NaCI.
 The impedance is found to fall rapidly to 40% between 7 to 30 min. Stabilization
occurs at about 30 to 45 min.
 An interesting observation from this figure is that while pre-rubbing the skin will
lower the initial impedance value, the final value after using a high concentration
electrolyte becomes nearly the same.
 Electrode jelly can be replaced in certain cases by using a conducting plastic as an
interface between the electrode and the surface of the body.

5. Explain the construction and working of Ag/AgCl electrode. (May 2017, Nov
2016, May 2016)

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  The silver/silver chloride or Ag/AgCl reference electrode is many electrochemists'
reference electrode of choice. It is easily and cheaply prepared. It is stable, and quite
robust.
 It is sometimes referred to as "SSCE" (Silver/Silver Chloride Electrode) but that
abbreviation has been used for Sodium Saturated Calomel Electrode also.
Construction:
 The figure to the right shows an easily constructed Ag/AgCl reference
electrode. The body of the electrode is made from 4 mm glass tube.
 A porous glass is available in 4 mm diameter rod and serves as the ionicly
conducting electrical pathway between the inside of the reference electrode and the
bulk of your cell.
 It has low electrical resistance (under 10 kohm for the common filling solutions)
and a modest leak rate.
 The electrical resistance of the reference electrode 'frit' is an important factor in
determining the stability and speed of your potentiostat in actual use.
 The leak rate may be important because of possible contamination of your solution
by the reference electrode filling solution and vice versa.
Production of Silver-Silver Chloride Electrodes:
 Silver-silver chloride electrodes are normally prepared by electrolysis. Two silver
discs are suspended in a saline solution.
 The positive pole of a dc supply is connected to the disc to be chlorided and the
negative pole goes to the other disc.
 A current at the rate of 1 mA/cm2 of surface area is passed through the electrode
for several minutes.
 A layer of silver chloride is thus deposited on the surface of the anode. The
chemical changes that take place at the anode and cathode respectively are:
 To prepare silver-silver chloride electrodes of good quality, only pure silver should
be used and the saline solution should be made from analar grade sodium chloride.
Before chloriding, silver must be cleaned—preferably by the electrolytic method.
 They demonstrated that the impedance was different for different layers of chloride
and that there is an optimum chloriding, which gives the lowest impedance.

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  A layer of silver chloride is thus deposited on the surface of the anode. The
chemical changes that take place at the anode and cathode respectively are:

NaCl = Na+ + Cl–

Cl– + Ag+ = AgCl

 The positively charged sodium ions generate hydrogen when they reach the
cathode surface.

2Na+ + 2H2O + 2 electrons = 2NaOH + H2

 They concluded that the lowest electrode-electrolyte impedance in the frequency

range of 10 Hz to 10 kHz was found to occur with a chloride deposit ranging between
100 and 500 mAs/cm2 of electrode area.
 Higher values may be used with a corresponding reduction in time to achieve the
100–500 mAs/cm2 chloride deposit.
 The use of a chloride deposit in excess of this range did not alter the resistive
nature of the electrode-electrolyte impedance although it increased its magnitude.

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  Geddes (1972) confirmed that an optimal coating of silver chloride applied to a
silver electrode minimizes the electrical impedance.
 This is supported by Getzel and Webster (1976) who concluded that silver chloride
may be applied to cleaned silver electrodes in the amount of 1050–1350 mA s/cm2 in
order to reduce the impedance of the electrodes.
 However, to further reduce the impedance of the electrodes, they should be coated
with at least 2000 mAs/cm2 of silver chloride followed by immersion in a
photographic developer for 3 minutes.
 Grubbs and Worley (1983) obtained a lower and more stable impedance electrode
by placing a heavier initial chloride coat on an etched silver electrode, and then
electrolytically removing a portion of that coat.
 The electrode functions as a redox electrode and the equilibrium is between
the silver metal (Ag) and its salt—silver chloride (AgCl, also called silver(I)
chloride).
The corresponding half-reactions can be presented as follows:
Ag+ + e- ----- Ag(s)
AgCl(s) + e- ------ Ag(s) + Cl-

or can be written together:

AgCl(s) + Ag(s) + e- --------- Ag(s) + e- + Ag+ + Cl-

which can be simplified:

AgCl(s) ------- Ag+ + Cl-

 This reaction is characterized by fast electrode kinetics, meaning that a sufficiently

high current can be passed through the electrode with 100% efficiency of
the redox reaction.
 The reaction has been proven to obey these equations in solutions of pH values
between 0 and 13.5.
 The Nernst equation below shows the dependence of the potential of the silver-
silver chloride electrode on the activity or effective concentration of chloride-ions:

E = E0 – RT/F ln(Cl-)

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  The standard electrode potential E0 against standard hydrogen electrode (SHE) is
0.230 V ± 10 mV. The potential is however very sensitive to traces of bromide ions
which make it more negative.

Application of Ag/AgCl

 An Ag/AgCl electrode used as a corrosion sensor in a reinforced concrete structure

is considered as having good application prospect.

6. Describe about the placement of electrodes for ECG and EEG. (May 2017, May 2015)
Placement of electrodes for ECG:

3-electrode system
 Uses 3 electrodes (RA, LA and LL)
 Monitor displays the bipolar leads (I, II and III)
 To get best results – Place electrodes on the chest wall equidistant from the heart
(rather than the specific limbs)

5-electrode system
 Uses 5 electrodes (RA, RL, LA, LL and Chest)
 Monitor displays the bipolar leads (I, II and III)
 AND a single unipolar lead (depending on position of the brown chest lead
(positions V1–6))

12-lead ECG
 10 electrodes required to produce 12-lead ECG
 4 Electrodes on all 4 limbs (RA, LL, LA, RL)6 Electrodes on precordium (V1–6)
 Monitors 12 leads (V1–6), (I, II, III) and (aVR, aVF, aVL)
 Allows interpretation of specific areas of the heart
 Inferior (II, III, aVF)Lateral (I, aVL, V5, V6)Anterior (V1–4)
12-lead Precordial lead placement
 V1: 4th intercostal space (ICS), RIGHT margin of the sternum
 V2: 4th ICS along the LEFT margin of the sternum
 V4: 5th ICS, mid-clavicular line

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  V3: midway between V2 and V4
 V5: 5th ICS, anterior axillary line (same level as V4)
 V6: 5th ICS, mid-axillary line (same level as V4)

Additional Lead placements

Right sided ECG electrode placement

There are several approaches to recording a right-sided ECG:

 A complete set of right-sided leads is obtained by placing leads V1-6 in a mirror-

image position on the right side of the chest (see diagram, below).
 It can be simpler to leave V1 and V2 in their usual positions and just transfer leads
V3-6 to the right side of the chest (i.e. V3R to V6R).
 The most useful lead is V4R, which is obtained by placing the V4 electrode in the
5th right intercostal space in the mid-clavicular line.
 ST elevation in V4R has a sensitivity of 88%, specificity of 78% and diagnostic
accuracy of 83% in the diagnosis of RV MI.

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V4R ECG lead placement
 Erhardt et al first described the use of a right sided precordial lead (CR4R or V4R)
in the diagnosis of right ventricular infarction which had previously been thought to
be electrocardiographically silent.
 Single right-sided precordial lead in the diagnosis of right ventricular involvement
in inferior myocardial infarction.

Posterior leads

Leads V7-9 are placed on the posterior chest wall in the following positions:

 V7 – Left posterior axillary line, in the same horizontal plane as V6.

 V8 – Tip of the left scapula, in the same horizontal plane as V6.
 V9 – Left paraspinal region, in the same horizontal plane as V6.

Lewis lead(S5-lead)

Lewis lead placement

 Right Arm (RA) electrode on manubrium
 Left Arm (LA) electrode over 5th ICS, right sternal border.
 Left Leg (LL) electrode over right lower costal margin.
Fontaine leads:
Fontaine bipolar precordial leads (F-ECG) are used to increase the sensitivity of epsilon
wave detection. Named after French cardiologist and electrophysiolgist Guy Hugues
Fontaine (1936-2018). Leads are placed as shown:

 Right Arm (RA) over the manubrium;

 Left Arm (LA) over the xiphoid process;
 and Left Leg (LL) in the standard V4 position (5th ICS MCL).
Instead of regular leads I, II, and III there are now three bipolar chest leads that are termed FI,
FII, and FIII which record the potentials developed in the right ventricle, from the
infundibulum to the diaphragm.

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The vertical bipolar lead FI, (similar to aVF) magnifies the atrial potentials and can be used
to record:

 Epsilon waves;
 Search for AV dissociation in ventricular tachycardia;
 And to study abnormal atrial rhythms when the P waves are too small on regular
leads.
Placement of electrodes for EEG:
 The Fp2, F8, T4, T6, and O2 electrodes are placed at intervals of 5%, 10%, 10%,
10%, 10%, and 5%, respectively, measured above the right ear, from front (Fpz) to
back (Oz).
 The same is done for the odd-numbered electrodes on the left side, to complete the
full circumference.

10–20 system (EEG)

 The 10–20 system or International 10–20 system is an internationally recognized

method to describe and apply the location of scalp electrodes in the context of
an EEG exam, polysomnograph sleep study, or voluntary lab research.
 This method was developed to maintain standardized testing methods ensuring that
a subject's study outcomes (clinical or research) could be compiled, reproduced, and
effectively analyzed and compared using the scientific method.
 The system is based on the relationship between the location of an electrode and
the underlying area of the brain, specifically the cerebral cortex.
 During sleep and wake cycles, the brain produces different, objectively recognized
and distinguishable electrical patterns, this can be detected by electrodes on the skin.
 The "10" and "20" refer to the fact that the actual distances between adjacent
electrodes are either 10% or 20% of the total front–back or right–left distance of the
skull. For example, a measurement is taken across the top of the head, from
the nasion to inion.
 Most other common measurements ('landmarking methods') start at one ear and
end at the other, normally over the top of the head. Specific anatomical locations of
the ear used include the tragus, the auricle and the mastoid.

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Electrode labeling:
 Each electrode placement site has a letter to identify the lobe, or area of the brain it
is reading from: pre-frontal (Fp), frontal (F), temporal (T), parietal (P), occipital (O),
and central (C).
 Note that there is no "central lobe"; due to their placement, and depending on the
individual, the "C" electrodes can exhibit/represent EEG activity more typical of
frontal, temporal, and some parietal-occipital activity, and are always utilized in
polysomnography sleep studies for the purpose of determining stages of sleep.
 There are also (Z) sites: A "Z" (zero) refers to an electrode placed on
the midline sagittal plane of the skull, (FpZ, Fz, Cz, Oz) and is present mostly for
reference/measurement points.
 These electrodes will not necessarily reflect or amplify lateral hemispheric cortical
activity as they are placed over the corpus callosum, and do not represent either
hemisphere adequately.
 Even-numbered electrodes (2,4,6,8) refer to electrode placement on the right side
of the head, whereas odd numbers (1,3,5,7) refer to those on the left; this applies to
both EEG and EOG (electrooculogram measurements of eyes) electrodes, as well as
ECG (electrocardiography measurements of the heart) electrode placement.

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  Chin, or EMG (electromyogram) electrodes are more commonly just referred to
with "right," "left," and "reference," or "common," as there are usually only three
placed, and they can be differentially referenced from the EEG and EOG reference
sites.
 The "A" (sometimes referred to as "M" for mastoid process) refers to the
prominent bone process usually found just behind the outer ear (less prominent in
children and some adults). In basic polysomnography, F3, F4, Fz, Cz, C3, C4, O1,
O2, A1, A2 (M1, M2), are used.
 Cz and Fz are 'ground' or 'common' reference points for all EEG and EOG
electrodes, and A1-A2 are used for contralateral referencing of all EEG electrodes.
 This EEG montage may be extended to utilize T3-T4, P3-P4, as well as others, if
an extended or "seizure montage" is called for.

Measurement:

 Specific anatomical landmarks are used for the essential measuring and positioning
of the EEG electrodes. These are found with a tape measure, and often marked with a
grease pencil, or "China marker."
 Nasion to Inion: the nasion is the distinctly depressed area between the eyes, just
above the bridge of the nose, and the inion, is the crest point of back of the skull,
often indicated by a bump (the prominent occipital ridge, can usually be located with
mild palpation). Marks for the Z electrodes are made between these points along the
midline, at intervals of 10%, 20%, 20%, 20%, 20% and 10%.
 Preauricular to preauricular (or tragus to tragus: the tragus refers to the small
portion of cartilage projecting anteriorly to the pinna).
 The preauricural point is in front of each ear, and can be more easily located with
mild palpation, and if necessary, requesting patient to open mouth slightly.
 The T3, C3, Cz, C4, and T4 electrodes are placed at marks made at intervals of
10%, 20%, 20%, 20%, 20% and 10%, respectively, measured across the top of the
head.
 Skull circumference is measured just above the ears (T3 and T4), just above the
bridge of the nose (at Fpz), and just above the occipital point (at Oz). The Fp2, F8,
T4, T6, and O2 electrodes are placed at intervals of 5%, 10%, 10%, 10%, 10%, and

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 5%, respectively, measured above the right ear, from front (Fpz) to back (Oz). The
same is done for the odd-numbered electrodes on the left side, to complete the full
circumference.
 Measurement methods for placement of the F3, F4, P3, and P4 points differ. If
measured front-to-back (Fp1-F3-C3-P3-O1 and Fp2-F4-C4-P4-O2 montages), they
can be 25% "up" from the front and back points (Fp1, Fp2, O1, and O2).
 If measured side-to-side (F7-F3-Fz-F4-F8 and T5-P3-Pz-P4-T6 montages), they
can be 25% "up" from the side points (F7, F8, T5, and T6).
 If measured diagonally, from Nasion to Inion through the C3 and C4 points, they
will be 20% in front of and behind the C3 and C4 points. Each of these measurement
methods results in different nominal electrode placements.

7. Write in detail notes on electrical conductivity of jellies and its types.(May 2017)
ELECTRICAL CONDUCTIVITY OF ELECTRODE JELLIES:

 Conducting creams and jellies have for long been used to facilitate a more intimate
contact between the subject’s skin and the recording electrodes.
 The outer horny layer of the skin is responsible for the bulk of the skin contact
impedance, and for this reason careful skin preparation is essential in order to obtain
the best results.
 The recording site should first be cleaned with an ether-meth mixture. In addition
to having good electrical conductivity, the electrode jelly must have a particular
chloride ion concentration (about 1%) close to the physiological chloride
concentration.
 This is primarily important for long-term monitoring because it should not produce
a harmful diffusion between the jelly and the body.
 It is to be particularly ensured that the jelly chosen is of a bland nature and does not
contain soap or phenol which can produce a marked irritation of the skin after a few
hours.
 The electrical conductivity of different makes of electrode cream can be measured
(Hill and Khandpur, 1969) by means of the Schering ac bridge circuit.

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  The cream is placed in a perspex conductivity cell of known dimensions and the
resistive component of the cell impedance is measured at 10 Hz, the conductivity
being calculated from the cell dimensions.
 The contact impedance of the skin depends upon the type of electrolyte used and
the time (Trimby, 1976)
 A low concentration sodium chloride electrolyte has 0.5% NaCI and a high
concentration electrolyte has a concentration in the range of 5 to 10% NaCI.
 The impedance is found to fall rapidly to 40% between 7 to 30 min. Stabilization
occurs at about 30 to 45 min.
 An interesting observation from this figure is that while pre-rubbing the skin will
lower the initial impedance value, the final value after using a high concentration
electrolyte becomes nearly the same.
 Electrode jelly can be replaced in certain cases by using a conducting plastic as
an interface between the electrode and the surface of the body.
 The impedance is found to fall rapidly to 40% between 7 to 30 min.
 Stabilization occurs at about 30 to 45 min.
 An interesting observation from this figure is that while pre-rubbing the skin will
lower the initial impedance value, the final value after using a high concentration
electrolyte becomes nearly the same.
 Electrode jelly can be replaced in certain cases by using a conducting plastic as an
interface between the electrode and the surface of the body.
 Jenkner (1967) used silastic S-2086 by Dow Corning with EEG electrodes and
showed that contact resistance was almost the same as with a conventional electrode
which make use of electrode jelly.

TYPES OF GEL:

PEO - BASED POLYMER GEL ELECTROLYTE:

 For preparation of this electrolyte EC and/or PC are taken as plasticizers. LiClO4,

LiCFSO3, LiN(SO2CF3)2 etc are used as the salt.

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  This electrolyte has a conductivity of 10-3 Scm-1 . The polymer used (i.e. PEO) is
not soluble to a greater extent with the solvent resulting in poor mechanical strength
of the gels.
PAN - BASED POLYMER GEL ELECTROLYTE:

 The polymer used is PAN (Polyacrylonitrile). Lithium salts are the principle salt
used here.
 The Li+ ion transference number were found to be very high in case of PAN- based
gel electrolyte.
PMMA - BASED POLYMER GEL ELECTROLYTE:

 The polymer, solvent and salt are PMMA, EC and/or PC and Lithium and sodium
salts respectively.
 An ionic conductivity of 10-3 S cm-1 has been achieved in this gel electrolyte.
PVDF - BASED POLYMER GEL ELECTROLYTE:

 PVDF is the polymer, EC/PC or DMF is the solvent and lithium salts such as
LiCF3SO3, LiPF6 or LiN(SO3CF3)2 or NaI is mainly taken as the salt.
 PVDF based gel has good electrochemical properties but they are reactive to
lithium and lithium salts.

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