Medical Instrumentation- Biosignals, ECG

UEEG002
Medical Instrumentation - ECG
D.Poornima,AP(Sr.Gr)/EEE,
Sri Ramakrishna Institute of Technology,
Coimbatore.

Bioelectrical signals
• Bioelectrical signals are very low amplitude and low frequency electrical
signals that can be measured from biological beings.
• They are generated from the functions of human body and can be measured
through changes in electrical potential across a cell or an organ.
• The bioelectrical signals of our interest are the electrocardiogram (ECG) and
the electroencephalogram (EEG) and Electromyography(EMG).
• An ECG measures the electrical manifestation of the ionic potential of the
heart
• An EEG measures the electrical activity evoked along the scalp of the brain.
• The ECG and the EEG are recorded using standard equipment in the
noninvasive fashion
• An EMG measures muscle response or electrical activity in response to a
nerve's stimulation of the muscle and is used to help detect neuromuscular
abnormalities.
• During the test, one or more small needles (also called electrodes) are
inserted through the skin into the muscle.

ELECTROCARDIOGRAM (ECG)
• The ECG is a noninvasive tool used to record
the electrical activity of the heart.
• Nobel laureate, Willem Einthoven, was the first
who had recorded the ECG in 1903 .
• It can be recorded with the surface electrodes
placed on the limbs and chest.
• ECG devices use varying number of electrodes
ranging from 3 to 12 for signal acquisition while
the system using more electrodes exceeding 12
and up to 120 is also available

Working of Heart
• The heart has four chambers -- the right
and left atrium and the right and left
ventricle.
• The right side of the heart collects blood
from the body and pumps it to
the lungs while the left side of the heart
receives blood from the lungs and pumps it
to the body.
• Electrically, the heart can be divided into
upper and lower chambers.
• An electrical impulse is generated in the
upper chambers of the heart that causes
the atria to squeeze and push blood into
the ventricles.
• There is a short delay to allow the
ventricles to fill.
• The ventricles then contract to pump blood
to the body and the lungs.

• Conducting system of the heart: SA means sinoatrial node. AV means atrioventricular
node. RB and LB mean right and left bundle, respectively, and are the nerves that spread
the electric impulse from the AV node into the ventricles.
• The heart has its own automatic pacemaker called the sinaoatrial, or SA node, located in
the right atrium. The SA node acts independently of the brain to generate electricity for
the heart to beat
• Normally, the impulse generated by the SA node runs through the heart's electrical grid
and signals the muscle cells in the atria to beat simultaneously, allowing for a
coordinated squeeze of the heart. Contraction of the atria pushes blood into the
ventricles.
• The electrical signal that was generated in the SA node travels to a junction box between
the atria and ventricles (the AV node) where it is delayed for a few milliseconds to allow
the ventricles to fill.
• The electrical signal then travels through the ventricles, stimulating those heart muscle
cells to contract. Ventricular contraction pumps blood to the body (from the left
ventricle) and the lungs (from the right ventricle).
• There is a short pause to allow blood to return to the heart and fill before the electrical
cycle repeats itself for the next heartbeat.

• The ECG waveform reflects the rhythmic electrical depolarization and repolarization
of the heart muscles associated with the contractions and relaxation of the atrium
and ventricles.
• The electrocardiogram gives details of the state of the heart and any disturbance in
the heart rhythmic(arrhythmia) can be diagnosed.
• The typical ECG wave consists of P wave, QRS complex and T wave.
• The electrical potentials of the heart are measured by placing suitable electrodes.
• Either surface electrodes with proper electrode paste or needle electrode can be
used

Difference between Electrode and Lead
• An electrode is a conductive pad attached to the
skin and enables recording of electrical currents
• An ECG Lead is the graphical representation of
the electrical activity of the heart and is created
by analyzing different electrodes.
• The standard ECG-12 Lead ECG includes 12
leads-obtained using 10 electrodes
• 12 lead ECG consists of two sets of ECG leads-
limb leads and chest leads (precordial leads)

Electrodes for ECG
• Limb Electrodes
– Most common type of electrodes
used are rectangular or circular
surface electrodes
– Materials used is German Silver (alloy
of zinc, copper and nickel) , Nickel
Silver, Nickel Plated Steel
– Applied to the surface of the body
with electrode jelly
– Held in position by clips
– Used in four limbs of the body

• Floating Electrodes
– Limb electrodes suffer from motion artefacts due to the
relative motion at the interface between the electrode
surface and electrode jelly
– The interface can be stabilized using floating electrodes
– No direct contact with the skin (Kahn and Boter)
– Consists of a lightweight metalled screen or plate held
away from the subject by a flat washer connected to the
skin
– Can be recharged by replenishing the jelly in the
electrodes

• A spray on chest electrode- a conducting spot is developed on the skin by
spraying a film of conducting adhesive (Pattern et al.)
• Connection with instrument is established with silver plated copper wires
fixed in the adhesive
• Extremely light weight, don't use electrode jelly
• Useful in monitoring exercising patients and pilots

• Pre gelled disposable electrodes

• Pasteless electrodes incorporate a bare
or dielectric-coated metal plate, in direct
contact with the skin, to form a very high
impedance interface.
• By using an integral high-input-
impedance amplifier, it is possible to
record a signal through the capacitive or
resistive interface.

Standard 12 Lead Configuration
• Numerous ECG lead systems and configurations have been
tested
• Standard 12 lead configuration is the most efficient to
diagnose abnormalities
• Has 12 leads derived by means of 10 electrodes
• 3 leads are result of comparing potentials recorded by two
electrodes (One electrode is exploring, one is reference)
• In remaining 9 leads one exploring electrode, but reference
is obtained by combining two or three electrodes
• At any given instant,all ECG leads analyse the same electrical
event

Electrode Placement
• To measure the heart's electrical activity accurately, proper electrode
placement is crucial.
• Chest (Precordial) Electrodes and Placement
– V1 - Fourth intercostal space on the right sternum
– V2 - Fourth intercostal space at the left sternum
– V3 - Midway between placement of V2 and V4
– V4 - Fifth intercostal space at the midclavicular line
– V5 - Anterior axillary line on the same horizontal level as V4
– V6 - Mid-axillary line on the same horizontal level as V4 and V5
•

• Limb (Extremity) Electrodes and Placement
– The limb electrodes can be far down on the limbs or close
to the hips/shoulders as long as they are placed
symmetrically.
• RA (Right Arm) - Anywhere between the right shoulder and right elbow
• RL (Right Leg) - Anywhere below the right torso and above the right ankle
• LA(Left Arm) - Anywhere between the left shoulder and the left elbow
• LL (Left Leg) - Anywhere below the left torso
and above the left ankle

THE 12 LEAD GROUPS
• There are 10 electrodes providing 12 perspectives of the heart's activity using different angles through two electrical
planes - vertical and horizontal planes.
• Vertical plane (Frontal Leads):
– By using 4 limb electrodes, you get 6 frontal leads that provide information about the heart's vertical plane:
• Lead I
• Lead II
• Lead III
• Augmented Vector Right (aVR)
• Augmented Vector Left (aVL)
• Augmented vector foot (aVF)
– Leads I, II, and III require a negative and positive electrode (bipolarity) foR
monitoring. On the other hand, the augmented leads-aVR, aVL, and aVF-are
unipolar and requires only a positive electrode for monitoring.
• Keep in mind that RL is neutral (also known as point zero where the electrical current is measured).
• RL doesn't come up in ECG readings, and is considered as a grounding lead that helps minimize ECG
artifact.

• Horizontal Plane (Transverse Leads)
– 6 chest electrodes provides 6 transverse leads
– Gives information about the heart's horizontal
plane: V1, V2, V3, V4, V5, and V6.
– Transverse leads are unipolar - requires only a
positive electrode.
– The negative pole of all 6 leads is at the center
of the heart.
– This is calculated with the ECG.

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.
• Named after Willem Einthoven, who theorized its existence.

• Lead I compares the electrode on
the left arm with the electrode on
the right arm - RA to LA
• The former is the exploring
electrode
• Lead I observes the heart from the
left
• Lead II compares the left leg with
the right arm and observes the
heart from an angle of 60°- RA to
LL
• Lead III compares the left leg with
the left arm, and observes the
heart from an angle of 120° - LL to
LA

• According to Kirchhoff’s law, the sum of all currents in a
closed circuit must be zero.
• Applying this to Einthoven's Triangle, emerges
Einthoven’s Law
𝐿𝑒𝑎𝑑 𝐼 + 𝐿𝑒𝑎𝑑 𝐼𝐼𝐼 = 𝐿𝑒𝑎𝑑 𝐼𝐼
• Means we need to know the information in two leads to
calculate the exact appearance of the third lead
Ex: amplitude of R wave in lead II is the sum of the
amplitudes of R wave in lead I and Lead III

ECG MACHINE
(ECG recording set-up / ECG Instrumentation)
• Electric field generated by the heart can be best characterized by vector
quantities
• But convenient to directly measure only scalar quantities, i.e. a voltage difference
of mV order between the given points of the body
• Diagnostically useful frequency range is usually accepted as 0.05 to 150 Hz
• Amplifier and writing part should faithfully reproduce signals in this range. A
good low frequency response is essential to ensure stability of the baseline
• Interference of nonbiological origin avoided by using modern differential
amplifiers, which are capable of providing excellent rejection capabilities
• A minimum of two paper speeds is used (25 and 50 mm per sec) for recording.

• The connecting wires for the patient electrode originate at the end of a patient cable.
• The wires from the electrodes connect to the lead selector switch
• A push button allows the insertion of a standardization voltage of 1mV to standardize
or calibrate the recorder.
• From the lead selector switch the ECG signal goes to a pre-amplifier
• Pre amplifier is a differential amplifier with high common mode rejection ratio, high
gain factor, high input impedance and low output impedance.
• It is AC coupled to avoid problems with small DC voltages that may originate from
polarization of the electrodes.
• The pre-amplifier, is followed by a DC amplifier called the pen amplifier or power
amplifier, which provides the power to drive the pen motor that records the actual ECG
trace.

• A position control centres the pen on the recording paper.
• Pen is an electrically heated stylus, the temperature of which can be adjusted with a stylus heat control
for optimal recording trace
• ECG recorders use heat sensitive paper
• The power switch of an ECG record has three positions.
– In the “ON” position the power to the amplifier is turned on but the paper drive is not running.
– In “RUN” position paper drive starts running.
– In “OFF” position power is switched off.
• A push button allows the operator to check whether the recorder is connected to the power line with
polarity.
• This is done to avoid any shock hazard for the patient.
• Isolated or floating input amplifiers are used for the safety aspect of electrical connections to the
patient.

Medical Instrumentation- Biosignals, ECG

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