EC8073 Medical Electronics - Hints for Slow Learner Nadar Saraswathi College of Engineering and Technology, Theni, Tamilnadu, India.

1. N.Mathavan || AP – ECE – NSCET
ELECTROCARDIOGRAPH (ECG)
This is the technique by which the electrical activities of the heart are studied.
Electrocardiograph:
This is the instrument by which the electrical activities of the heart are recorded.
Electrocardiogram:
This is the record or graphical registration of electrical activities of the heart.
1. ECG Waveforms
 P Wave
 Q Wave
 R Wave
 S Wave
 T Wave
2. Einthoven Triangle
3. Bipolar limb leads or Standard leads or Einthoven lead system
 Lead I
 Lead II
 Lead III
4. Augmented unipolar limb lead or Wilson Lead System
 Augmented Voltage Right Arm (aVR)
 Augmented Voltage Left Arm (aVL)
 Augmented Voltage Foot (aVF)
5. Unipolar chest leads
6. ECG Recording Method
 Defibrillator Protection Circuit
 Lead Selection Logic
 Calibration Circuit
 Pre Amplifier
 Power Amplifier
 Auxiliary Amplifier
 Feedback Network
 Display Unit

2. N.Mathavan || AP – ECE – NSCET
ECG Waveforms
 Einthoven was the 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 &T.
 The width of a wave on the horizontal axis represents a measure of time.
 The height and depth of a wave represent a measure of voltage.
An upward deflection of a wave is called positive deflection
A downward deflection is called negative deflection.
S.
No
Wave/ Segment Cause
Amplitude
(mV)
Duration
(sec)
1 P Wave Depolarization of atria 0.25 0.12 to 0.22
2
R Wave
(QRS Complex)
Re-polarization of atria and Depolarization of
ventricles 1.6 0.07 to 0.1
3 T Wave Re-polarization of ventricles 0.1 to 0.5 0.05 to0.15
4 U Wave
Slow re-polarization of
Intra ventricular(Purkinje fibers) system <0.1 0.2

3. N.Mathavan || AP – ECE – NSCET
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.
The R wave nominal voltage from different
lead is as given below.
Lead I - 0.53 mv (0.07 - 1.13) mv
Lead II - 0.71 mv (0.18 - 1.68) mv
Lead III - 0.38 mv (0.03 - 1.31) mv
By KVL VII = VI + VIII
Bipolar limb leads or Standard Leads or Einthoven lead system
In this lead system, the potentials are tapped from four locations of our body.
They are: i) Right arm (RA) ii) Left arm (LA) iii) Right Leg (RL) iv) Left Leg (LL)
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 (RA) and a positive
electrode placed on the
left arm (LA). It gives voltage VI
the voltage drop from the
left arm (LA) to the
right arm (RA).
 Lead II:
It is a lead obtained between a negative electrode placed on the right arm (RA) and a
positive electrode placed on the left leg (LL). It gives voltage VII, the voltage drop from
the left leg (LL) to the right arm (RA).

4. N.Mathavan || AP – ECE – NSCET
 Lead III:
It is a lead obtained between a negative electrode placed on the left arm (LA) and
a positive electrode placed on the left Leg (LL). It gives voltage VIII, the voltage drop
from the left leg (LL) to the left arm (LA).
Augmented Unipolar Limb Leads
Wilson introduced augmented unipolar limb lead system. The electrocardiogram is
recorded between a single electrode and the central terminal, which has a potential
corresponding to the center of the body.
Thus, two equal and large resistors are connected to a pair of limb electrodes and the
center of this resistive network.
The 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 lead connections are,
 Augmented voltage Right arm (aVR)
It is a 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
 Augmented voltage Left arm (aVL)
It is a lead obtained between the average signal obtained from three negative electrodes
(right arm, left foot and right foot) and the signal obtained from a positive electrode placed
on the left arm
 Augmented voltage Foot arm (aVF)
It is a lead obtained between the average signal obtained from three negative electrodes
(left arm, right arm and right foot) and the signal obtained from a positive electrode placed
on the left foot

5. N.Mathavan || AP – ECE – NSCET
Unipolar Chest Lead
In unipolar chest leads, the exploratory electrode is obtained from one of the chest
electrodes. 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.
Vl: is a lead obtained between the reference negative electrode and a positive electrode
placed on the chest in the V1 position
V2: is a lead obtained between the reference negative electrode and a positive electrode
placed on the chest in the V2 position
V3: is a lead obtained between the reference negative electrode and a positive electrode
placed on the chest in the V3 position
V4: is a lead obtained between the reference negative electrode and a positive electrode
placed on the chest in the V4 position
V5: is a lead obtained between the reference negative electrode and a positive electrode
placed on the chest in the V5 position.

6. N.Mathavan || AP – ECE – NSCET
V6: is a lead obtained between the reference negative electrode and a positive electrode
placed on the chest in the V6 position.
ECG Recording Method
 Defibrillator Protection Circuit
The protection circuit
Employs
• Buffer Amplifier
(To Increase I/P Impedance)
• Over Voltage Protection Circuits
(Protect Pre & Power Amplifier
from over voltage)
 Lead Selection Logic
It s used to select Lead type either
Bipolar or Augmented
 Calibration Circuit
While selecting lead some artifact is introduce at output, by using calibration circuit can
calibrate the O/P unit.
 Pre Amplifier
From the lead selector switch, the ECG signal goes to a preamplifier, a differential
amplifier with high common-mode rejection. It is ac-coupled to avoid problems with

7. N.Mathavan || AP – ECE – NSCET
small dc voltages that may originate from polarization of the electrodes.
 Power Amplifier
The preamplifier is followed by a dc amplifier called the power (Pen) amplifier, which
provides the power to drive the pen motor that records the actual ECG trace.
 Auxiliary Amplifier
Pre Amplifier are not sufficient for eliminate the common mode signal, because impedance
of all electrodes are not equal. So auxiliary amplifier is used.
 Feedback Network
Feedback N/W consist of R-C circuit. It provides necessary damping to the pen motor
 Display Unit
CRO or Paper chart recorder is used as O/P unit, generally, paper chart is used.
Electrocardiograms are recorded at a paper speed of 25 mm/sec