1) The ECG records electrical activity of the heart through electrodes placed on the skin. It represents the summation of action potentials from myocardial fibers. 2) Einthoven's triangle uses the right arm, left arm, and left leg as electrode placements approximating the heart's position in the center. These produce the standard limb leads I, II, and III in bipolar recordings. 3) Unipolar precordial leads V1-V6 are obtained by placing a exploring electrode on the chest and connecting it to limb electrodes as indifferent electrodes, producing signals between the chest and each limb.

1. DASAR-DASAR EKG
Rahmatina B. Herman
Bagian Fisiologi
Fakultas Kedokteran - Unand

2. Introduction
Body fluids are good conductors → fluctuations in
potential that represent the algebraic sum of action
potentials of myocardial fibers can be recorded
extracellularly
The record is the electrocardiogram (ECG)
The ECG may be recorded by using an active or exploring
electrode connected to an indifferent electrode at zero
potential (unipolar recording) or by using two active
electrodes (bipolar recording)
In a volume conductor, the sum of the potentials at the
points of an equilateral triangle with a current source in
the center is zero at all times

3. Introduction…..
A triangle with the heart at its center (Einthoven’s triangle)
can be approximated by placing electrodes on both arms
and on the left leg
These are the three standard limb leads used in ECG
If these electrodes are connected to a common terminal,
an indifferent electrode that stays near zero potential is
obtained
Depolarization moving toward an active electrode in a
volume conductor produces a positive deflection, whereas
depolarization moving in the opposite direction produces a
negative deflection

4. Flow of current in the chest around partially depolarized ventricles

5. Conventional arrangement of electrodes for recording the standard electro-
cardiographic leads. Einthoven’s triangle is superimposed on the chest

6. Recording depolarization wave (A and B) and repolarization wave (C
and D) from a cardiac muscle fiber

7. Above, monophasic action potential from a ventricular muscle fiber during normal
cardiac function, showing rapid depolarization and then repolarization occurring
slowly during the plateau stage but rapidly toward the end
Below, Electrocardiogram recorded simultaneously

8. Relationship of Contraction to ECG Waves
Before contraction can occur, depolarization must spread
to initiate chemical processes of contraction
P wave occurs at beginning of contraction of atria
QRS complex occurs at beginning of ventricles
contraction
Ventricles remain contracted until after ventricles
repolarization → until after the end of T wave
Atria repolarize ± 0.15 to 0.20 sec after termination of P
wave, approximately when QRS complex is being
recorded → atrial repolarization (atrial T wave) is usually
obscured by larger QRS complex → seldom is observed in
ECG

9. Relationship of Contraction to ECG Waves …..
Ordinarily ventricular muscle begins to repolarize in
some fibers ± 0.20 sec after the beginning of
depolarization (QRS complex), but in many other
fibers, it takes as long as 0.35 sec → process of
ventricular repolarization extends over a long period,
about 0.15 sec → T wave in normal ECG is a prolonged
wave, but the voltage of T wave is considerably less
than the voltage of the QRS complex, partly because of
its prolonged length

10. Voltage and Time Calibration ECG
All recordings of ECG are made with appropriate calibration
lines on recording paper
The calibration lines are already ruled on the paper
The horizontal lines are voltage calibration lines, arranged
so that 10 of the small line divisions upward or downward
in the standard ECG represent 1 mV, with positivity in the
upward direction and negativity in the downward
direction.
The vertical lines are time calibration lines; each inch in the
horizontal direction is 1 sec, and each inch is usually broken
into five segments by dark vertical lines; the intervals
between these dark lines represent 0.20 sec; the 0.20 sec
intervals are then broken into five smaller intervals by thin
lines, each of which represents 0.04 sec

11. Normal electrocardiogram

12. Normal Voltages in the ECG
When ECG are recorded from electrodes on the two arms
or on one arm and one leg:
- the voltage of QRS complex usually is 1.0 to 1.5 mV
from the top of R wave to the bottom of S wave
- the voltage of P wave is between 0.1 and 0.3 mV
- the voltage T wave is between 0.2 and 0.3 mV
P-Q or P-R Interval ± 0.16 sec:
from beginning of P wave and beginning of QRS complex ,
called P-Q interval (P-R interval if Q wave is absent)
Q-T Interval ± 0.35 sec:
from beginning of Q wave (or R wave, if Q wave is absent)
to the end of T wave

13. TABLE ECG intervals
a Measured from the beginning of the P wave to the beginning of the QRS complex.
b Shortens as heart rate increases from average of 0.18 s at a rate of 70 beats/min to
0.14 s at a rate of 130 beats/min.

14. Heartbeat Rate as Determined from ECG
The rate of heartbeat can be determined easily from
an ECG because the heart rate is the reciprocal of the
time interval between two successive heartbeats.
If the interval between two beats as determined from
the time calibration lines is 1 sec, the heart rate is 60
beats per minute
The normal interval between two successive QRS
complexes in the adult person is about 0.83 sec →
heart rate: 60/0.83 times per minute = 72 beats per
minute

15. ELECTROCARDIOGRAPHIC
LEADS

16. Bipolar Limb Leads
Bipolar leads were used before unipolar leads were
developed
The term “bipolar” means that ECG is recorded from two
electrodes located on different sides of the heart, in this
case, on the limbs
Thus, a “lead” is not a single wire connecting from the
body but a combination of two wires and their electrodes
to make a complete circuit between the body and the
electrocardiograph
The electrocardiograph in each instance is represented by
an electrical meter in the diagram, although the actual
electrocardiograph is a high-speed recording meter with a
moving paper

17. Bipolar Limb Leads…..
The standard limb leads —leads I, II, and III—each record
the differences in potential between two limbs
Because current flows only in the body fluids → the
electrodes were at the points of attachment of the limbs,
no matter where on the limbs the electrodes are placed
In lead I, the electrodes are on the right arm and left arm
with the left arm positive
In lead II, the electrodes are on the right arm and left leg,
with the leg positive
In lead III, the electrodes are on the left arm and left leg,
with the leg positive.

18. Einthoven’s Triangle
Einthoven’s triangle is drawn around the area of heart
This illustrates that the two arms and the left leg form
apices of a triangle surrounding the heart
The two apices at the upper part of the triangle
represent the points at which the two arms connect
electrically with the fluids around the heart, and the
lower apex is the point at which the left leg connects
with the fluids

19. Einthoven’s Law
Einthoven’s law states that if electrical potentials of any
two of the three bipolar limb electrocardiographic leads
are known → the third one can be determined
mathematically by simply summing the first two
For instance the right arm is -0.2 mV, the left arm is + 0.3
mV, and the left leg is +1.0 mV
- lead I records +0.5 mV (difference between -0.2 mV on right
arm and +0.3 mV on left arm
- lead II records +1.2 mV (differences between -0.2 mV on right
arm and +1 mV on left leg)
- lead III records +0.7 mV (differences between +0.3 mV on left
arm and +1 mV on left leg)

20. Unipolar (V) Leads
An additional nine unipolar leads, that is, leads that
record the potential difference between an exploring
electrode and an indifferent electrode
Commonly used in clinical electrocardiography
Often electrocardiograms are recorded with one
electrode placed on the anterior surface of the chest
directly over the heart
The electrode is connected to the positive terminal of
the electrocardiograph, and the negative electrode,
called the indifferent electrode, is connected to the
right arm, left arm, and left leg

21. Unipolar (V) Leads
There are six unipolar chest leads (precordial leads)
designated V1–V6
Three unipolar limb leads: VR (right arm), VL (left arm),
and VF (left foot)
Augmented limb leads, designated by the letter a (aVR,
aVL, aVF) are generally used
The augmented limb leads are recordings between one
limb and the other two limbs → increases the size of
the potentials by 50% without any change in
configuration from the nonaugmented record.

22. Unipolar electrocardiographic leads.

23. Connections of the body with the electrocardiograph for recording
chest leads. LA, left arm; RA, right arm

25. Monitoring
ECG is often recorded continuously in hospital coronary
care units, with alarms arranged to sound at the onset of
life-threatening arrhythmias
Using a small portable tape recorder (Holter monitor), it is
also possible to record the ECG in ambulatory individuals as
they go about their normal activities
The recording is later played back at high speed and
analyzed
Long-term continuous records can be obtained
Recordings obtained with monitors have proved valuable in
the diagnosis of arrhythmias and in planning the treatment
of patients recovering from myocardial infarctions