The electrocardiogram(ECG) provides a graphic depiction of the electric forces generated by the heart . The ECG graph appear as a series of deflections and waves produced by each cardiac cycle. During activation of the myocardium, electrical forces or action potentials are propagated in various directions. These electrical forces can be picked up from the surface of the body by means of electrodes and recorded in the form of an electrocardiogram.

1. BASICS OF ECG
SANDHIYA. R M.Sc
CLINICAL INSTRUCTOR
FAHS, CARE

4. THE ELECTROCARDIOGRAM
The electrocardiogram(ECG) provides a graphic depiction of the
electric forces generated by the heart . The ECG graph appear as a
series of deflections and waves produced by each cardiac cycle.

5. THE ELECTROCARDIOGRAPHIC PAPER
The electrocardiographic recording paper is divided into small and large squares.
The small squares are 1 mm & the large squares are 5 mm
The squares form a grid which facilitates the measurement of (i) time parameters
(horizontal measurement) and (ii) deflection amplitudes (vertical measurement)
In the clinical context , the electrocardiogram is nearly always conventionally
recorded at a paper speed of 25 mm/s .
At this paper speed, five large squares represent 1 s , one large square represents
0.20 s or 1/5 of a s or 200 ms and one small square represents 0.04 s or 1/25 of a
s or 40 ms .

6. Normally , the ECG machine is standardized in such a way that a 1 mV signal
from the machine produces a 10 mm vertical deflection . In other words ,
each small square on the vertical axis represents 0.1 mV and each large
square represents 0.5 mV .
Most graph papers used for the recording of electrocardiograms have every
15 th large square ( a period of 3 s) marked by a vertical line on the upper
border . This facilitates the quick assessment of the heart rate.

9. THE ELECTROCARDIOGRAPH
The electrocardiograph is a sophisticated galvanometer , a sensitive electromagnet ,
which can detect and record changes in electromagnetic potential .
It has positive and negative poles.
The wire extensions from these poles have electrodes at each end ; a positive
electrode at the end of the extension from the positive pole and the negative
electrode at the end of the extension from the negative pole . The paired
electrodes together constitute an electrocardiographic lead.
when the paired electrodes are oriented in any particular direction , the theoretical
straight line joining the electrodes is known as the axis of that lead or the lead
axis.
A lead so placed will detect and transmit any changes in electric potential which
occur between in electrodes.

11. THE ELECTRICAL FIELD OF THE HEART
The heart is situated at the centre of the electrical field which it generates.
The intensity of this electrical field diminishes algebraically with the distance from its centre.
Thus the electrical intensity recorded by an electrode diminishes rapidly when the electrode is
moved a short distance from the heart, and less as the electrode is moved still further away
from the heart .
With distances greater than 15 cm from the heart, the decrement in the intensity of the
electrical field is hardly noticeable.
Consequently ,all electrodes placed at a distance greater than 15 cm from the heart may, in an
electrical sense , be considered to be equidistant from the heart.
For example, an electrode placed at 25 cm from the heart records about the same potential as
one placed 35 cm from the heart.

12. ELECTROCARDIOGRAPHIC LEADS
 During activation of the myocardium, electrical forces or action potentials
are propagated in various directions. These electrical forces can be picked up
from the surface of the body by means of electrodes and recorded in the
form of an electrocardiogram.
A pair of electrodes, that consists of a positive and a negative electrode
constitutes an electrocardiographic lead. Each lead is oriented to record
electrical forces as viewed from one aspect of the heart.
The position of these electrodes can be changed so that different leads are
obtained. The angle of electrical activity recorded changes with each lead.
Several angles of recording provide a detailed perspective the heart.

13. There are twelve conventional leads, which may be physiologically divided into
two groups depending upon their orientation to the heart
1. The frontal plane leads (limb / extremity leads) : These are oriented in
the frontal or coronal plane of the body and consist of standard leads I, II and
III and leads aVR, aVL & aVF
2. The horizontal plane leads(chest /precordial leads) : These are oriented
in the transverse or horizontal plane of the body and are formed by the
precordial leads – V1 to V6

16. THE LIMB LEADS
The limb leads are derived from electrodes placed on the limbs. An electrode
is placed on each of the three limbs namely right arm, left arm and left leg.
The right leg electrode acts as the grounding electrode
1. Standard limb leads—three in number
2. Augmented limb leads—three in number
STANDARD LIMB LEADS :
The standard limb leads obtain a graph of the electrical forces as recorded
between two limbs at a time. Therefore, the standard limb leads are also called
bipolar leads. In these leads, one limb carries a positive electrode and the other
limb carries a negative electrode. There are three standard limb leads
Lead I
Lead II
Lead III

17. The leads derived from these three electrodes(limb) are conventionally as
follows
1. Standard lead I – This lead is derived from the placement of the negative
electrode on the right arm and the positive electrode on the left arm.
2. Standard lead II -This lead is derived from the placement of the negative
electrode on the right arm and the positive electrode on the left leg.
3. Standard lead III -This lead is derived from the placement of the negative
electrode on the left arm and the positive electrode on the left leg.
LEAD POSITIVE
ELECTRODE
NEGATIVE
ELECTRODE
I LA RA
II LL RA
III LL LA

20. AUGMENTED LIMB LEADS :
The augmented limb leads obtain a graph of the electrical forces as recorded
from one limb at a time. Therefore, the augmented limb leads are also called
unipolar leads. In these leads, one limb carries a positive electrode, while a
central terminal represents the negative pole which is actually at zero
potential. There are three augmented limb leads
Lead aVR (Right arm)
Lead aVL (Left arm)
Lead aVF (Foot left).
LEAD POSITIVE ELECTRODE
aVR RA
aVL LA
aVF LL

22. THE CHEST LEADS :
The chest leads are obtained from electrodes placed on the precordium in
designated areas. An electrode can be placed on six different positions on the
left side of the chest, each position representing one lead .Accordingly, there
are six chest leads namely:
Lead V1 : Over the fourth intercostal space, just to the right of sternal
border.
Lead V2 : Over the fourth intercostal space, just to the left of sternal
border.
Lead V3 : Over a point midway between V2 and V4
Lead V4 : Over the fifth intercostal space in the midclavicular line.
Lead V5 : Over the anterior axillary line, at the same level as lead V4.
Lead V6 : Over the midaxillary line, at the same level as leads V4 and V5

24. Sometimes, the chest leads are obtained from electrodes placed on the right
side of the chest. The right-sided chest leads are V1R, V2R, V3R, V4R, V5R and
V6R. These leads are mirror-images of the standard left-sided chest leads.
V1R : 4th intercostal space to left of sternum.
V2R : 4th intercostal space to right of sternum.
V3R : Point mid-way between V2R and V4R.
V4R : 5th intercostal space in midclavicular line, and so on.
The right-sided chest leads are useful in cases of:
True mirror-image dextrocardia.
Acute inferior wall myocardial infarction
(to diagnose right ventricular infarction).

26. DEFLECTIONS
By convention, a deflection above the baseline or isoelectric (neutral) line is a
positive deflection while one below the isoelectric line is a negative deflection
The direction of a deflection depends upon two factors namely, the direction
of spread of the electrical force and the location of the recording electrode.
In other words, an electrical impulse moving towards an electrode creates a
positive deflection while an impulse moving away from an electrode creates a
negative deflection

28. The ECG graph consists of a series of deflections or waves. Each
electrocardiographic deflection has been arbitrarily assigned a letter of the
alphabet. Accordingly, a sequence of wave that represents a single cardiac
cycle is sequentially termed as P Q R S T and U
By convention, P, T and U waves are always denoted by capital letters while
the Q, R and S waves can be represented by either a capital letter or a small
letter depending upon their relative or absolute magnitude.

29. Significance of ECG Deflections
 P wave : Produced by atrial depolarization.
 QRS complex : Produced by ventricular depolarization.
It consists of: Q wave : First negative deflection before R wave.
R wave : First positive deflection after Q wave.
S wave : First negative deflection after R wave.
 T wave : Produced by ventricular repolarization.
 U wave : Produced by Purkinje repolarization
You would be wondering where is atrial repolarization. Well, it is represented by
the Ta wave which occurs just after the P wave. The Ta wave is generally not
seen on the ECG as it coincides with (lies buried in) the larger QRS complex.

34. THE DOMINANCE OF THE LEFT VENTRICLE
• The ventricles consist essentially of three muscle masses: the IVS & the free
walls of a right and left ventricles
• The LV is a dominant anatomical structure and is also the main haemodynamic
pump of the heart. The IVS consequently forms a continuum with the free wall
of the LV
• Electrocardiologically, and electrophysiologically too,the left ventricle, including
the IVS , is the dominant ventricle. The free wall of the RV plays a relatively
minor role.
• Furthermore, while the free wall of the RV constitutes the anatomical anterior
wall of the heart, the electrocardiological anterior wall of the heart is, in
effect, the interventricular septum.
• For example, an anterior wall infarction refers to infarction of the IVS and not
the free wall of the RV
• It should also be borne in mind that the electromagnetic forces generated by
the free wall of the RV are relatively minor compared with those generated by
the free wall of the LV

36. NORMAL P WAVE
• The P wave is a small rounded wave produced by atrial depolarization. In
fact, it reflects the sum of right and left atrial activation,the right
preceding the left since the pacemaker is located in the right atrium.
• The P wave is normally upright in most of the ECG leads with two exceptions.
In lead aVR, it is inverted along with inversion of the QRS complex and the T
wave , since the direction of atrial activation is away from this lead .
• In lead V1, it is generally biphasic that is, upright but with a small terminal
negative deflection, representing left atrial activation in a reverse direction
• Normally , the P wave has a single peak without a gap or notch b/w right and
left atrial components. A normal P wave meets the following criteria :
- Less than 2.5mm (0.25mV) in height
- Less than 2.5mm (0.10 sec) in width

38. THE POTENTIAL FORMS OF THE QRS DEFLECTIONS AND
THEIR NOMENCLATURE
• The QRS complex reflects ventricular activation or depolarization. An initial
downward deflection after the P wave is termed as Q wave.
• An initial upwards deflection after the P wave termed as R wave
• The S wave usually represents the terminal part of ventricular activation
• The relative sizes of the QRS deflections are usually reflected by uppercase
and lower case lettering: capital and small letters.
• Thus, a small initial r wave followed by a relatively large S wave is termed an rS
compex
• A complex with an R and S wave of approximately equal amplitudes is termed an
RS complex
• A large R wave followed by a relatively small s wave is termed an Rs complex
• A single wave complex that is completely positive is termed an R wave complex

39. • A small initial downwards deflection followed by a relatively tall upwards
deflection, which, in turn, is followed by a relatively large terminal downwards
deflection, is termed a qRS complex
• A complex with a relatively deep and wide initial negative deflection followed
by a small terminal positivity is labelled as Qr complex
• A complex with complete negativity is termed a QS complex
• A second positivity of the QRS complex is termed an r’-r prime - deflection
• Thus, an rS complex followed by a small terminal positivity is termed an rSr’
complex
• When this terminal sond positivity is relatively tall, the complex is termed
rSR’.
• When the deflection is completely positive and notched , it is termed an RR’
complex

41. THE GENESIS OF THE QRS COMPLEX
• Activation of the ventricles begins in the left subendocardial region of the
lower third of the IVS, spreading transversely from L-> R
• It is opposed by a smaller activation force, which occurs almost synchronously
but fractionally later, and which arises in the right subendocardial region of
the IVS, spreading transversely through the septum from R-> L
• The large L->R force dominates, counteracting the smaller R->L force and
resulting in an effective vector that is directed transversely from L->R
through the lower third of the IVS. This is sometimes referred to as the
septal force or septal vector

42. • Activation of the IVS is then followed by the activation of free walls of both
ventricles. This occurs transversely from the subendocardial to the
subepicardial regions of the free walls of both ventricles.
• This may be represented, in simplified form, by a relatively large force from
R->L through the larger free wall of the LV that occurs synchronously with a
smaller opposing force that is directed from L-> R through the smaller free
wall of the RV.
• The larger R->L force of the free LV wall dominates & counteracts the smaller
L->R force of the free RV wall. This results in an effective or net resultant
vector that is directed from R->L through the free wall of the LV.
• Thus, in very oversimplified terms, activation of the ventricles may be
depicted as a small initial vector from L->R through the IVS, followed by a
larger vector from R->L through the free wall of the LV.

44. THE EFFECT ON A LEFT- ORIENTATED LEAD
A lead orientated to the LV,such as lead V6,lead aVL, or standard lead I, first
senses the relatively small resultant septal vector , which is directed away
from the positive pole of such a lead resulting in a small initial download
deflection - a small q wave
This is followed by the larger resultant vector of the free left wall, which is
directed towards the positive pole of such a lead resulting in a large upward
deflection - a tall R wave
A left -orientated lead thus normally reflects a qR complex (eg.standard laed I
& leads V4,V5 and V6)

45. THE EFFECT ON A RIGHT- ORIENTATED LEAD
A lead orientated to the RV,such as lead V1 or lead V2 ,will first sensethe small
resultant septal vector , which is directed towards it, and will consequently
reflect a small initial upwards deflection - a small r wave
This is followed by the larger resultant vector of the free left wall, which is
directed away from the right orientated lead resulting in a large downwards
deflection - a S wave
A right -orientated lead thus normally reflects a rS complex (eg. leads V1 and
V2)

47. THE TRANSITION ZONE
The transition zone represents the electrocardiographic zone or lead that
reflects the transition from the rS pattern (recorded by right -orientated
laed) to the qR pattern(recorded by left -orientated laed)
It commonly occurs in one or occasionally two, of the midprecordial leads-
generally lead V3 and/or lead V4
The transition pattern is usually an RS complex but, at times, the pattern
may be relatively bizarre,

48. NORMAL T WAVE
• The T wave is a large rounded wave produced by the rapid phase of ventricular
repolarization. The T wave is normally upright in most leads with certain
exceptions.
• It is invariably inverted in lead aVR along with inversion of the P wave and QRS
complex. It is often inverted in lead V1, occasionally in lead V2 V3 and
sometimes in lead LIII
• The normal T wave is taller in lead V6 than in lead V1. The amplitude of the
normal T wave does not generally exceed 5mm in the limb leads and 10mm in the
precordial leads

59. THE EINTHOVEN TRIANGLE
• We have seen that the standard limb leads are recorded from two limbs at a
time, one carrying the positive electrode and the other,the negative electrode.
• The three standard limb leads (I,II,III) can be seen to form an equilateral
triangle with the heart at the center. This triangle is called the Einthoven
triangle
• To facilitate the graphic representation of electrical forces, the three limbs
of the Einthoven triangle can be redrawn in such a way that the three leads
they represent bisect each other and pass through a common central point.
• This produces a triaxial reference system with each axis seperated by 600
from the other,the lead polarity (+ or -) and direction remaining the same

61. • We have also seen that the augmented limb leads are recorded from one limb at
a time, the limb carrying the positive electrode and the negative pole being
represented by the central point.
• The three augmented limb leads (aVR,aVL,aVF) can be seen to form another
triaxial reference system with each axis being seperated by 600 from one other
• When this triaxial system of unipolar leads is superimposed on the triaxial
system of limb leads, we can derive a hexaxial reference system with each axis
being seperated by 300 from the other.
• Note carefully that each of the six leads retain its polarity(positive and negative
poles) and orientation(lead direction).
• The hexaxial reference system concept is important in determining the major
direction of the heart’s electrical forces p

63. HEART RATE CALCULATION

65. ELECTRICAL AXIS
• The 12 lead ECG can measure the axis of the electrical
flow of energy during the cardiac cycle.
• Cardiac cell depolarization & repolarization produces a
many small electrical currents
• Sum of these currents are called instantaneous vectors
• Average of the instantaneous vectors called mean
vector

66. MEAN ELECTRICAL AXIS
• Direction of the mean vector called the
mean electrical axis
• Axis is defined in the frontal plane only

73. DETERMINATION OF QRS AXIS
METHOD-1
• Find the lead with smallest or equiphasic deflection
• Determine the lead at right angles to the first lead
• See the net deflection in the second lead
• The axis is directed towards the positive or negative pole of the second
lead

76. METHOD-2
For rapid and easy estimation of QRS axis,just scan the direction of the
dominant deflection in leads L1 and aVF whether positive or negative. This
gives us the quadrant in which the QRS axis is located

79. ABNORMALITIES OF QRS AXIS
Normal QRS axis
-300 to +900
Right axis deviation
+900 to +1800
CAUSES :
- Thin tall built
- Chronic lung disease
- Pulmonary embolism
- Ostium secundum ASD
- RVH
- Lateral wall infarction

80. Left axis deviation
-300 to -900
CAUSES:
- Obese stocky built
- WPW syndrome
- Ostium primum ASD
- LVH
- IWMI
North- West QRS axis
-900 to -1800
CAUSES:
- Congenital heart disease
- Left vevtricular aneurysm
SYNONYMS: Indeterminate QRS axis, Extreme right axis deviation, NO MAN’S LAND