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Cardiovascular Module
Cardiovascular Physiology
Lect. Eight
Electrocardiogram
(ECG)
Prof. Dr. Najeeb Hassan Mohammed
Electrical potential of the heart:
Electrocardiogram (ECG):
 ECG is the recording of electrical potential of the heart that
extend to the body surface placing surface electrodes on the skin.
 It records the waves of depolarization and repolarization that are
generated by the cardiac muscle.
 The apparatus used is called the electrocardiograph which is a
sensitive galvanometer with an amplifier.
The electrocardiogram (ECG):
Objectives:

Draw an ECG classical waveform and label
each component (P, QRS, T).

Draw diagrams indicating the 6 standarad
limb leads axes (I,II,III,aVR,aVL,aVF).
The electrocardiogram (ECG):
Standard ECG (12 leads):
• 3 Bipolar standard limb
leads (I, II, III).
• 3 unipolar limb leads
(aVR, aVL, aVF).
• 6 unipolar chest leads.
3 Bipolar standard
limb leads (I, II, III):
These leads record the differences between the
potentials in 2 limbs, by applying electrodes
usually at the wrist and ankle.
• Lead I: This records the difference between the
potential in the left arm (LA) and that in the
right arm (RA).
• Lead 11: This records the difference between
the potential in the right arm (RA) and that in
the left leg (LL).
• Lead III: This records the difference between
the potential in the left leg (LL) and that in the
left arm (LA).
Einthoven's triangle: This is an equilateral
triangle, the sides of which represent the 3
bipolar standard limb leads while the heart lies
at its centre.
RA
–
+
I
–
LA
–
II
III
+
+
LL
3 Unipolar limb leads
(aVR, aVL, aVF):
• They are augmented unipolar
limb leads that have magnified
amplitudes by about 50 %.
a = augmented.
• They measure the absolute
(actual) potential at a certain
point as a potential differences
between an active, +ve or
exploring electrode and
common reference (-ve)
electrode (zero potential).
6 Unipolar chest leads:
Unipolar leads (chest leads) record the
absolute potential at 6 standard points
on the anterior chest wall designated
as V1 to V6, as follows:
•
•
•
•
•
•
V1: At the right margin of the
sternum in the 4th right
intercostal space.
V2: At the left margin of the sternum
in the 4th left intercostal space.
V3: Midway between V2 and V4.
V4: At the left midclavicular line in
the 5th intercostal space.
V5: At the left anterior axillary line in
the 5th intercostal space.
V6: At the left midaxillary line in the
5th intercostal space.
ECG waves:
Normal ECG consists of
the following waves:
 P wave caused by
atrial depolarization.
 QRS waves caused by
ventricular
depolarization.
 T wave caused by
ventricular
repolarization.
ECG

P wave:
Atrial
depolarization.

QRS complex:
Ventricular
depolarization.

T wave:
Ventricular
repolarization.
 Phase 1;left to
right across the
septum, produce Q
wave in V6 and an
R wave in V1.
 Phase 2;
depolarization of
left ventricle from
endocardium to
epicardium, results
in a tall R wave in
V6 and a deep S
wave in V1.
 Phase 3;
depolarization of
the basal parts of
ventricles
producing a
terminal S wave in
V6 and a terminal
R wave in V1.
Genesis of the QRS
complex.
• The ECG paper is calibrated
so that a change of 1 mV
upward or downward
produces a deflection of 10
mm amplitude (10 small
squares= 2 large squares).
• each mm between the thin
horizontal lines (voltage
calibration lines) equals 0.1
mV. In other words, the thick
horizontal lines calibrated at 5
mm intervals.
• The vertical lines are time
calibration lines in which
duration of each small square
(1mm) equals 0.04 second,
each large square (5 small
squares) represents 0.20
second, and 5 large squares is
1 sec
Calibration of the
ECG
ECG Calibration
Calculation of heart rate from ECG paper:

If the heart rhythm is regular, the heart rate
(HR) ran be counted by dividing the number
of large squares between two consecutive R
waves into 300 or small squares into 1500.

If the rhythm is irregular, one can multiply
the number of complexes (R) in 6 seconds
(30 large squares) by 10.
Duration and intervals:





P wave; 0.07-0.14 seconds.
PR interval; 0.12- 0.21 seconds.
beginning of the P wave to the
onset of the Q wave or onset of
the R wave Abnormal PR
interval is either long or short.
QRS complex; 0.06 – 0.10
seconds. Abnormality; wide
QRS.
T wave; 0.25 -0.35 seconds.
QT interval; 0.28 – 0.44 seconds.
Electrical axis and cardiac vector
Objectives:
State the relationship between the
direction of cardiac vector with the
direction (-ve, +ve) and amplitude of an
ECG waves.
Draw diagram indicating the axes of
limb leads.
Electrical axis of the ventricular QRS
 The axis of each
standard bipolar
and unipolar
limb leads can be
presented in the
following
diagram:
 In relation to the
bipolar limb
leads (I, II, III),
the cardiac
vector or axis can
be calculated.
Normal mean QRS vector
• In a normal heart, the average
direction of the vector of the
heart through the ventricles;
(mean QRS vector) is about +59
degrees.
Axis deviation
 The cardiac vector affects the
configuration of the ECG waves in
the various leads.
 The normal direction of the mean
QRS vector is downwards and to the
left and is generally between –30
and +110 degrees.
 Axis deviation occurs if the electric
axis of the heart is beyond the
normal range.
 QRS axis further right that of +110
constitutes Right axis deviation
(RAD).
 QRS axis left that of –30 constitutes
Left axis deviation (LAD).
Left axis deviation (LAD)
 the mean QRS axis is
toward the +ve pole in
lead I and toward the -ve
pole in lead III.
 there are high +ve waves
(R waves) in lead I and
deep -ve waves (S waves)
in lead III.
 LAD normally occurs in
horizontal hearts (e.g. in
short obese subjects and
pregnant women).
 pathologically, it is
common in left
ventricular hypertrophy
and left bundle branch
block.
Left axis deviation
Right axis deviation (RAD)
 the mean QRS axis is
toward the -ve pole in
lead I and toward the
+ve pole in lead III.
 there are deep -ve waves
(S waves) in lead I and
high +ve waves (R
waves) in lead III.
 RAD normally occurs in
vertical hearts (e.g. in
tall slender subjects).
 pathologically, it is
common in right
ventricular hypertrophy
and right bundle branch
block.
Right axis deviation