The document discusses electrocardiography (ECG), including its objectives, principles, and how it is used to analyze cardiac electrical activity. An ECG records from electrodes on the body and displays the P, QRS, and T waves representing atrial and ventricular depolarization and repolarization. The timing and amplitudes of the waves provide information on heart rate, rhythm, and conduction. Analysis of 12-lead ECGs can identify normal sinus rhythm as well as arrhythmias and conduction abnormalities associated with cardiac conditions.

1. ELECTROCARDIOGRAPHY
(ECG)
Prof. Vajira Weerasinghe
Professor of Physiology

2. Objectives for theory and practical
• Explain the principles of electrocardiography (ECG)
• Describe the changes in cardiac rate & rhythm
• Describe re-entry phenomenon
• Identify sinus tachycardia and bradycardia on an ECG strip
• Calculate heart rate on an ECG strip with regular and irregular
rhythm
• Draw and identify the waveforms of a typical ECG
• Describe variations in the ECG in health & disease
• Identify basic arrhythmias and explain the physiological basis

3. ECG
• Recording of cardiac electrical activity from
electrodes placed on the limbs and the
precordial surface
• Principles
Body acts as a volume conductor
Cardiac vector
Einthoven’s triangle
Differential amplifier effect

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5. Spread of electrical activity
• Myocardium depolarizes from endocardium to epicardium,
but repolarises in the opposite direction (from epicardium to
endocardium)
• SA node -> atrial muscle -> AV node -> bundle of His -> Left
and Right Bundle Branches -> Purkinje fibres -> Ventricular
muscle Physiological%20basis%20of%20ECG.pptx#7. Slide 7
• AV septum non conducting – AV node is a shuttle between
atria and ventricles
• Speed of conduction :
SA Node
100mph
AV Node
60mph
Purkinje
30mph
Contractile
cells
Contractile
cells
Contractile
cells

6. P wave: depolarization
of the atria
QRS complex:
depolarization of
ventricles
S-T and T wave:
repolarization
of ventricles
P - R interval : interval from onset of atrial depolarization to ventricular
depolarization
Q-T interval : duration of ventricular depolarization and repolarization

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14. Naming of waves

15. 1 sec
Each large square is 0.2 s
Small square is 0.04 s
Calculate heart rate ?
0.2 s
0.04 s

16. Calculations
Paper speed 25 mm/sec
small square = 0.04 sec
large square = 0.2 sec
Sensitivity 10 mm = 1 mV
Calculate heart rate
Tachycardia (increase heart rate)
Bradycardia (decrease heart rate)

17. Rate calculation
• When regular
= 300
large squares in R-R interval
= 1500
small squares in R-R interval

18. Rate calculation
• When irregular
= number of cardiac cycles x 10
for 6 seconds

20. Naming of waves
PR interval: 0.12 – 0.2 s (3-5 small squares) important
regarding AV conduction

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22. Cardiac vectorCardiac vector
 Cardiac impulse is a vectorCardiac impulse is a vector
 Vector has magnitude and a directionVector has magnitude and a direction
 eg. Weight is a vector whereas mass is a scalar quantityeg. Weight is a vector whereas mass is a scalar quantity

23. 3D object3D object2D object2D object

24.  In typical ECG, 12 tracings are taken fromIn typical ECG, 12 tracings are taken from
different sites to represent the cardiac vectordifferent sites to represent the cardiac vector
12 lead ECG

25. ECG leads
• Conventionally use 12 leads
• It is actually 12 electrical views of the heart
6 Limb leads 6 chest leads
Bipolar Unipolar
(Augmented)
Lead I
Lead II
Lead III
Lead aVL
Lead aVR
Lead aVF
Lead V1
Lead V2
Lead V3
Lead V4
Lead V5
Lead V6

26. Einthoven Triangle
• 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 electrocardiography
• Lead I, Lead II and Lead III
• Ground electrode (earth
electrode) is connected to the
right leg

27. 1. Upward deflection – if the wave of depolarization is travelling towards the positive pole of
that lead
2. Downward deflection – if the wave of depolarization is travelling towards the negative pole
of that lead
3. Biphasic deflection – if the path of the depolarization wave is perpendicular to that lead. A
small biphasic wave results

28. Augmented Limb Leads
• These record the potential
difference between an
exploring electrode and an
indifferent electrode
• Augmented limb leads,
designated by the letter a (aVR,
aVL, aVF), are generally used
• They are recordings between
the one, augmented limb and
the other two limbs which
increases the size of the
potentials by 50%

29. Unipolar limb leads

30. Augmented limb leads
aVR “looks at” the cavities of the
ventricles
– Atrial depolarization, ventricular
depolarization, and ventricular
repolarization move away from the
exploring electrode, and the P wave,
QRS complex, and T wave are
therefore all negative (downward)
deflections
aVL and aVF look at the
ventricles, and the deflections
are therefore predominantly
positive or biphasic

31. Chest leads
• Precordial leads records the potential on a point in
the chest against a zero
• Views the heart in the horizontal plane
V1 V2
V3
V4
V5
V6
anterior

32. • V1: 4th
intercostal space,
right sternal edge
• V2: 4th
intercostal space,
left sternal edge
• V3: Midway between V2
and V4
• V4: 5th
intercostal space
midaxillary line
• V5: same horizontal plane,
anterior axillary line
• V6: same horizontal plane,
mid axillary line

33. Chest leads

34. • There is no Q wave in V1 and V2, and
the initial portion of the QRS complex is
a small upward deflection because
ventricular depolarization first moves
across the midportion of the septum
from left to right toward the exploring
electrode
• The wave of excitation then moves
down the septum and into the left
ventricle away from the electrode,
producing a large S wave. Finally, it
moves back along the ventricular wall
toward the electrode, producing the
return to the isoelectric line
• Conversely, in the left ventricular leads
(V4 – V6) there may be an initial small Q
wave (left to right septal
depolarization), and there is a large R
wave (septal and left ventricular
depolarization) followed in V4 and V5 by
a moderate S wave (late depolarization
of the ventricular walls moving back
toward the AV junction)

35. Electrical axis of the Heart
• Is the mean direction of the action potentials
travelling through the ventricles during
ventricular depolarization
• Important to determine in case of diseases
• It is the mean direction of the QRS axis that is
usually determined

36. Electrical axis
• Normal axis is between
-30o
& +90o
• Determined by the QRS
axis
• It is the general direction
in the frontal plane
towards which the QRS is
predominantly pointing
I
III
IIaVF
aVR
aVL-30o
0o
+60o
+90o
+120o
-150o
Left axis
deviation
Right axis
deviation

37. • Important to remember in QRS axis calculation
1. The mean QRS axis points midway between
any two leads that shows tall R waves of equal
height
2. The mean QRS is oriented at right angles to
any lead showing a biphasic complex and towards
the lead which has a tall R wave
Generally use Lead I and aVF
Electrical axis

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41. + 30o

42. + 90o

43. 0o

44. + 120o

45. InterpretationInterpretation
 Calculate heart rateCalculate heart rate
 Rate: slow or rapid ?Rate: slow or rapid ?
 Rhythm: regular or irregular or irregularly irregular?Rhythm: regular or irregular or irregularly irregular?
 P Wave:P Wave: present or absentpresent or absent
 if present, normal or abnormal in configurationif present, normal or abnormal in configuration
 PR interval: prolonged or shortened ?PR interval: prolonged or shortened ?
 QRST complex: normal or abnormal in configurationQRST complex: normal or abnormal in configuration
 ST segment depression or elevation?ST segment depression or elevation?
 T inversion present?T inversion present?
 Q waves prominent?Q waves prominent?
 Cardiac axis determinationCardiac axis determination
 Any other features (missed beats, delta waves, tall tentedAny other features (missed beats, delta waves, tall tented
waves, bifid waves etc)waves, bifid waves etc)
 www.slideshare.net/vajira54www.slideshare.net/vajira54 or moodleor moodle