This document provides an overview of electrocardiography and the cardiac electrical system. It discusses the components of the cardiac conduction system including the sinoatrial node, atrioventricular node, bundle of His, bundle branches and purkinje fibers. It also describes the waves that make up the electrocardiogram including the P wave, QRS complex and T wave. Additionally, it discusses electrocardiograph use and placement of electrodes for recording the standard 12-lead electrocardiogram. Key intervals measured on the ECG like the PR interval, QT interval and segments are also outlined.

1. OGUNLADE O., MBChB., M.Sc., PhD., FWACP
Department of Physiological Sciences,
Faculty of Basic Medical Sciences,
College of Health Sciences
Obafemi Awolowo University, Ile-Ife
15 June 2021
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Cardiovascular Physiology:
Electrocardiography 1

2. Introduction
 Electrocardiography is the aspect of physiology that
deals with the recording and analysis of the electrical
activities of the heart
 Electrocardiogram(ECG): graphic record of the electrical
activities of the heart obtained at the body surface.
 Electrogram: graphic record of the electrical activities of
the heart obtained within the cardiac chambers or on the
pericardium.
 Electrocardiograph(ECG machine): instrument used
for recording the cardiac electrical activities.
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3. Cardiac Electrical System or Cardiac
Conduction System
Cardiac electrical system or cardiac conduction system is
structurally made up of the following components;
1. Sinoatrial node
2. Internodal and interatrial pathways
3. Atrioventricular node(A-V node)
4. His bundle(A-V bundle)
5. Bundle branches
6. Fascicles
7. Purkinje fibres
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4. Cardiac Electrical System
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5. Organization of Cardiac Electrical System
Sinoatrial Node
Internodal fibres &
interatrial pathways
Atrioventricular Node
Bundle of His
Bundle Branches
Fascicles
Punkinje fibres
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6. Sinoatrial Node-The Cardiac
Pacemaker
 A collection of central stellate pacemaker ( P) cells which possess
intrinsic ability for spontaneous depolarisation and
repolarization(automaticity)
 Discovered in 1906 but first described(published) in 1907 in the
countryside of Kent (UK) by Arthur Keith and his laboratory
assistance; Martin Flack (a young medical student).
 The human sinoatrial node measures approximately 1.5cm long and
0.5cm wide.
 Located in the upper wall of the right atrium, close to the interatrial
septum at the entrance of superior vena cava into the right atrium
 Sinoatrial node is the cardiac pacemaker
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7. PQRST Complex: Building Block of ECG
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8. ECG Denotations
Denotations Descriptions
P wave atrial depolarization
QRS complex ventricular depolarization
Q wave first negative deflection in ventricular
depolarization
R wave first positive deflection in ventricular
depolarization
S wave second negative deflection in ventricular
depolarization or the first negative deflection
after R wave
T wave ventricular repolarization June 15, 2021
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9. ECG Paper
 The graph paper consists of horizontal and vertical lines at
1mm apart that intersect to form small (1mm) and large
(5mm) squares
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10. ECG Graph:Time and Voltage
Assessment
Time Measurement : time is assessment is done horizontally;
 1mm represents 0.04s (40ms)
Voltage Measurement: voltage assessment is done vertically.
 1mm represents 0.1mV
Standardization
 Paper speed: 25mm/s
 Calibration signal or amplitude indicator: 10mm/mV
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11. Electrocardiogram
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12. ECG Denotations
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13. Intervals and Segments in ECG
 PR interval: period from the onset of atrial depolarization
to the beginning of ventricular depolarization
 QT interval: period from the onset of the ventricular
depolarization to the end of ventricular repolarization
 RR interval: period from the peak of R wave to the other
 PR segment: isoelectric line from the end of atrial
depolarization to the beginning of ventricular depolarization
 ST segment: isoelectric line from the end of ventricular
depolarization to the beginning of ventricular repolarization
 J-Junction: the junction between the end of ventricular
depolarization and the beginning of S-T segment
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14. ST Segment
 ST segment : isoelectric line between the end of ventricular
depolarisation and the beginning of ventricular repolarization
 It is the isoelectric line between QRS complex andT wave
 ST segment can be elevated or depressed in some cardiac
disorders
 Causes of ST segment elevation include; early repolarization,
acute myocardial infarction and acute pericarditis
 Causes of ST segment depression include; myocardia
ischaemia(angina pectoris) and hypokalaemia
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15. U wave
 An inconstant wave
 Exact mechanism of its genesis is still controversial however
suggested mechanisms;
➢ repolarization of papillary muscle and neighbouring
structures(old concept)
➢ repolarization of mid-myocardial cells (M-cells)
➢ an afterpotentials
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16. QTc Interval
QT interval : period from the onset of ventricular
depolarisation to the end of ventricular repolarization
 QT interval varies with the heart rate
 When QT is measured , it should be adjusted for heart rate using
Bazzet’s Formula;
QTc = QTm /RRint ½ ,
QTc = QT corrected, QTm = measured QT & RRint.= RR interval
 Normal QTc : 0.36 – 0.44s for male,
: 0.36 – 0.46s for female
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17. Estimation of QT Interval
 Bazzet Formula
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18. Heart Rate(HR) Estimation
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 Regular RR Interval
a. HR = 300/x or 1500/y
where x is the number of big squares within RR interval
y is number of small squares within RR interval
b. HR = 60/RR interval
The normal heart rate: 60-100bpm

19. ECG Baseline Characteristics
Rate 60-100beats/min
P wave Duration:<0.12s
Amplitude: <2.5mm
Configuration: mostly monophasic & upright
PR segment isoelectric
P-R interval 0.12-0.2s
Q wave Duration: <0.04s
Amplitude:<1/4R
QRS Complex Duration:0.06-0.1s
J-junction isoelectric
ST segment isoelectric
T wave Varies ;mostly monophasic & upright
QTc 0.36-0.44s-male
0.36-0.46s-female
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20. STANDARD 12-LEAD ECG
 Standard 12-lead ECG is recorded by the aids of 10
electrodes arranged on specific locations on the body surface
 One electrode is attached to each limb to produce 6 limb
leads
 6 electrodes are attached to the chest to produce 6 chest or
precordial leads
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21. ECG Machine and Its Accessories
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22. Limb Electrodes
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23. Chest Electrode Placement
V1-4th intercostal space, right sternal edge
V2-4th intercostal space, left sternal edge
V3-midway betweenV2 &V4
V4-5th intercostal space,left midclavicular line
V5-5th intercostal space, left anterior axillary line
V6-5th intercostals space,left mid-axillary line
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24. Placement of Chest Electrodes
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25. Classifications of Standard 12
Leads
Types of Lead Leads
Bipolar limb leads I, II, III
Augmented limb
leads
aVF, aVL, aVR
Chest or Precordial
leads
V1,V2,V3,V4,V5,V
6
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26. Standard 12-Lead ECG
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27. Lead Classification Based on Planes of Location
 Frontal Plane Leads
➢ These leads are located in the frontal plane
➢ They include all the limb leads; I,II,III, aVL ,aVF & aVR
 Horizontal/Transverse Plane Leads:
➢ These leads are located in the horizontal/transverse plane
➢ They include all the chest leads;V1-V6
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28. Frontal and Horizontal Planes
Frontal plane
Horizontal plane
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29. Lead Classification based on cardiac
anatomy
Classification Leads
Left lateral leads I, aVL,V5 &V6
Inferior leads II,III & aVF
Anteroseptal leads V3,V4
Anterior leads V1-V6
Right ventricular cavity
leads
aVR,V1 &V2
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30. 
THANKYOU
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