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11. CVS ECG Introduction.pdf
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
1
Cardiovascular Physiology:
Electrocardiography 1
2. Introduction
ď Electrocardiography is the aspect of physiology that
deals with the recording and analysis of the electrical
activities of the heart
ď Electrocardiogram(ECG): graphic record of the electrical
activities of the heart obtained at the body surface.
ď Electrogram: graphic record of the electrical activities of
the heart obtained within the cardiac chambers or on the
pericardium.
ď 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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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
ď A collection of central stellate pacemaker ( P) cells which possess
intrinsic ability for spontaneous depolarisation and
repolarization(automaticity)
ď 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).
ď The human sinoatrial node measures approximately 1.5cm long and
0.5cm wide.
ď 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
ď Sinoatrial node is the cardiac pacemaker
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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
ď 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;
ď 1mm represents 0.04s (40ms)
Voltage Measurement: voltage assessment is done vertically.
ď 1mm represents 0.1mV
Standardization
ď Paper speed: 25mm/s
ď Calibration signal or amplitude indicator: 10mm/mV
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13. Intervals and Segments in ECG
ď PR interval: period from the onset of atrial depolarization
to the beginning of ventricular depolarization
ď QT interval: period from the onset of the ventricular
depolarization to the end of ventricular repolarization
ď RR interval: period from the peak of R wave to the other
ď PR segment: isoelectric line from the end of atrial
depolarization to the beginning of ventricular depolarization
ď ST segment: isoelectric line from the end of ventricular
depolarization to the beginning of ventricular repolarization
ď J-Junction: the junction between the end of ventricular
depolarization and the beginning of S-T segment
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14. ST Segment
ď ST segment : isoelectric line between the end of ventricular
depolarisation and the beginning of ventricular repolarization
ď It is the isoelectric line between QRS complex andT wave
ď ST segment can be elevated or depressed in some cardiac
disorders
ď Causes of ST segment elevation include; early repolarization,
acute myocardial infarction and acute pericarditis
ď Causes of ST segment depression include; myocardia
ischaemia(angina pectoris) and hypokalaemia
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15. U wave
ď An inconstant wave
ď 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
ď QT interval varies with the heart rate
ď When QT is measured , it should be adjusted for heart rate using
Bazzetâs Formula;
QTc = QTm /RRint ½ ,
QTc = QT corrected, QTm = measured QT & RRint.= RR interval
ď Normal QTc : 0.36 â 0.44s for male,
: 0.36 â 0.46s for female
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17. Estimation of QT Interval
ď Bazzet Formula
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18. Heart Rate(HR) Estimation
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ď 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
20. STANDARD 12-LEAD ECG
ď Standard 12-lead ECG is recorded by the aids of 10
electrodes arranged on specific locations on the body surface
ď One electrode is attached to each limb to produce 6 limb
leads
ď 6 electrodes are attached to the chest to produce 6 chest or
precordial leads
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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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27. Lead Classification Based on Planes of Location
ď Frontal Plane Leads
⢠These leads are located in the frontal plane
⢠They include all the limb leads; I,II,III, aVL ,aVF & aVR
ď 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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