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Introduction to
ECG
EEE 491 Biomedical Engineering
Compiled by Dr. Khawza I Ahmed
What is ECG?
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The electrocardiogram (ECG) is a time-varying signal reflecting
the ionic current flow which causes the cardiac fibers to contract
and subsequently relax. The surface ECG is obtained by
recording the potential difference between two electrodes placed
on the surface of the skin. A single normal cycle of the ECG
represents the successive atrial depolarisation/repolarisation and
ventricular depolarisation/repolarisation which occurs with every
heart beat.
Simply put, the ECG (EKG) is a device that measures and
records the electrical activity of the heart from electrodes placed
on the skin in specific locations
What the ECG is used for?
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Screening test for coronary artery disease, cardiomyopathies, left
ventricular hypertrophy
Preoperatively to rule out coronary artery disease
Can provide information in the precence of metabolic alterations
such has hyper/hypo calcemia/kalemia etc.
With known heart disease, monitor progression of the disease
Discovery of heart disease; infarction, coronal insufficiency as
well as myocardial, valvular and cognitial heart disease
Evaluation of ryhthm disorders
All in all, it is the basic cardiologic test and is widely applied in
patients with suspected or known heart disease
Measuring ECG
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ECG commonly measured via 12 specifically
placed leads
The 12-Leads
The 12-leads include:
–3 Limb leads
(I, II, III)
–3 Augmented leads
(aVR, aVL, aVF)
–6 Precordial leads
(V1- V6)
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Typical ECG
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A typical ECG period consists of P,Q,R,S,T and U
waves
ECG Intervals / Segments
PR: 0.12-0.20 sec
QRS: <0.10 sec
QT: 0.30-0.40 sec (60 bpm)
ECG Waves
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P wave: the sequential
activation (depolarization)
of the right and left atria
QRS comples: right and
left ventricular
depolarization
T wave: ventricular
repolarization
U wave: origin not clear,
probably
”afterdepolarizations” in
the ventrices
ECG Example
ECG Signal Generation
Action Potential
Electrocardiogram (ECG/EKG)
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Is a recording of electrical activity of heart conducted thru ions in body to surface
13-60
Cardiac Cellular Electrical
Activity
Typical Equilibrium Potentials:
Na(+) ~+40mV
K(+) ~-90mV
C(++) ~+150mV
ECG recording
ECG recording
ECG recording
ECG recording
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ECG
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3 distinct waves are
produced during cardiac
cycle
P wave caused by atrial
depolarization
QRS complex caused by
ventricular depolarization
T wave results from
ventricular repolarization
Fig 13.24
13-63
Elements of the ECG:
• P wave: Depolarization of both atria;
• Relationship between P and QRS helps distinguish various cardiac
• Shape and duration of P may indicate atrial enlargement
arrhythmias
• PR interval: from onset of P wave to onset of QRS
• Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)
• Represents atria to ventricular conduction time (through His bundle)
• Prolonged PR interval may indicate a 1st degree heart block
• QRS complex: Ventricular depolarization
• Larger than P wave because of greater muscle mass of ventricles
• Normal duration = 0.08-0.12 seconds
• Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular
hypertrophy, MI, electrolyte derangement, etc.
• Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may
represent MI
ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 msec
T wave:
• Represents repolarization or recovery of ventricles
• Interval from beginning of QRS to apex of T is referred to as the absolute refractory
period
QT Interval
• Measured from beginning of QRS to the end of the T wave
• Normal QT is usually about 0.40 sec
• QT interval varies based on heart rate
Fig. 13.24b
Fig. 13.24c
Fig. 13.24d
Elements of the ECG:
• P wave
• Depolarization of both atria;
• Relationship between P and QRS helps distinguish various
• Shape and duration of P may indicate atrial enlargement
cardiac arrhythmias
•QRS complex:
• Represents ventricular depolarization
• Larger than P wave because of greater muscle mass of ventricles
• Normal duration = 0.08-0.12 seconds
• Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular
hypertrophy, MI, electrolyte derangement, etc.
• Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may
represent MI
• PR interval:
• From onset of P wave to onset of QRS
• Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)
• Represents atria to ventricular conduction time (through His bundle)
• Prolonged PR interval may indicate a 1st degree heart block
Fig. 13.24g
T wave:
• Represents repolarization or recovery of ventricles
• Interval from beginning of QRS to apex of T is referred to as the absolute refractory
period
ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 msec
QT Interval
• Measured from beginning of QRS to the end of the T wave
• Normal QT is usually about 0.40 sec
• QT interval varies based on heart rate
ECG recording
ECG recording
ECG recording
ECG recording
ECG recording
ECG recording
ECG recording