An electrocardiogram (ECG) records the electrical activity of the heart. Electrodes placed on the skin detect small electric currents from the heart and transmit them to an amplifier. The amplified currents are recorded as wavy lines on paper or displayed on a screen. A normal ECG shows three recognizable waves - P, QRS, and T - which represent atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively. The ECG provides information about the heart's rate, rhythm, and any damage or abnormalities. It is used to diagnose heart conditions like heart attacks, arrhythmias, and effects of diseases.

1. HT503: Module 7 | Medical
Instruments
ECG
Instructor: Dr. Rajiv K. Kar
Presented By: Shubham Sanjay Agrawal (224159011)
Neeraj Jaswal (224159005)
Jyoti and Bhupat Mehta School of Health Sciences and Technology
Indian Institute of Technology Guwahati
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2. What is ECG?
 An Electrocardiogram (ECG) is the
recording(gram) of the electrical activity
(electro) generated by the cells of the heart
(cardio) that reaches the body surface.
 The electrocardiograph machine is a
sophisticated galvanometer , a sensitive
electromagnet, which can detect and record
small changes in electromagnetic potential.
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William Einthoven
(1924 Nobel prize)

3. Working principle of electrocardiograph
 It works on the principle that a contracting cardiac muscle
generates a small electric current that can be detected and
measured through electrodes suitably placed on the body.
 For a resting electrocardiogram, a person is made to lie in the resting
position and electrodes are placed on arms, legs and at six places on
the chest over the area of the heart. The electrodes are attached to
the person’s skin with the help of a special jelly.
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4. Working principle of electrocardiograph
 The electrode picks up the current and transmit them to an
amplifier inside the electrocardiograph. Then electrocardiograph
amplifies the current and records them on a paper as a wavy line.
In an electrocardiograph, a sensitive lever traces the changes in
current on a moving sheet of paper.
A modern electrocardiograph may also be connected to an
oscilloscope, an instrument that display the current on a screen.
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5. Leads of ECG
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6. Block Diagram of ECG
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7. Normal ECG wave
 A normal ECG makes a specific pattern of
three recognizable waves in a cardiac
cycle. These wave are- P wave, QRS wave
and T-wave, P-R interval, S-T segment.
P-wave:
It is a small upward wave that appears first
It indicates atrial depolarization (systole),
during which excitation spreads from SA
node to all over atrium
About 0.1 second after P-wave begins, atria
contracts. Hence P-wave represents atrial
systole
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8. Normal ECG wave
QRS wave:
It is the second wave that begins as a
little downward wave but continues as
a large upright triangular wave and
ends as downward wave.
It represents the ventricular
depolarization (systole).
Just after QRS wave begins, ventricles
starts to contracts. Hence QRS wave
represents ventricular systole
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9. Normal ECG wave
T- wave:
It is third small wave in the form of a
dome-shaped upward deflection.
It indicates ventricular repolarization
(diastole) and also represents the
beginning of ventricular diastole.
** ATRIAL DIASTOLE MERGES WITH
QRS-WAVE
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10. Normal ECG wave
 P-R interval:
It represents the time required for an impulse
to travel through the atria, AV node and bundle
of his to reach ventricles.
S-T segment:
It is measured from the end of S to the
beginning of T- wave.
It represents the time when ventricular fibres
are fully depolarized.
About 0.1 second after P-wave begins, atria
contracts. Hence P-wave represents atrial
systole
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11. Application of ECG
It indicates the rate and rhythm or pattern of contraction of heart.
It gives a clue about the condition of heart muscle and is used to
diagnose heart disorders.
It helps the doctors to determine whether the heart is normal, enlarged
or if its certain regions are damaged.
It can also reveal irregularities in heart’s rhythm known as ‘arrhythmia’.
It is used by doctors to diagnose heart damage in conditions like high
blood pressure, rheumatic fever and birth defects.
An ECG also helps to determine the location and amount of injury
caused by heart attack and later helps to assess the extent of recovery.
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12. Significance of different waves in an ECG
deviating from normal ECG
Enlarged P-wave:
It indicates enlarged atrium (it occurs in a condition called mitral stenosis in which
due to narrowing of mitral valve, blood backs up into left atrium).
Enlarged Q-wave: downward wave
It indicates a myocardial infraction ( heart attack).
Enlarged R- wave:
It indicates enlarged ventricles.
Long P-Q interval:
It indicates more time taken by impulse to travel through atria and reach ventricles
It happens in coronary artery disease and rheumatic fever when a scar tissue may
form in heart.
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13. Significance of different waves in an ECG
deviating from normal ECG
Elevated S-T segment:
When S-T segment is above the base line, it may indicates acute myocardial
infraction.
Depressed S-T segment:
It indicates that heart muscles receive insufficient oxygen.
Flatter T-wave:
It indicates insufficient supply of oxygen to heart muscle as it occurs in
coronary artery disease.
Elevated T-wave:
It may indicates increased level of potassium ions in blood as in
hyperkalemia.
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14. Reference
 Carr, Joseph J., and Brown, John Michael. Introduction to Biomedical
Equipment Technology. India, Prentice Hall, 2001.
 Narsimhan , Francis, Schamroth; LeoSchamroth An Introduction to
Electrocardiography; 8th Edition; Wiley India. 2013
 Karki G., Electrocardiogram (ECG): working principle, normal ECG
wave, application of ECG, August 2018. Visited on: 27th Oct, 2022
https://www.onlinebiologynotes.com/electrocardiogram-ecg-
working-principle-normal-ecg-wave-application-of-ecg/
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15. Thank You!
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