The property of automaticity of the sinus node is responsible foe the impulse initiation and travels along the cardiac tissue as depolarizations which result in its contraction. So, when activated, the heart is a concentrated locus of time varying potentials in the body. These voltage fluctuations can be measured by the placement of electrodes on the surface of the body. This forms the basis of electrocardiography. In this presentation we will see the basics, the lead systems and the principles behind recording of ECG.

1. Fundamentals of
ELECTROCARDIOGRAPHY
Dr Saran A K
Lead Systems and Principles of ECG Recording
1

2. • Introduction
• History of ECG/EKG
• Components of an Electrocardiograph
• Normal ECG Patterns
• Clinical Applications
2

3. Heart as a Dipole
• When activated, the heart is a concentrated
locus of time varying electrical potentials in
the body.
• This difference in polarity between two
neighbouring locations constitutes a dipole.
(momentary dipoles)
• Electrical Currents (weak) readily flow from
one pole of dipole to other through any
conducting media. Fig. 12.6 Example of Cardiac Dipole
Medical Physiology Rodney & Bell 4th Ed
3

4. Body as a Volume Conductor
• Because the body fluids are good conductors,
fluctuations in potential that represent the
algebraic sum of the action potentials of
myocardial fibres can be recorded
extracellularly.
• These currents radiate outward through the
body all the way to the surface of the skin and
can be picked up by electrodes. Fig. 11-5 Flow of current in chest around
partially depolarised ventricles
Guyton & Hall Medical Physiology 14th Ed
4

5. Electrocardiogram (ECG)
• An amplified, timed recording of the
fluctuating potentials of the heart, as
detected on the surface of the body during
cardiac cycle.
• The recording gives a plot of voltage as a
function of time.
Fig. 11-2 Recording of De/Repolarisation Waves
Guyton & Hall Medical Physiology 14th Ed
5

6. Fig. 11-3 Monophasic AP and ECG (left)
Fig. 11-1 Normal ECG (right)
Guyton & Hall Med Physiology 14th Ed
6

7. Early Precursors and History of ECG
Fig. 1 A brief review: history to understand
fundamentals of electrocardiography
Majd AlGhatrif, MD and Joseph Lindsay, MD
7

8. Birth of Clinical Electrocardiogram
8
• Dr. Willem Einthoven, refined the capillary electrometer even further and was
able to demonstrate five deflections and it resulted in the curves that we see
today.
• In 1901, he successfully developed a new string galvanometer with very high
sensitivity, which he used in his electrocardiograph.

9. Components of an Electrocardiograph
9
Parts Function
Leads
1. Bipolar
• Standard Limb Leads
2. Unipolar
• Chest Leads
• Limb Leads
Pick up electric potential from body surface
Amplifier Amplifies the electrical activity
Recording Device
• Pen Recording Device
• Oscilloscope
Makes record of the electrical activity

10. • Leads are formed by electrodes, placed on body
surface which measure potential fluctuations between
the two points
• A 12 lead electrocardiogram is the standard.
• The ECG can be recorded by using
1. two active electrodes (bipolar recording)
2. an active or exploring electrode connected
to an indifferent electrode at zero potential
(unipolar recording)
10
Leads used in Electrocardiography
Fig. 1.8 Optimal Sites for Recording Cardiac
Electrical Activity
Wagner Marriott’s Electrocardiography

11. Einthoven’s Triangle
• Kirchoff’s voltage law states that for a closed loop series
path the algebraic sum of all the voltages around any closed
loop in a circuit is equal to zero.
L1+L2+L3 = 0
• Einthoven codified the analysis of the electrical activity of
heart by proposing certain conventions.
• The heart is considered to be at the centre of a
equilateral triangle, corner serves as the location
for electrodes
• The resulting three bipolar leads are designated as I,
II and III.
11
Fig. 12.10 The Einthoven’s Triangle
Medical Physiology Rodney & Bell 4th Ed

12. Einthoven's Law
• In electrocardiogram, for standard
limb leads, the algebraic sum of
potential of any wave or complex in
lead II is equal to the sum of
potentials of leads I and III.
12
Lead I + Lead III = Lead II
Fig 28-8 Calculation of mean QRS Vector
Ganong’s Review of Medical Physiology 22nd Ed

13. A. Bipolar Lead
• The electrocardiogram is recorded from
two electrodes placed on different
sides of heart .
• Thus lead is not a single wire but a
combination of two wires and their
electrodes ,to make a complete circuit
between body and electrocardiograph.
13
The Standard 12 Lead Electrocardiograph

14. 1. Bipolar/Standard Limb Leads
oConventional method
o2 active electrodes are used potential
difference between these is measured.
oLI, LII and LIII each record the differences in
potential between two limbs.
• L1 : right and left arm
• LII : right arm and left foot
• L III : left arm and left leg
14

15. B. Unipolar Leads
• The exploring electrode is placed on
the precordium or a limb and voltage is
measured w.r.t the indifferent
electrode. Here potential is compared
to zero, not to potential at another
site.
1. Unipolar Chest Leads (V1-V6)
2. Augmented Unipolar Limb Leads
(aVR, aVL, aVF)
15

16. 1. Unipolar Chest Leads
• 6 Active electrodes are placed in different parts of the chest and is connected
to positive terminals of ECG
16
Fig 4.8 Location of Precordial Leads
Goldberger Clinical Electrocardiography
Lead Position
V1 4th ICS just right of sternum
V2 4th ICS just left of sternum
V3 Midway between V2 and V4
V4 5th ICS in the mid-clavicular line
V5 Same level as V4 in the anterior axillary line
V6 Same level as V4 and V5 in the mid axillary line

17. Wilson’s Central Terminal
• Dr. Frank N. Wilson of the University of Michigan
developed the concept of the ‘central terminal’. By
connecting the three limb electrodes, a central
negative lead reflecting a ‘ground’ or reference
terminal was created.
• Artificially constructed reference for
surface electrocardiography, which is
assumed to be near zero and steady
during the cardiac cycle
17

18. 2. Augmented Unipolar Limb Leads
• Allow other vantage points from which to view
the cardiac vectors and they are called
Augmented unipolar limb leads
• They are unipolar because the potential is
compared to zero and not to potential at
another site.
• This increases the size of the potentials by 50%
without any change in the configuration from
the non augmented record.
18

19. 19
Derivation of Hexaxial Lead System
A. Triaxial diagram of the bipolar leads I, II, III
B. Triaxial diagram of the augmented limb leads (aVR, aVL and aVF)
C. The two triaxial diagrams can be combined into a hexaxial diagram
that shows the relationship of all the six limb leads.
Fig 4.7,4.10 Goldberger's Clinical Electrocardiography

20. Additional Leads
1. Oesophageal Leads
• Unipolar leads placed at tips of catheters are
inserted into the esophagus
• Useful to record atrial complex and to explore
posterior left ventricular surfaces
2. Endocardial direct leads
• Exploring Electrode introduced by cardiac
catheterization in to different cardiac chambers
20

21. 21
Lead II – Standard Limb Lead
Negative Waves in avR

22. ECG Paper
22
Horizontal and vertical sq. by lines at 1 mm distance
A heavier or dark line is present after every 5 mm
o Horizontal measurements : units of time
o Vertical measurements : voltage.
Speed of the paper : 25mm/s
A faster speed is used to visualise wave deflection better

23. • Horizontal : 1small division( 1mm) - 0.04 sec
• Vertical : 1 small division(1 mm) - 0.1 mV
• Standardization
23
Standardization of ECG To adjust the voltage of QRS in order to make its interpretation easier.
Normal or Full Standardization 10mm equals 1mV (upward deflection) Normal
Half Standardization 5mm equals 1mV Hypertrophy of ventricles
Double Standardization 20mm equals 1mV Too small waveforms

24. Normal Pattern of ECG
24
Waves
1. P Wave
2. QRS Complex
3. T Wave
4. U Wave
Segments
1. PR / PQ Segments
2. ST Segment
3. TP Segment
Intervals
1. PR /PQ interval
2. QT interval
3. ST interval
4. R-R interval

25. 25
P Wave
Q Wave
R Wave
S Wave

26. P Wave
26
• Positive wave except in lead aVR
• Due to depolarization of both the
left and right atrium
• Duration : 0.08- 0.12 sec
• Amplitude : 0.12 – 0.3mV
Significance : Magnitude of P wave
is a guide to atrial electrical activity.

27. QRS Complex
• Has 3 waves
• First negative after P - Q wave
• First positive after P - R wave
• Firstnegative below baseline after R: S wave
• Small letters (q,r,s) small amplitude
• No R Wave – QS Complex
• Duration : 0.08- 0.1sec
• Voltage : 1 mV
• Inverted in lead aVR
27

28. Q Wave • Septal activation
• Lead I, aVL, V5, V6
• Do not exceed 25% R wave and 0.04 sec
R Wave • Prominent positive wave
• Antero-septal region of ventricular
myocardium
• Upstroke coincides with ventricular systole
and is directly proportional to functional
ability of ventricles
• do not exceed 25 mV in normal individual
• Progression of R wave from V1 to V6 due to
increased voltage
S Wave • Posterobasal portion of left ventricle and
pulmonary conus.
• Amplitude deceases from V1 to V6
28

29. 29
Fig. R Wave Progression Goldeberger’s Electrocardiography

30. • Since voltage increases, Progression of
R wave is from V1 –V6
• Amplitude of S wave decreases from
V1 –V6
• In V3 and V4 , R and S wave amplitudes
are more or less equal, this is called
transition zone
30
• R in V6 , S in V1 show left ventricular activity.
• R in V1 and S in V6 show right ventricular activity
• R Wave should not exceed 4mm in V1 and 25 mm in V6.

31. T Wave
• Due to ventricular repolarization
• Positive wave in most of the leads
except aVR
• The last cells to depolarize in the
ventricles are the first to
repolarize
• Duration : 0.27 sec
• Amplitude : 0.2 -0.3 mV
• There is no definite wave to show
repolarization of atria.
• Its buried inside QRS complex
31

32. U Wave
• Not always present
• Due to slow repolarization of
papillary muscles
• Normally an upright deflection
and smaller than T Wave.
• Best appreciated in precordial
leads V2 to V4
• Duration : 0.05 sec
• Voltage : 0.2 mV
32

33. PR Segment
• The PR segment is the flat, usually
isoelectric segment between the end
of the P wave and the start of the
QRS complex.
• The event during this time is AV
nodal delay+ conduction through
bundle of His
• Duration : 0.04- 0.1 sec
33

34. ST Segment
• J point is the point at which the S
wave meets the base line.
• ST segment iso electric segment that
follows QRS complex from J point to
beginning of T wave
• Duration : 0.05- 0.1 sec
TP Segment
• Iso electric segment from the end of T
wave to the beginning of next P wave.
34

35. PR/PQ Interval
• The time between the beginning of the
P wave and the beginning of the QRS
complex .
• If Q wave is not present, it is up to the
beginning of R wave, P wave + PR
segment
• Duration : 0.12 -0.2 sec
• Event : atrial depolarization + AV Nodal
delay +conduction through Bundle of
His just before ventricular
depolarization
• Inversely proportional to Heart rate
35

36. QT Interval
• From the beginning of the Q wave to the end of
the T wave
• QRS Complex + ST Segment +T wave
• Event : ventricular depolarization
and repolarization
• Duration : 0.42- 0.43 sec
ST Interval
• J point to end of T wave
• Late part of ventricular depolarisation and
Ventricular repolarization
• ST Segment + T wave
• Duration : 0.32 sec
36

37. R-R Interval
37
The normal interval between
two successive R waves in the
adult person is about 0.83
second.
• Heart rate =60/RR Interval
• Heart Rate =1500/no of small squares b/w RR
• Heart Rate =300/no of large squares b/w RR

38. •Prerequisite for a Good ECG Recording
1. Relaxed lying down posture
2. Good contact between the skin and the electrode.
3. Properly Standardised ECG machine, Proper grounding
to avoid alternating current interference.
4. No electronic equipment in contact
5. Proper Electrode Placement
38

39. Types of Electrocardiograms
39
1. Resting ECG
2. Exercise ECG (Exercise Stress Test)
• For the evaluation of exercise capacity
• Detection of coronary disease and the
assessment of its severity
• Prediction of cardiovascular risk
• Monitoring the response to treatment

40. 3. Signal Averaged ECG
• It is a high-resolution, non-invasive
electrocardiographic method
• Performed for 15-20 minutes and the
information obtained from this test is
processed by a computer.
Uses
• Late ventricular potentials (LVP)
detection
• Re-entry ventricular arrhythmias, and
sudden cardiac death (SCD)
40

41. 4. Holter (Ambulatory) Monitoring
Portable electrocardiograph machine in
which readings to be made over a 24-
hour period while doing regular daily
activities
Uses
Detecting occasional cardiac arrythmias
which would be difficult to identify in a
shorter period.
41

42. Clinical Applications of ECG
• Electrocardiography today is an essential part of the initial
evaluation for patients presenting with cardiac complaints.
• Specifically, it plays an important role as a non-invasive, cost-
effective tool in the diagnosis of diseases such as
1. Chamber Hypertrophy and Dilatation
2. Myocardial ischemia and infarction
3. Arrythmias : It serves as a gold standard for diagnosis
4. Myopericardial Disease or Pericarditis
42

43. 5. Conduction disturbances at various stages from SA Node
to Purkunjee system
6. Effect of drugs and monitoring the drug therapy such as
quinine, quinidine and procainamide
7. Electrolyte disturbances especially Hypokalaemia,
Hyperkalaemia, Hypocalcaemia and Hypercalcemia.
8. Detection of efficacy of various cardiac intervention
procedures viz. angioplasty
9. Continuous ambulatory electrocardiography helps in
evaluation of symptoms related to daily activity.
43

44. References
44
1. Ganong. Review of Medical Physiology 22nd Edition McGraw Hill
2. Guyton & Hall. Textbook of Medical Physiology 11th Edition Saunders
Elsevier
3. Rodney & Bell. Medical Physiology Principles for Clinical Medicine Wolters
Kluwer
4. Goldberger. Clinical Electrocardiography A Simplified Approach Elsevier
5. SN Chugh. Textbook of Clinical Electrocardiography 3rd Edition Jaypee
6. Ganong. Review of Medical Physiology 26th Edition McGraw Hill

45. THANK YOU !
saran.adhoc@gmail.com
45