This document provides an overview of the basics of electrocardiography (ECG). It discusses the principles of cardiac activation and repolarization that underlie the ECG. It describes the components of an ECG machine and how it detects cardiac electrical signals. It explains the standard 12-lead ECG system and the orientation and views of the heart provided by each lead. Key waves, intervals, and parameters measured from the ECG are defined. The roles of cardiac anatomy and physiology in generating the ECG are also outlined.

1. BASICS OF ECG
Dr. Nagula Praveen
MD,DM
Associate Professor of Cardiology
Osmania General Hospital
Hyderabad

2.  Principles of ECG
 Atrial and Ventricular Activation
 Deflexions
 Electrodes
 Leads

3. Principles of ECG
 In the resting state all the cardiac cells are deemed to be polarized.
 When cardiac cells are activated they are deemed to be depolarized.
 The negative ions migrate to the outer surface of the cell and the positive
charges pass into the cell, i.e., the polarity is reversed.
 With recovery positive charges return to the outer surface and negative charges
migrate into the cell – this process is termed repolarization.

5. Dipole and doublet.
 When two electrical charges of equal and opposite direction, i.e. one positive
ion and one negative ion, are juxtaposed on either side of a membrane, they
constitute a dipole.
 When two charged ions of equal and opposite direction are situated next to
each other on the surface of an excitable tissue, they constitute a doublet.

6. Current flows only when there is difference in electric potential
 A series of cells in the resting state will all have positive surface charges.
 There is consequently no difference in surface electrical potential and no
current flows.
 If a stimulus travels through these resting polarized cells, those cells initially
activated or depolarized will have negative charges whilst those not yet
activated will have positive surface charges.
 A potential electrical difference will therefore exist between the surface of the
excitable cells and the surface of the adjacent resting cells and a current will
flow i.e., the surface boundary between excitable and non-excitable tissue is
characterized by a doublet.
 A doublet will always exist between the surfaces of the excited and resting cells,
the flow of an electrical current may be viewed as a series of doublets.

7. Two important principles
 The current will have a positive head and a negative tail.
 A unipolar electrode or the positive pole of a bipolar electrode, orientated
towards the oncoming head will record a positive or upward deflexion.
 A unipolar electrode or the negative pole of a bipolar electrode, orientated
towards the receding tail will record as negative or downward deflexion.
 The electromagnetic force is a vector (has both magnitude and direction).
 All the electrocardiographic deflexions are the expression of such forces or
vectors.

9. The electrocardiograph
 Sophisticated galvanometer with a sensitive electromagnet - detect and record
changes in the electromagnetic potential.
 It has a positive pole and negative pole.
 The wire extensions from these poles have electrodes at each end:
a positive electrode at positive pole, negative electrode at negative pole.
 The paired electrodes together taken as an electrocardiographic lead.

11.  When the paired electrodes are oriented in any particular direction, the
theoretical straight line joining the electrodes is known as the axis of that lead,
or lead axis.
 A lead so placed will detect and transmit any changes in electrical potential
which occur between its electrodes.

12. Electrical field of the heart
 The heart is situated in the centre of the electrical field which it generates.
 The intensity of this electrical filed diminishes algebraically with the distance
from its centre.
 The electrical intensity recorded by an electrode diminishes rapidly when the
electrode is moved a short distance from the heart and less and less as the
electrode is moved still further away from the heart.

13.  With distances greater than 15 cm from the heart, the decrement in the intensity
of the electrical filed is hardly noticeable. – all electrodes placed at a distance
greater than 15 cm from the heart, may in an electrical sense, be considered to
be equidistant from the heart.
 Ex: an electrode placed at 25 cm from the heart will record about the same
potential as one placed 35 cm from the heart.
 This applies only to the standard leads.

16. Characteristics of each wave
 Duration – measured in fractions of a second.
 Amplitude- measured in millivolts.
 Configuration – a more subjective criterion referring to the shape and
appearance of a wave.

17. Electrocardiological significance of the
cardiac anatomy
 Heart is a four chambered organ.
 In an electrophysiological sense, only two chambers: atria and ventricle.
 The two atria function as a single electrophysiological chamber – (an unit):
there is no electrical boundary between them, and are activated by a single
activation process – bi atrial chamber.
 Similarly the ventricles as biventricular chamber.
 Electrically inert conduction barrier formed by the fibrous atrioventricular ring.
 Communication is only through AV node, the bundle of His, the bundle
branches and their ramifications.

18. Dominance of the left ventricle
 The interventricular septum
 The free wall of right ventricle
 The free wall of left ventricle
 Left ventricle – main hemodynamic pump of the heart.
 The IVS forms a continuum with the free wall of the left ventricle.
 RV free wall as appendage to the left ventricle.
 The free wall of the right ventricle plays a relatively minor role.
 Electrocardiological anterior wall of the heart is in effect, the interventricular
septum.
 Ex: AWMI = infarction of the IVS and not the free wall of the right ventricle.

19. The mode of atrial activation
 Bi-atrial chamber is a relatively thin walled structure and is not equipped with
the highly specialised conducting system of the ventricles.
 Activation of the bi-atrial chamber occurs longitudinally and by contiguity,
spreading from its point of origin in the sino-atrial node to engulf the whole
chamber, each fibre in turn activating the adjacent fibre.
 Proximal parts are activated before the distal parts.

20. The mode of ventricular activation
 Activation of the ventricles is effected through the specialised and highly
efficient conduction system which transmits the supraventricular impulse very
rapidly to all the endocardial regions of the chamber.
 The muscle is then activated from endocardial to epicardial surfaces through
the terminal ramifications of the conducting system.
 Excitation therefore occurs transversely through the ventricular myocardium,
and this enables the whole chamber to be activated near synchronously.

21. The electrocardiographic paper
 The electrocardiogram is nearly always conventionally recorded at a paper
speed of 25 mm per second.
 At this paper speed,
 Five large squares represent one second,
 One large square represents 0.20 or 1/5 of a second,
 And one small square represents 0.04 or 1/25 of a second.
 Most graph papers used for the recording of electrocardiogram have every
fifteenth large square (a three second period) marked by a vertical line on the
upper border.

22.  Time parameters (horizontal measurement)
 Deflexion amplitudes (vertical measurement)

24. The conventional electrocardiographic leads
 An electrocardiographic lead can be placed on the body in any three
dimensional relationship to the heart.
 12 coventional leads based on their orientation to the heart.
 Frontal plane leads: standard leads I,II, and III, and leads AVR,AVL and AVF.
 Horizontal plane leads: precordial leads – leads V1 to V6.

25. The Frontal plane leads
 Electrically equidistant from the heart.
 They are called as bipolar leads.
 Standard lead I . The lead is derived from the placement of the negative
electrode on the right arm and the positive electrode on the left arm.
 Standard lead II. The lead is derived from the placement of the negative
electrode on the right arm and the positive electrode on the left foot.
 Standard lead III. The lead is derived from the placement of the negative
electrode on the left arm and the positive electrode on the left foot.

27. Unipolar limb leads
 The electrode of this lead is called as exploring electrode (positive) – reflects
the true potential.
 The negative electrode is so constructed that it is considered to be at zero
potential.
 All unipolar leads are termed V leads
 extremity or limb leads and
 precordial, or chest leads.
 Extremity leads are of low electrical potential and are therefore instrumentally
augmented.
 These augmented extremity leads are thus prefixed by the letter “A”.

29.  Lead AVR
 Augmented unipolar right arm lead
 faces the heart from the right shoulder.
 orientated to the cavity of the heart.
 All the deflexions of the heart are normally negative in this lead.

30.  Lead AVL
 Augmented unipolar left arm lead.
 Face the heart from the left shoulder.
 Anterolateral or superior surface of the left ventricle.
 Lead AVF
 Augmented unipolar left leg lead.
 Inferior surface of the heart.

32. Horizontal plane leads
 Precordial leads
 Designated by the letter “V”
 Placement of the precordial electrodes
 Lead V1: fourth intercostal space, immediately to the right of the sternum.
 Lead V2: fourth intercostal space, immediately to the left of the sternum.
 Lead V4: fifth intercostal space in the midclavicular line.
 Lead V3: placed on the chest exactly midway between the V2 and V4
electrode position.
 Lead V5: fifth intercostal space in the anterior axillary line.
 Lead V6: fifth intercostal space in the mid axillary line.

34. Orientation of the leads
 Lead II and III and AVF – Inferior surface of the heart.
 Lead I, AVL – High or superior left lateral wall ( lateral leads).
 Lead AVR – Cavity of the heart.
 Lead V1 – Cavity of the heart
 Lead V1-V6 Anterior wall of the heart.
 Anteroseptal leads – V1 toV4,
 Apical or lateral V5 and V6.
 Left oriented leads –I,AVL,V5 and V6.
 No lead orientated directly to the posterior wall of the heart.

35. Right sided leads

36. Posterior leads

39. Einthoven triangle

40. Einthoven law

41. Modified leads
 Mason Likar Modification
 Frank’s Leads

44. Standardization
 One millivolt will result in a 10 mm deflexion.
 Standardization signal will have clear and perfect right angles at each corner.
 Overdamping - pressure of the writing stylus is too firm on the platform or
writing edge of the electrocardiography so that its excursions are somewhat
retarded.
 Rounding of the transition from the upstroke to the horizontal part of the
standardization signal and downstroke to the horizontal part.
 Wider complexes.
 Diminished amplitude.
 Underdamping or overshoot – sharp spikes at these corners
 Narrow complexes
 Exaggerated waves.

46. Why ECG reading is important?
 Gold standard for the diagnosis of arrhythmias
 Helps in identification and management of STEMI
 Helps detect electrolyte imbalances

47. THANKYOU