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1. ECG BASICS
BY, DR MARIA

2. • The specialized cells of the heart’s electrical conduction
system work together to generate impulses that cause the
heart to contract and relax, resulting in the rhythmic beating
of the heart.
• The heart is composed of specialized cells that work
together to maintain a rhythmic heartbeat.
• The conduction system of the heart is responsible for
generating electrical signals that cause the heart to contract
and relax.
• The depolarization and repolarization of these cells play a
crucial role in this process.

3. Depolarization
• Depolarization is the process by which the cells of the heart become less negative and
contract. When the cells are at rest, they are negatively charged or polarized.
• However, when an electrical impulse is generated, the cells become depolarized. This occurs
when the concentration of ions changes, specifically when sodium ions rush into the cells.
• The sodium ions move into the cells through ion channels, which are specialized proteins
embedded in the cell membrane.
• Once the sodium ions enter the cells, they cause the cells to become positively charged,
which results in the depolarization of the cells. This causes the cells to contract and push
blood through the heart.

4. Repolarization
• Repolarization is the process by which the cells return to
their negatively charged state. This occurs when the cell
membrane becomes more permeable to potassium ions,
which exit the cell.
• The loss of positive ions from the cell causes the cells to become
negatively charged again, leading to relaxation.
• The process of depolarization and repolarization creates the
electrical activity of the heart, which is represented as a
PQRST waveform on an electrocardiogram (EKG).

6. • Cardiac Electrophysiology
• Cardiac electrophysiology focuses on the study of the electrical properties and activity of the
heart, including the following:
• Sinoatrial (SA) node
• Atrioventricular (AV) node
• Purkinje fibers
• Bundle of His
• The sinoatrial node, also known as the pacemaker, is responsible for setting the heart’s
rhythm. The wave of depolarization that originates from the SA node is responsible for causing
the atria to contract.
• This is visualized as the P wave on an EKG tracing.
• The impulse is received by the AV node, which causes a short delay. This delay is visualized as
the PR interval on the EKG tracing.
• Then the stimulus moves through the bundle of His, through the left and right bundle
branches, and into the Purkinje fibers.
• This produces ventricular depolarization, and contraction occurs, which can be seen as the QRS
complex.

8. • What is an Action Potential?
• An action potential is a rapid, temporary change in the electrical
potential of a cell caused by the movement of ions across the cell
membrane.
• In the context of the heart, an action potential refers to the
electrical changes that occur in the heart cells during each
heartbeat.
• When the heart muscle cells are stimulated by an electrical impulse,
such as from the sinoatrial (SA) node, the action potential is initiated,
resulting in the depolarization and repolarization of the cells.
• The sequence of depolarization and repolarization produces the
electrical signal that triggers the contraction of the heart and the
ejection of blood.

9. • What is an ECG?
• ECG is the abbreviated term for an electrocardiogram. During an
ECG test, electrodes are placed on the skin of the chest, arms,
and legs, and connected to a machine that records the electrical
signals produced by the heart.
• The test is painless and takes only a few minutes to
complete. The recorded signals are then used to create a
visual representation of the heart’s electrical activity, called
an ECG waveform.

10. Parts of the ECG explained
• P waves
• P waves represent atrial depolarisation.
• In healthy individuals, there should be a P wave
preceding each QRS complex.
• QRS complex
• The QRS complex represents the depolarisation of
the ventricles.
• It appears as three closely related waves on the ECG
(the Q, R and S wave).
• T wave

11. • T wave
• The T wave represents ventricular
repolarisation.
• It appears as a small wave after the
QRS complex.
•U Wave
• Hypothesised to be Purkinje repolarisation

13. INTERVALS
1. PR interval
The PR interval begins at the start of the P wave and ends at
the beginning of the Q wave.
It represents the time for electrical activity to move between
the atria and the ventricles.
RR interval
2. The RR interval begins at the peak of one R
wave and ends at the peak of the next R wave.
It represents the time between two QRS complexes.
QT interval
3. The QT interval begins at the start of the QRS
complex and finishes at the end of the T wave.
It represents the time taken for
the ventricles to depolarise and then repolarise.

14. SEGMENTS
ST segment
The ST segment starts at the end of the S
wave and ends at the beginning of the T wave.
The ST segment is an isoelectric line representing the
time between depolarisation and repolarisation of
the ventricles (i.e. ventricular contraction).
•PR segment = depolarization of the AV node. I.e.
When current is passing through the AV node. It’s a
flat line because the wave is not strong enough to be
recorded on the voltmeter.

15. HOW TO RECORD ECG
• Wash your hands and don PPE if appropriate.
• Introduce yourself to the patient including your name and role.
• Confirm the patient’s name and date of birth.
• Gain consent to proceed with ECG recording.
• Adequately expose the patient’s chest for the procedure (offer a blanket
to allow exposure only when required). Exposure of the patient’s lower
legs and wrists is also necessary to apply the limb leads.
• Ask the patient to lay on the clinical examination couch with the head of
the couch at a 45° angle.
• Check if the patient has any pain before continuing with the clinical
procedure.

16. •Electrode placement
• A 12-lead ECG involves the use of 10 electrodes, six on the
chest and four on the limbs.
• Begin by checking the expiry date of the electrodes to ensure
they are within date.
• It is important to ensure each electrode has good skin
contact, which may involve cleaning or shaving the areas
where you need to place electrodes. If this is the case, make
sure to explain this clearly to the patient and gain
consent before proceeding

17. •Electrodes
• An ECG electrode is
a conductive pad attached to the
skin to record electrical activity.
• The data gathered from these
electrodes allows the 12 leads of
the ECG to
be calculated (e.g. lead
I is calculated using data from
the electrodes on both
the right and left arm).
• The electrodes used to generate
a 12-lead ECG are described
below.
• Chest electrodes
• Table 1. The chest electrodes and
their placement.
Electrode Location on the body
V1 4th intercostal space at the right sternal edge
V2 4th intercostal space at the left sternal edge
V3 Midway between the V2 and V4 electrodes
V4 5th intercostal space in the midclavicular line
V5 Left anterior axillary line at the same horizontal level as V4
V6
Left mid-axillary line at the same horizontal level as V4
and V5

19. Electro
de
Location on the body
V1 4th intercostal space at the right sternal edge
V2 4th intercostal space at the left sternal edge
V3 Midway between the V2 and V4 electrodes
V4 5th intercostal space in the midclavicular line
V5
Left anterior axillary line at the same horizontal
level as V4
V6
Left mid-axillary line at the same horizontal
level as V4 and V5

20. Electrod
e
View of the heart
V1 Septal view of the heart
V2 Septal view of the heart
V3 Anterior view of the heart
V4 Anterior view of the heart
V5 Lateral view of the heart
V6 Lateral view of the heart
Chest leads
Table 3. The chest leads.

21. Electrode Location on the body
Red (RA) Ulnar styloid process of the right arm
Yellow
(LA)
Ulnar styloid process of the left arm
Green
(LL)
Medial or lateral malleolus of the left leg
Black
(RL)
Medial or lateral malleolus of the right leg
Limb electrodes
There are four limb electrodes.
Table 2. The limb electrodes and their placement

22. •How the 12-lead ECG works
• Understanding the difference between an ECG
electrode and an ECG lead is important:
• An ECG electrode is a conductive pad attached to the skin
to record electrical activity.
• An ECG lead is a graphical representation of
the heart’s electrical activity which is calculated by
analysing data from several ECG electrodes.

24. How to read ECG paper
The paper used to record ECGs is standardised across most hospitals and has the following characteristics:
•Each small square represents 0.04 seconds
•Each large square represents 0.2 seconds
•5 large squares = 1 second
•300 large squares = 1 minute

25. •Heart rate
• What is a normal adult heart rate?
• Normal: 60-100 bpm
• Tachycardia: > 100 bpm
• Bradycardia: < 60 bpm
• If a patient has a regular heart rhythm, their heart rate can
be calculated using the following method:
• Count the number of large squares present within one R-R
interval.
• Divide 300 by this number to calculate heart rate.

26. • Heart rate calculation example
• 4 large squares in an R-R interval
• 300/4 = 75 beats per minute

27. Cardiac axis
In healthy individuals, the electrical activity of the
heart begins at
the sinoatrial node then spreads to
the atrioventricular (AV) node. It then spreads
down the bundle of His and Purkinje fibres to
cause ventricular contraction.
Whenever the direction of electrical activity
moves towards a lead, a positive deflection is
produced.
Whenever the direction of electrical activity
moves away from a lead, a negative deflection is
produced.
The cardiac axis gives us an idea of the overall
direction of electrical activity.

28. • Normal cardiac axis
• In healthy individuals, you would
expect the cardiac axis to lie between
-30°and +90º. The overall direction of
electrical activity is therefore towards
leads I, II and III (the yellow arrow
below). As a result, you see a positive
deflection in all these leads, with lead
II showing the most positive
deflection as it is the most closely
aligned to the overall direction of
electrical spread. You would expect
to see the most negative
deflection in aVR. This is due
to aVR providing a viewpoint of the
heart from the opposite direction.

29. • Right axis deviation
• Right axis deviation (RAD) involves the
direction of depolarisation being distorted
to the right (between +90º and +180º).
• The most common cause of RAD is right
ventricular hypertrophy. Extra right
ventricular tissue generates a stronger
electrical signal by the right side of the
heart. This causes the deflection in lead I to
become negative and the deflection
in lead aVF/III to be more positive.
• RAD is commonly associated with conditions
such as pulmonary hypertension, as they
cause right ventricular hypertrophy. RAD
can, however, be
a normal finding in very tall individuals.

30. •Left axis deviation
• Left axis deviation (LAD) involves the direction
of depolarisation being distorted to the
left (between -30° and -90°). This results in
the deflection of lead III becoming negative
(this is only considered significant if
the deflection of lead II also
becomes negative). Conduction
abnormalities usually cause left axis
deviation.