First Lecture

ECG

1
 Contents
1- Introduction
2- Action Potential
3- Conduction of Electricity
4- Electrodes
5- Leads
6- ECG paper
7- Abmormal ECG

2
Introduction
To measure heart electricity, We should measure 3
things
1- Strength
It is indicated by the amplitude of the waves

2- Speed
It is indicated by duration
Whenever the electricity is fast, We will have narrow wave,
which means less time to end and vice versa for the slow
one.

3- Direction
If the electricity is towards the electrode, it gives Positive
wave, But if it is in opposite direction, It gives Negative
wave.
When it is perpendicular on the electrode, we call it “
equiphasic wave”, which means Positive = Negative

3
Action Potential
Resting membrane potential: This voltage difference exists
across the cell membrane when the cell is not electrically
excited. Normally, RMP= -90, where the sign indicates the
state of the interior of cardiac cells.

Main ions related to myocardial cell

Action potential: This voltage difference exists across the cell

membrane when the cell is electrically excited “During
action”, which leads in changes in ions concentration.
Action potential of Myocardial cells
Depolarization:
Phase 0: Sodium influx

Repolarization:
Phase 1: Potassium outflux, Chloride influx
Phase 2: Potassium outflux, Calcium influx
Phase 3: K outflux
Phase 4: Na-K pump to restore ion balance

4
Action potential of SA and AV nodes
As we see there is a difference between the action
potential of the contractile cells and the pacemaker.
Depolarization:
Phase 0: Calcium influx

Repolarization:
Phase 3: Potassium outflux
But Phase 4 here is different, as there is the funny channels “if
channels” which helps in slow sodium influx, these channels are
responsible for the spontaneous depolarization of the SAN, that’s
why we call it Automatic cells. Note:

Contractile cells depolarization occurs by Na influx,

But SAN depolarization occurs by Ca influx

Ivabradine is a medicine used for tachycardia, As it works on If

channels by preventing Na influx leading to decrease SAN firing.

Conduction of electricity
Depolarization of cells transfers from one cell to another,
Starting from cells of the right atrium, While Repolarization
starts in reverse from the last cell to be depolarized.
Velocity of conduction in contractile cells depends
on Na+ while in SAN depends on Ca++ influx
5
Electrodes
They are conductive pads attached to the skin.They
enable the recording of electrical currents produced by
the heart. "‫"كاميرا بتصور‬ ‫صحرا وخضرناها‬
‫نار وطفيناها‬

We have two types of electrodes,

Limb electrodes:
Red electrode: Placed on the right arm.
Yellow electrode: Placed on the left arm.
Green electrode: Positioned on the left leg.
Black electrode (also known as the indifferent electrode): Placed on
the right leg, It is called that because it has high resistance so a
negligible amount of potential difference is measured through it so
we use it just to connect other to ECG apparatus.
Chest Electrodes:
They are arranged from chest from V1 to V6 “ V for Vector” or C1 to
C6 “C for chest”

Leads
"‫" الصورة ال طلعت من الكاميرا‬

They are the tracing potential difference between two points.

We have 12 leads.

6
Standard leads ‫كامرتين بيدوا صورة‬
There’re 3 leads:
Lead I: Right hand to left hand
“Red to Yellow”

Lead II: Right hand to left foot

“Red to green”

Lead III: Left hand to left foot

“Yellow to green”

Since each lead here has 2 electrodes which has opposite polarity “
+ve or -ve according to direction of electricity”, We call them bipolar
leads.
Now if we put the Standard leads on a graph
We will notice that Lead I makes angle 0o,
Lead 2 makes angle 60o and Lead III makes angle 120o,
So each one let us see the heart from different angle.
But what if we want to see from only one limb?

We use one electrode, Those we call Augmented vector leads

“Unipolar leads, the limb is always positive since we compare it with
indifferent electrode”‫كاميرا بصورة‬
But Using one electrode makes it weak electrical picture, so
augmentation was needed.
7
Augmented vector leads
aVR: Right hand is the +ve
‫نقطة التقاء‬
aVL: Left hand is the +ve
aVF: Left foot is the +ve
Let’s put them in graph with the standard leads
We will notice that aVR makes angle -150O, aVL makes angle -30o
and aVF makes angle 90O

These 6 leads are called Coronal leads,

We have in total 12 leads, The other six
are chest leads and they are unipolar leads
as augmented vector leads.

8
ECG paper
ECG paper is thermal. It is coated with a special chemical that
reacts to heat, allowing the ECG machine to create tracings of
the heart’s electrical activity. This type of paper is designed to
be resistant to changes in temperature and humidity, ensuring
accurate recordings.
This paper is divided by heavy line into big squares and each
big square is divided into 5 small squares.(1 small square = 1 mm)
Horizontal axis (Time)
1 minute = 300 squares (divided by 60)
1 second = 5 big square (divided by 5)
0.2 second = 1 big square
0.2 second = 5 small squares (divided by 5)
0.04 second = 1 small square
Vertical axis (force of electricity)
1mV =2 big squares
1 mV = 10 small squares
0.1 mV = 1 small square

9
Waves
PQRST
-P wave represents atrial depolarization
which is 2.5 mm X 2.5 mm
- PR segment which is the isoelectric
Line between P wave and QRS
represents AV nodal delay
(This delay is important to protect Segment is isoelectric line
Interval is wave + segment
ventricles and for coronaries reperfusion)

- PR interval = P wave + PR segment

- QRS complex represents ventricular depolarization
Phase 3 mainly
Phase 2 = ST segment
It is 2.5 mm wide but its amplitude if in limb
leads is 5 small squares but in chest leads,
the amplitude will be 10 small squares.
So if we add amplitudes in leads I, II and III > 15 m

- T wave represents ventricular repolarization

- Atrial repolarization is hidden as it is at
the same time of ventricular depolarization

10
QRS

These 3 waves could be Monophasic with one wave only,

Biphasic or Triphasic.
To identify each wave:
1st -ve wave = Q
1st +ve wave = R
1st -ve wave after R = S
1st +ve wave after S = R"

Atrial depolarization
It starts from SA then internodal pathway to depolarize the
right atrium then left atrium through Bachmann bundle.

Ventricular depolarization
SA --- AV --- Bundle of his ---Right and left bundle branch ---
Purkinje system

11
As we see, The depolarization of
The septum and ventricular cells are in
2 different directions, where the septum
depolarize to right while ventricular cells to the
left, that will affect the QRS from different
Chest leads, As V1,V2 are on the right they
can read the septal depolarization by small r
“septal R” while in V5,V6 , the R become larger
As it reads the ventricular depolarization.
Concurrently, the S wave decreases in amplitude
from the right to left.

12
T wave:
is normally asymmetrical
Its peak is nearer to the end of the wave

Its amplitude :
5 squares in Limb leads
10 squares in the chest leads

If elevated symmetrically: hyperacute T wave

If elevated asymmetrically : normal variate

P line or segment : end of T wave to the beginning of P wave

J point : junction between end of S segment and the St segment

If the J point is higher than the level of TP line : ST elevation

If the J point is lower than the level of TP line : ST depression

13
Abnormal ECG
1- Spot diagnosis
2- Follow the scheme to confirm, correct, complete
diagnosis

Scheme:
1- AV block & arrhythmia: long strip Lead II, V1, V5
2- Atrial enlargement: Lead II, V1
3- BBB
4- Ventricular enlargement
3 & 4: V1,V2,V5,V6

5- Axis & hemiblock: Limb leads (Lead I, aVF)

6- Infarction & ischemia: Wall to wall

7- Miscellaneous
- Low voltage
- Electrolyte disturbances (hypokalemia)
- Drugs (digitalis)
- Preexcitation syndrome (WPW)

14
Each lead has a view for the heart:
Inferior leads : aVF, Lead II, Lead III
Lateral leads : Lead I, aVL, V5, V6
Septal leads : V1, V2
Anterior leads : V3, V4
Posterior leads : V7, V8, V9
Right leads : V3R, V4R

15