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Farah Khreisat

Raghad Abu Jebbeh

Faisal Mohammad

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بسم هللا الرحمن الرحيم

Hello guys, hope you're doing well, as you've seen in the previous

lecture, the Dr started with an extremely important topic which is the

ECG, the Dr emphasized the importance of this topic, that's why it's

gonna take us about 3-4 lectures explaining it.

I've used many sources to make this sheet as clear as possible, these

include: The record, Dr's slides, Guyton and Hall, BRS in addition to some

information from Dr Najeeb's lectures.

First, let's revise what has been previously explained:

-ECG (ElectroCardioGram): is simply an electrical recording of the

electrical events that occur in the heart. These events are basically: the

depolarization and repolarization of both atria and ventricles.

- The Depolarization wave is an upward (positive)deflection while the

repolarization wave is a downward (negative)deflection. (This is not

always the case! Since it depends on the arrangement of the two

electrodes, but it has been internationally accepted - standardized- that

depolarization is denoted by an upward deflection and repolarization is

denoted by a downward deflection).

-Before we start, I need you to understand one important thing: How

impulses travel throughout the different chambers of the heart?

Watch this video please (don’t worry, it's only 1 minute long)

https://www.youtube.com/watch?v=te_SY3MeWys

-In the physiology of this system, we are concerned with the normal ECG

of a healthy person, now let's revise the normal ECG waves, we're gonna

need them! So look carefully at the following figure while reading the

notes below:

Figure 1

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There are mainly 4 electrical events that occur in the heart:

1) Atrial depolarization.

2) Atrial repolarization.

3) Ventricular depolarization.

4) Ventricular repolarization.

Interpretation of the waves:

- The P wave = depolarization of atrial myocardium (Signals the onset of

atrial contraction).

-The QRS complex =depolarization of the ventricular myocardium

(Signals the onset of ventricular contraction).

- The T wave = ventricular repolarization.

Important note: The electrical activity always precedes the mechanical

activity:

1-The P wave immediately precedes atrial contraction.

2-The QRS complex immediately precedes ventricular contraction.

3-The T wave precedes ventricular relaxation (Not immediately though

since the ventricles remain contracted until a few milliseconds after

the end of the T repolarization wave).

4-Atria relax after they get repolarized.

-There are also intervals and segments that will be discussed later in this

sheet.

Note: this recording doesn’t stop! If the recording stops and a

continuous straight line appears, this means death.

Now, three questions might pop up in your mind:

1-Why are all the electrical events represented by one wave except for

ventricular depolarization which is represented by the QRS complex?

This is due to the fact that there are 3 stages of depolarization and

contraction in the ventricles:

Depolarization starts at the interventricular septum as you've seen in the

video, then travels rapidly to the apex of both ventricles and

then back to the base (posterior aspect of the ventricle).

So, the three stages of ventricular depolarization are:

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a) The Q wave which corresponds to the depolarization of the

interventricular septum (septal depolarization).

b) The R wave which represents the depolarization of the main mass of

the ventricles, that's why it is the largest wave. (Major ventricular

depolarization).

c) The S wave which represents the final depolarization of the ventricles,

at the base of the heart (basal depolarization).

2- We said that there are 4 main electrical events that occur in the

heart, but only 3 of them are represented on the ECG, why is atrial

repolarization missing ?

Atrial repolarization and ventricular depolarization “occur at the same

time! So atrial repolarization doesn't show up on ECG, since it gets

masked by the electrically stronger ventricular depolarization (Ventricles

generate higher electrical activity than atria since they have larger

muscle mass than that of atria).

3- The T wave, which represents ventricular repolarization, is shown as

an upward deflection! While it is expected to appear as a downward

deflection since that's what is internationally accepted as mentioned

earlier, what's the explanation?

This is due to the fact that depolarization and repolarization of cardiac

myocytes don’t start from the same point!

(Depolarization starts from the endocardium to the epicardium and from

the base to the apex, while repolarization goes from the epicardium to

the endocardium and from the apex to the base).

So, we can say that repolarization starts retrospectively from the last

depolarized site. This means that depolarization and repolarization

propagate in opposite directions (Repolarization starts from the place

where depolarization ended).

Hmmm... I can see you wondering, why does that occur? Look at the

following figure (figure 2) and read the notes that follow:

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Before explaining what actually happens, remember what we said in

page 2, that the direction of the deflection of the depolarization and

repolarization waves depends on how the electrodes are arranged

(because here in ECG, we're recording electrical currents and current is a

vector with both value and direction!) so changing the electrodes'

direction changes the direction of the wave. But logically this is not the

case here! This means that the change happened from within the heart

itself and not from the electrodes…

Physiologically, we have two possible explanations:

1- From the figure, we notice that the action potential of the endocardial

fiber (black line) is longer, so it depolarizes first but repolarizes last.

Whereas, the action potential of the epicardial fiber (blue line) is

shorter, so it depolarizes last but repolarizes first. So this is an intrinsic

property of the cardiac muscle fibers that may be due to the difference

in electrolytes environment between epicardium and endocardium.

2- The second and most important reason is based on the fact that

-usually- depolarization is followed by systole (contraction) and

repolarization is followed by diastole (relaxation).

So, after the QRS complex, we'll have ventricular systole and during

ventricular contraction, intraventricular pressure (i.e. pressure inside the

centre of the ventricle) increases greatly! This pressure is the highest

close to the endocardium and the lowest close to the epicardium, and

this high pressure near the endocardium might change the electrolyte

environment in the endocardium (such as changes in sodium and

potassium concentrations) to the extent that delays repolarization of the

endocardial fiber.

Figure2

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So now we've finally understood why the T wave appeared as a positive

(upward) deflection.

-Another important thing to notice from figure 2 is that both phase 3 of

atrial action potential (atrial repolarization) and phase 0 of ventricular

action potential (ventricular depolarization) happen at the same time,

and that's why atrial repolarization doesn’t show on the ECG.

Now, in which condition we might see atrial repolarization on the ECG?

This happens when ventricular depolarization is delayed (if the time

difference between the P wave and the QRS complex is longer than

normal). For example, AV block delays ventricular depolarization, so

both events don't occur simultaneously and hence they both appear on

ECG.

But the question here would be: If atrial repolarization appears as a

wave on the ECG in the previously mentioned condition, would it appear

as an upward or downward deflection?!

It will appear as a downward deflection, why? Because atrial pressure is

much lesser than ventricular pressure! And the high ventricular pressure

was the main reason behind the upward deflection of the T wave as we

said previously.

-Atrial pressure is extremely low (almost zero), and when it rises during

atrial contraction, it wouldn’t exceed 5mmHg (in the hospital, the

maximum value that is accepted for normal atrial pressure is 20mmHg),

on the other hand, ventricular pressure after contraction is normally

equal to 120 mmHg (see the huge difference!). So, atrial pressure isn’t

high enough to cause changes in electrolytes environment that lead to

positive deflection of the repolarization wave.

How is atrial pressure measured?

-Atrial pressure is usually measured by CVP line (Central Venous

Pressure line): where a catheter (tube) is inserted into the superior vena

cava and it goes down to end up in the right atrium, this tube is

connected with a barometer on the outside to measure pressure (right

atrial pressure).

-An example on the clinical importance of the CVP line:

Intravenous (IV) fluids are frequently administered during early

hospitalization in patients with acute heart failure who are receiving

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diuretics, but this IV fluid must be monitored since too much fluid may

result in cardiac arrest, so we can monitor the patient while

administering IV fluids by continuously measuring the right atrial

pressure using CVP line.

ECG paper: Fasten your seatbelts guys.. This part of the lecture is really important

and requires you to focus.

-The ECG paper is simply a strip of graph paper with large and small grids

(squares).

Important notes:

- The horizontal line (X axis) represents time.

- The vertical line (Y axis) represents voltage.

- The paper is divided into (big) squares by dark lines, each one of these

big squares is divided into 5 (small) squares by lighter lines (as you can

see in the picture).

Note: in the next pages, when I say (square) without saying big or small,

I'm referring to the small one.

On the X axis:

- Each one of the small squares is equal to 1mm.

Figure 3

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-The speed of the machine (the cardiac monitor) is 25mm per second,

which means that 1 second is represented by 25 small squares on the

ECG paper.

- So what does each square represent in seconds?

25 squares 1 second

1 square ?

Each square = 1/25 = 0.04 seconds

So, to know what a certain number of squares on the ECG paper

represent in seconds, there are two ways:

Either divide the number of squares over 25 OR multiply the number of

squares by 0.04

Examples:

5 squares (1 big square) 5/25= 0.20sec OR 5*0.04 = 0.20sec

15 squares 15/25= 0.6sec OR 15* 0. 04= 0.6sec

-On the Y axis, every 10 small squares are equal to 1 millivolt (mV), so

each small square equals 1/10= 0.1 mV.

We've discussed ECG waves, now let's talk about intervals:

Figure 4

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First you should understand that in each interval, there MUST be at least

one wave between the two points.

- The cardiac cycle is basically the time (interval) between two

successive R waves (or two successive P or T waves).

-We usually use the R-R interval since the R wave is more sharp (more

obvious).

- Each cardiac cycle represents one heart beat, SO we can measure the

heart rate (heart beats per minute). How? Follow these steps:

1) If we're given the number of squares between the two successive R

waves. 2) we calculate how many seconds they're equal to as we

learned above by multiplying with 0.04(or by dividing over 25)

3)Then the number that results is simply the time (in seconds) that is

required for the heart to beat one time (seconds per 1 beat) 4) we

divide 60 over this number to get the number of heart beats per minute

(heart rate).

So, Heart rate (beats/ minute) = 60 (sec/min)

Cardiac cycle time (RR interval (sec/ beat))

-From this equation we can deduce that the time of the cardiac cycle

and the heart rate are inversely related (When the duration of the

cardiac cycle “or R-R interval” decreases, heart rate increases and vice

versa).

This is all theoretical, let's apply this with the following examples to

understand:

- Example1: If there were 25 small squares between successive R waves

(RR interval), What's the heart rate?

The cardiac cycle takes ( 25/25 = 1 second),

To calculate the heart rate (beats/min) we divide 60sec over the cardiac

cycle time 60/1=60 beats /minute.

- Example 2:If the RR interval was 15 small squares, What's the heart

rate?

The cardiac cycle takes (15*0.04 =0.6 seconds),

The heart rate = 60/0.6 =100 beats/min.

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-Example 3: If the RR interval was 20 small squares, calculate the heart

rate?

The cardiac cycle takes ( 20*0.04 =0.8 seconds),

The heart rate = 60/0.8 =75 beats/min. Note: for teaching purposes, we usually take the cardiac cycle time as 0.8 seconds

and the heart rate as 75 beats per minute. Normally however, There is normal

variation between people so there could be any other values.

- Advanced example:(you can skip it, it's just a small challenge)

If the heart rate is 150beats/min (obviously abnormal), what is the RR

interval (how many small squares are present between the two

successive R waves)?

1) First we should calculate how much time in seconds does the cardiac

cycle (one heart beat) take:

150 beats 60 seconds

1 beat ?

= 60/150 = 0.4 seconds (cardiac cycle takes 0.4 seconds).

2) Then we calculate how many squares does this time represent in the

ECG paper?

1 second 25 squares

0.4 seconds ?

= 0.4*25= 10 squares.

An important thing to note is that measuring the heart rate is NOT the

aim of doing ECG! It doesn’t make sense that each time we want to

measure the heart rate we should do an ECG! If our aim is just to

measure the heart rate, this is easily done by the following steps

(palpitation):

1- Place two fingers over your radial artery (which is located on the

thumb side (lateral side) of your wrist).

2- When you feel your pulse, count the number of beats in 15 seconds.

3- Multiply this number by four to calculate your beats per minute (or

you can count the number of beats in 30 seconds and multiply by 2).

But if we do an ECG we can:

a) measure the heart rate. (Normal range of heart rate (60-100

beats/min). Below this is bradycardia and above it is tachycardia).

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b) know if the heart rate is regular (rhythmic, all cardiac cycles have the

same duration) or irregular ( Arrhythmia), you can test the regularity of

the heart rate by labelling the two successive R waves and see if the

interval between them fits in each cardiac cycle, if they fit then the heart

rate is rhythmic, if they don’t, it’s arrhythmic.

So, we've learned how to calculate the heart rate, now let's learn how to

use the ECG to measure other intervals:

1) P-R interval:

- Extends from the start of atrial depolarization to the start of ventricular

depolarization -QRS complex- (represents the duration of atrial

depolarization and repolarization).

- Also known as P-Q interval, but because the Q wave is small and may

be absent on most normal ECGs, we prefer to use P-R to standardize the

terminology.

- The importance of this interval is that it represents the conduction of

depolarization between the atria and ventricles (the transmission of

impulses from the SA node down to the ventricles through the AV node).

- Normal duration: 0.16 seconds.

- P-R interval shouldn't exceed 0.20 sec (5 small squares (1 big square)).

If it exceeds this range, this indicates that there's a delay in AV

conduction (AV block).

What is AV block? Go to the next page to find out..

Figure 5

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AV block: it has 3 grades (degrees), in ALL 3 of them, the P-R interval

exceeds 0.20 sec.

- 1st and 2nd degrees are known as incomplete heart block, but 3rd degree

is known as complete heart block.

1st degree heart block: AV node is not conducting the impulse fast

enough. There is some damage in the AV but it still conducts

every beat, every P wave is followed by a QRS complex.

2nd degree heart block: AV node is a little bit more damaged than

the in 1st degree but not completely damaged! So it sometimes

passes the impulses to the ventricles and sometimes does not

(not all p waves are followed by a QRS complex).

-But there is a rhythm, what do we mean by that? This means that

there is regularity of this abnormality (the P-R interval is abnormal

but this abnormality repeats itself -regular irregularity!-)

Sometimes for example the patient has (3:2), what does this

number mean? The first number represents the no. of P waves

and the second represents the QRS complexes, which means that

there are 2 Ps followed by QRS and 1 P is not!

-If it's (4:2), this means that there are 2 Ps followed by QRS (each),

and 2 Ps are not (in other words, there are 2 QRS for every 4 P

waves).

- If it's (2:1), this means that there is one P followed by QRS and

one P is not.

Note: The first number (the number of P waves) is always bigger

than the second one (the number of QRS waves).

-Second-degree AV block is characterized by intermittently

“dropped” QRS complexes (the patient can feel that as dropped

heart beats) at the time AV doesn’t conduct impulse to ventricles.

(3:2) second degree AV block (red arrows = p waves, blue arrows = QRS complexes)

Figure 6

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3rd degree heart block: (complete heart block),

-AV node is completely damaged! The atria and ventricles

are completely dissociated from each other, the nerve

impulse generated in the sinoatrial node (SA node) in

the atrium of the heart does not propagate to

the ventricles.

-Extra note: because the impulse is blocked, and

the electrical conduction system of the heart fails to

stimulate the ventricles, in this case, to prevent cardiac

arrest, what happens is known as ventricular escape beat

which is a self-generated electrical discharge initiated by

the ventricles and causing their contraction.

-The PR interval will be variable (unlike the regularity seen

in 2nd degree), as the hallmark of complete heart block is

lack of any apparent relationship between P waves and

QRS complexes.

- In this case, atrial repolarization can be seen on the ECG

as a downward deflection as explained earlier in the sheet.

- These patients need Artificial Pacemaker, and we implant

it in the right ventricle (not the right atrium since the AV

node is blocked).

2) QRS interval : -It denotes (represents) the depolarization of the ventricles.

- It shouldn’t exceed 0.12 sec (really fast since the spread of

depolarization through ventricles occurs through purkinji fibers and they

are very fast conducting fibers (about 4 meters per second!)).

- This interval may be prolonged (may exceed 0.12 sec) in these cases:

If one of the AV bundle branches is damaged, then the ventricle

on the side where the bundle branch is damaged will receive

impulses through muscles but we know that the conduction

through the ventricular muscles is slow (about 0.5 meters/sec), so

the QRS interval will be prolonged.

In cases of hypertrophy (enlargement) of ventricular muscles such

as: 1) in athletes, which is known as athlete heart syndrome (AHS)

in which we may see prolonged QRS interval with a very high peak

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of the R wave caused by high voltage. And 2) In case of

hypertension.

3- Q-T interval: - Starts from the beginning of the Q wave to the end of T wave (it

represents the duration required for the ventricle to undergo a single

cycle of depolarization and repolarization).

- The Q-T interval is usually half the cardiac cycle (half the R-R interval)

and it’s normally 0.35 seconds.

Now we've understood the ECG waves and intervals, one thing left

which is the ECG segments (take a look back at figure 5).

-A segment is basically an isoelectric line which represents either

complete depolarization or complete repolarization (NO WAVES).

We have two segments on the ECG:

P-R segment: starts from the end of the P wave to the beginning

of the R or Q wave (remember that the P-R interval starts from the

beginning of the P wave not the end, be careful!).

-It resembles phase 2 of atrial action potential (plateau

(completely depolarized atrium)). This segment is short.

S-T segment: starts from the end of the S wave to the beginning of

the T wave (the time between the end of depolarization and the

start of repolarization in the ventricles).

-It represents phase 2 of ventricular action potential (plateau

(completely depolarized ventricle)).

-In the intervals, we used to measure their duration to know if there is

an abnormality or not, but here in segments we don’t care about time or

number of small squares, we care about whether there is a deflection

or not! It's easier to look at the S-T segment since it's more obvious than

the short P-R segment.

-Both elevation and depression of the S-T segment indicate ischemia,

Ischemia may develop into infarction. So, a patient with S-T elevation or

depression on ECG should be kept under control to avoid the

development of MI. Ischemia: decreased blood flow. Infarction: complete stop in blood flow.

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Flow of Electrical Currents in the Chest around the Heart

-The electrical current flows from a negatively charged area to a

positively charged one.

-As we said before, the depolarization of ventricles starts in the

interventricular septum.

-The electrical current spreads this depolarization from the depolarized

area to the still polarized area.

-The last part of the heart that gets depolarized is the posterior aspect

of the left ventricle.

-So, we have many currents that are spreading in different directions

from the depolarized areas to the still polarized areas. Each current is a

vector (has a magnitude and a direction), so having different vectors

means that we must calculate a resultant vector.

-When the heart reaches a point where it is completely depolarized, NO

vectors are present, No currents are flowing (Isoelectric line).

We've understood the ECG paper, but haven’t you guys wondered about

how is the ECG recorded?

To record ECG, we have to look at the electricity of the heart from

different views, because if we only use one galvanometer to record the

electricity of the heart, it will record it but it wouldn't be enough to give

us a complete picture of what's going on in the heart.

Figure 7

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Nowadays, we use a 12-lead ECG (12 galvanometers present in the ECG

machine), they are important in representing the electrical activity of the

heart from 12 different angles, these 12 leads are as the following:

1- Six leads on the limbs.

2- Six leads on the chest.

- Each lead provides a view of the electrical activity of the heart between

two points.

EXTRA: I can see you are wondering, what is an ECG lead?

You can think of it as a connection between two ECG electrodes placed

on two different parts of the body, each electrode is connected through

a wire to the ECG machine, and this whole system is called a lead circuit.

So a lead is basically the indirect connection of two electrodes put on

the body through wires and a galvanometer between them.

-A lead that is composed of two electrodes of opposite polarity (one

positive and one negative) is called bipolar lead.

-A lead that is composed of a single positive electrode and a reference

point is called a unipolar lead.

Now let's start with our topic:

Bipolar limb leads

-There are 3 bipolar limb leads (Leads I, II and III).

- In each lead, we have a galvanometer with two electrodes, the positive

attached to one limb and the negative is attached to another, that's

where its name comes from (bipolarlimb).

- In all leads, we use the right arm, left arm and the left foot. The right

foot is considered an earth (ground lead).

What are the two points that each lead connects together?

1) Lead I: We put one electrode on the right arm and the other on the

left arm.

2) Lead II: We put one electrode on the right arm and the other on the

left foot.

3) Lead III: We put one electrode on the left arm and the other on the

left foot.

Now, on which limb we put the positive electrode and on which limb we

put the negative electrode in each lead?

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Here's a tip to remember where we put each electrode in these leads:

The left foot is always positive and the right arm is always negative.

So, the leads are like this:

But why did we put them this way? Depending on what??

Einthoven, the man who invented this, put the electrodes in these

directions to have positive recordings in all 3 leads. If we reversed the

electrodes in the lab, we would get negative readings, so, the

arrangement of the electrodes in each lead has been internationally

accepted –standardized- .

The 3 bipolar limb leads make a triangle called Einthoven's triangle.

Characteristics of Einthoven's triangle:

1) The heads of this triangle represent the RA (right arm) , LA (left arm)

and LL (left leg) , with the heart at the center.

2) It is an equilateral triangle, which means that:

It is also equiangular (all three internal angles are each 60

degrees)

The centre of this triangle is the centre of a circle that can be

drawn around it.

If we draw perpendicular lines from the centre toward the sides

they will halve them.

Lead I :

- Positive electrode on left arm.

- Negative electrode on right arm.

Lead II :

- Positive electrode on left foot.

- Negative electrode on right arm.

Lead III:

- Positive electrode on left foot.

- Negative electrode on let arm.

Figure 8

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Einthoven’s Law:

It states that the electrical potential of any limb equals the sum of the

other two (+ and - signs of leads must be observed).

L II= L I + L III

-If lead I = 1.0 mV, Lead III = 0.5 mV, then Lead II = 1.0 + 0.5 = 1.5 mV

- Take a look back at figure 8:

Lead I = +0.5 mV

Lead III = +0.7 mV

Then lead II = Lead I + Lead III = +1.2 mV.

The physical basis of Einthoven's law:

Einthoven's law depends on Kirchoff's second law, which states that the

directed sum of voltages in any closed network = 0.

What Einthoven did is that he changed the direction of electrodes in

lead II, so as to make it opposite to the closed circle.

(Look at the following two figures, they will clarify this point).

According to Kirchoff's

law: LI + LII + LIII = 0

Because the three vectors

are forming a closed

network.

This is what Einthoven did,

he changed the direction of

electrodes in lead II:

LI+ LIII + (-LII) = 0

LII = LI + LIII

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We've finally reached the end of this lecture, I tried my best to make it

as clear as possible, hope it was, best of luck guys..

Your colleague, Farah Khreisat..