Snímka 1

Electrocardiography basics

This presentation will deal with the

basics of ECG description as well as

the physiological basics of

Snímka 2 Lecture overview

1. Cardiac conduction system

– functional anatomy

2. ECG basics

– Lead placement

– peaks and waves

– electrical axis

– conduction system and ECG

3. ECG description methodology

– approaches to ECG description

– Example of ECG description

In the first section we will be talking

about the heart conduction system and

how is electric impulse transmitted

through cardiac muscle.

Understanding this point is important

in order to understand the

electrocardiography itself. In the

second part, we will try to explain

what peaks and waves

electrocardiogram (ECG) consists of.

In addition, conduction system in

relation to ECG will be described.

Lastly, the ways of ECG description

are going to be introduced.

Snímka 3 1. Cardiac conduction system

The components of cardiac

conduction system includes:

• Sinoatrial node

• Atrioventricular node

• His bundle

• Left bundle branch

- Left anterior fascicle

- Left posterior fascicle

E. Right bundle branch

F. Purkinje fibers

Snímka 4

Sinoatrial

node

Atrioventricular

node

1. Cardiac conduction system

Action potential and the subsequent

electrical impulse is created in

sinoatrial node which is also referred

to as primary heart pacemaker, and

further propagated into

atrioventricular node. Here the

electrical impulse is slowed down in

order to let the ventricles fill with

blood due to atria contraction, and

further transmitted to His bundle and

left and right bundle branches, where

the later one speeds up the electrical

impulse again, so all muscle fibers in

ventricles are stimulated/depolarized

at the same time.

Snímka 5 2. Electrocardiography basics

The heart generates its own electrical

signal as mentioned before, (also

called an electrical impulse), and this

can be recorded by placing electrodes

on the chest or limbs. This is called an

electrocardiography and the result of

electrocardiography is

electrocardiogram.

Snímka 6 2. Lead placement

The standard 12-lead

electrocardiogram is a representation

of the heart's electrical activity

recorded from electrodes on the body

surface

Generally, we recognize the so called

bipolar and unipolar leads. To bipolar

leads, the standard limb leads I, II,

III (full circle) belong. The unipolar

leads include the augmented leads

aVR, aVL and aVF (dashed circle)

and chest leads V1-V6 (dotted

circle).

Snímka 7 2. Bipolar lead placement

For bipolar leads, we place electrodes

as follows:

The red color electrode goes to right

arm, yellow goes to left arm, green

goes to left foot, and black (which is

earthed electrode) goes to right foot.

The standard lead I looks to electrical

activity of heart from the right arm to

left arm (or lateral direction), lead II

looks to heart from right arm to left

foot (or left superior to inferior

direction), and lead III reflects the

electrical activity from left arm to left

foot (or right superior to inferior

direction).

Note: The colors of electrodes are

typical and standard for a particular

region, i.e. in Europe. Nevertheless,

in other regions (e.g. in USA) these

colors may be different, but they

remain the same throughout the

region.

Snímka 8 2. Unipolar lead placement

Augmented leads are named as aVR,

aVF and aVL, where

a stands for augmented

V stands for voltage

R stands for right arm

L stands for left arm

F stands for foot

Practically, the augmented leads are

„hidden“ inside the standard leads, so

the correct positioning of standard

limb leads (and vice versa) ensures

also the correct position of augmented

leads. Augmented leads looks to

electrical activity of heart from frontal

plane.

Snímka 9 V1

V6

2. Unipolar leads placement

The unipolar chest leads (or

precordial leads) look to electrical

activity of heart from the horizontal

plane. The proper positioning of chest

leads ensures the proper interpretation

of ECG. The positioning of electrodes

is as follows:

V1 (red color electrode) - right

4th intercostal space

V2 (yellow color electrode) - left

4th intercostal space

V3 (green color electrode) - halfway

between V2 and V4

V4 (brown color electrode) - left

5th intercostal space, mid-clavicular

line

V5 (black color electrode) -

horizontal to V4, anterior axillary line

V6 (violet color electrode) -

horizontal to V5, mid-axillary line

Note: The colors of electrodes are

typical and standard for a particular

region, i.e. in Europe. Nevertheless,

in other regions (e.g. in USA) these

colors may be different, but they

remain the same throughout the

region.

Snímka 10 2. Electrocardiogram

J point

Generally, there are 2 waves and 3

peaks on the ECG. The waves

represents the P and T wave, peaks

are Q, R, and S. Besides we also

recognize (important for ECG

description) PQ/PR segment, ST

segment, QT interval and RR interval.

PQ/PR segment – from the beginning

of P wave to Q peak

ST segment – from the end of S peak

(J point) to the beginning of T wave

QT interval – from the beginning of

QRS to the end of T wave

PQ/PR segment – from the end of P

wave, to the beginning of QRS

complex

RR interval – distance between 2

consecutive R peaks

Note: Sometimes after T wave,

another smaller wave can be found.

This is a U wave, and its origin is not

clear. However, it is hypothesized,

that this U wave represents either

„afterdepolarizations" in the

ventricles, or repolarization of

interventricular septum.

Snímka 11 2. Principle of electric activity detection

• The reference point is

electrode

• If the depolarization

wave is moving towards

electrode, the amplitude

is positive

• If the depolarization

wave is moving away

from electrode, the

amplitude is negative

What is important for electric activity

detection, is the fact, that the

electrode is a reference point.

Whenever the electric impulse

(depolarization wave) travels towards

this electrode, the amplitude of the

ECG amplitude (peak or wave) is

mostly positive (upwards on ECG).

Whenever the depolarization wave

travels away from this electrode, the

ECG amplitude (peak or wave) is

mostly negative (downwards on

ECG).

Note: Since we have a 12-lead ECG,

each lead represents a reference point.

In other words, we have 12 reference

points, each looking on heart from

different side. Since the ventricle

myocardium has the largest amount of

muscle fibers and therefore electrical

activity, these rules reflect mainly the

ventricle depolarization through QRS

complex.

Snímka 12 2. Principle of electric activity detection

• If the electrode is in the

middle of the electrical

axis, the amplitude is

biphasic (postive and

negative)

When this electrode is in the middle

of electrical activity of heart, the

amplitude is biphasic (both negative

and positive). The reason for this, is,

that the impulse travels to the

electrode (positive amplitude) and

then it turns away from the reference

point (negative amplitude).

Note: Since we have a 12-lead ECG,

each lead represents a reference point.

In other words, we have 12 reference

points, each looking on heart from

different side. Since the ventricle

myocardium has the largest amount of

muscle fibers and therefore electrical

activity, these rules reflect mainly the

ventricle depolarization through QRS

complex.

Snímka 13 2. Principle of electric activity detection

The above mentioned can be best

presented on this ECG. Look for the

precordial leads V1 to V6. Each lead

represents one reference point.

V1 is positioned in the 4th intercostal

space parasternally right (see previous

slides) in the region of sinoatrial

node. This means that the electrical

impulse or depolarization wave would

move away from this electrode.

Therefore QRS complex in V1 would

be mostly negative.

On the other hand, lead V6 is

positioned in 5th intercostal space

midline axillary line, in the region of

lateral-posterior wall of left ventricle,

therefore the QRS complex would be

mostly positive.

Note: The lead V3 has equally

positive and negative amplitude in

QRS complex. This is because the

lead lies exactly in the middle of

electrical activity, thus the impulse

travels equally towards and equally

away from the V3 reference point.

This is referred to also as to

transition zone.

Snímka 14 2. Electrical axis• Is given by the sum of

vectors of electrical potentials at the given time

• The heart's electrical axis refers to the general direction of the heart's depolarization wavefront (or mean electrical vector) in the frontal plane.

RA LA

LF

II

I

III

The electrical axis is a sum of

electrical potentials at a given time.

Under normal circumstances, the

impulse travels from sinoatrial node

(right up side) to the left ventricle

(left bottom side). This is the normal

electrical axis of the heart, reflected

by the QRS complex in standard and

augmented unipolar leads, reflected

by the most positive QRS complex

which would be in lead II.

Snímka 15 2. Electrical axis

A panel• lead I & aVF are +ve = normotype• lead I & aVF are -ve = os v NW zone• lead I -ve & aVF are +ve = rightward axis

• lead I +ve & aVF is -ve - than the lead II is important

B panel • lead II +ve = normotype• lead II -ve = leftward axis

Approach to electrical axis

determination

+ve stands for positive QRS complex

- ve stands for negative QRS complex

Firstly, start with A panel, when the

last criteria (lead I +ve and aVF –ve)

is met, then continue to panel B.

Snímka 16

Applying the above mentioned rule,

this is the normal electrical axis. The

lead I and lead aVF, both are +ve

Snímka 17

P

Q

R

S

T

2. Conduction system and ECG

The conjunction of impulse

conductance through heart muscle to

ECG output is as follows:

Sinoatrial node depolarizes and send

electrical impulse to atrioventricular

node. Atria depolarize. This is on

ECG represented by the P wave. In

the atrioventricular node, the impulse

is slowed down, in order to let the

ventricles fill with blood from atria

contraction. This on ECG is

represented by PQ segment (from the

end of P wave to beginning of QRS

complex). The electrical impulse is

then quickly propagated through the

His bundle to both bundle branches

and ventricle myocardium. Fast

ventricle depolarization is reflected

by the QRS complex, where Q

represents depolarization of

interventricular septum. RS then

depolarization of the ventricles

themselves. After ventricle

depolarization, ventricle

repolarization occurs. This is

represented by ST segment on ECG.

Snímka 18

P

Q

R

S

T

2. Conduction system and ECG

Snímka 19 3. Electrocardiogram – description methodology1. Rhythm

– sinus, non-sinus – atrial fibrillation?, AV junction?, malign rhythm?

2. Action

– regular, irregular

3. Heart rate

4. Electrical axis

– rightward, leftward, normal

5. Conduction times PQ, QRS, QT

6. Deformities of P, QRS

– mitral P, biphasic P, QRS deformities – rSR config., delta wave

7. Transition zone in precordial leads

8. ST segment

– isoelectric, depression and elevation

9. T wave

– positive, negative, flattened, peak

10. Other

This represents a step by step guide

how to correctly describe ECG

without possibility to omit some

important stuff. This detailed ECG

description is a need-to-know basis

for all medical students.

1. First point includes the rhythm, the

basic rhythm is a sinus rhythm. The

definition of sinus rhythm is when we

see a P wave in front of each QRS

complex in any, but at least one lead.

If You cannot see a sinus rhythm, then

think describe the rhythm as non-

sinus. (The special attention goes to

so-called malign rhythms, which

include asystole, pulseless electric

activity, ventricular tachycardia and

ventricular fibrillation. This is

because a patient with such rhythm

requires immediate cardiopulmonary

resuscitation).

2. Secondly, action is described. This

can be either regular or irregular. You

can find out regularity by counting

RR distance between 2 consecutive

QRS complexes, in at least 3 different

places within one lead. If the distance

is all the same, the action is regular

3. Heart rate, can be calculated by

formula 300 divided by [number of

big (5mm) squares between two

consecutive R peaks]

4. Electrical axis is determined by the

formerly mentioned rules.

5. Conduction times in PQ, QRS, and

QT segments/intervals. One mm on

ECG paper represents 0.04s, if the

paper is moving with speed 25mm/s.

The PQ interval should for example

measure between 0.12-0.20 s, what

means 3-5 mm on ECG paper.

6. Deformities of P wave and QRS

complex– in each lead we determine,

whether the P and QRS are not

deformed. The P wave for example

may be mitral, biphasic or peaked, on

the other hand, the QRS may be

deformed by delta wave, or may have

a RSR configuration (please refer the

next chapter).

7. Transition zone in precordial leads

– we trie to find where is the middle

of the electrical axis, i.e. where is the

QRS complex equally positive AND

negative.

8. ST segment evaluation – we try to

find in each lead separately, whether

the ST segment isoelectric, depressed

or elevated.

9. T wave configuration – T wave

may be positive, negative, flattened or

peaked. Again, we observe T wave in

each lead separately.

Snímka 20 3. Electrocardiogram – description methodology

1. Heart rate

(slow-normal-fast)

2. Rhythm

(regular-irregular)

3. QRS width

(narrow-broad)

4. ST segment

(elevation, depression)

The above mentioned method is an

ultimate method for ECG description.

It represents complete view, however,

requires much skill, experience, and

still is also time consuming.

Nowadays, in hastened world, much

faster but safe methods of ECG

evaluation were developed. For

clinical practice, especially when You

are not going to be an internist or

cardiologist, this method should be

sufficient to exclude most life

threatening situations.

• You look to heart rate and You try

to answer the most relevant

question...is it slow, normal, or

fast?

• You look for rhythm – is it regular

or irregular? If irregular, then

arrhythmias may be the answer

• QRS width – if the QRS complex

is widened, then this could mean a

bundle branch blockade

• ST segment – is it elevated or

depressed? ST segment points

towards cardiac ischemia (ST

depression) or myocardial

infarction (ST elevation).

Answering these 4 questions should

rule out or confirm the most

dangerous ECG diagnosis. Still, if it

is not Your specialty and You are in

doubt, call a specialist.

Snímka 21

ECG description for surgeons

ECG description methodology for

surgeons...

NB: more of a joke, so please do not

take this slide seriously

Snímka 22 3. Some physiological values

• PQ interval – 0.12 – 0.20 sec.

• QRS complex < 0.12 sec

• QT interval - < 0.36 sec

– QTc = (QT/RR2) = <0.44 sec

• Heart rate – 60-90 beats per minute

• Transition zone – V3, V3/V4, V4

Just some basic physiologic values,

You should be aware of, when want to

describe the ECG.

Snímka 23

300/ 4 = 75 bpm

4

Example ECG the description step by

step

Please find attached ECG protocol

In this case:

• Sinus rhythm (P wave is in front

of each QRS complex), best seen

in lead II

• Regular action (distance between

RR in lead II is 20 mm

• Heart rate is 75/min (there are 4

five mm squares between two R

peaks, what means 300/4 = 75)

• Normal electrical axis

• PQ interval is 0,2sec (5 small

squares), QRS interval is 0,1sec

(2,5 small square) and QT interval

is 0,36sec (9 small squares)

• P negative in aVR (however aVR

is a mirror reflection of lead II,

therefore, no abnormalities are

observed). No QRS abnormalities

in all leads

• Transition zone is in V3 (QRS

equally positive and negative)

• ST segment is in isoelectric line,

seems to be elevated by 2mm in

V2

• T wave flattened in aVL,

otherwise positive

• Little high-frequency noise in

lead III.

The final ECG description should

read: sinus rhythm, regular action, HR

75/min, normal axis, PQ0,2s, QRS

0,1s, QT 0,36s, P and QRS without

deformities, transition zone V3, ST

segment elevation in V2 by 2mm

otherwise isoelectric, T wave

flattened in aVL otherwise positive,

noise in lead III. Conclusion: normal

ECG.

Snímka 24 Interesting ECG - STEMI

Snímka 25 Interesting ECG - STEMI

Snímka 26 Interesting ECG – Ventricular tachycardia

This is a 12-lead ECG with

monomorphic ventricular tachycardia.

Although it might be difficult to

describe this ECG according to 10-

points description, it is easy to

describe with the 4-point description.

• Heart rate Fast/Slow? – fast

(approx. 130/min)

• Rhythm regular/irregular? –

regular

• QRS width narrow or broad? –

broad

• ST elevation, yes/no? – cannot

determine

Result: broad QRS complex

tachycardia, i.e. ventricular

tachycardia. Since QRS complexes

look the same, it is monomorphic

tachycardia. HR is around 130/min,

the patient will be most probably

stable, i.e. suitable for pharmacology

conversion.

Snímka 27 Interesting ECG – Ventricular fibrilation