Share this:

ECG Interpretation

 Watch Free Online Medical Video Lecture on ECG Interpretation

Part 1


Part 2


Part 3


Part 4


What is Electrocardiography?

Graphical representation of the electrical activity of the heart is called electrocardiography.

Function of Electrical Activity in Ventricular Myocardium

Resting membrane potential (RMP) is the electrical voltage difference between inside and outside of the membrane.

When the myocardial cell is not stimulated, it is in state of resting membrane potential (RMP). Normally, resting ventricular myocardial cells are electrically negative inside.

For example; RMP of ventricular myocardial cell is -90mV, it means that in the resting condition, the myocardial cell internally has negative polarized membrane. As soon as myocardial cell is stimulated, some cations will move into to the cells, this positive charged ions moving into the cells will take RMP towards less negative state, because cations will neutralize the electronegativity.

Threshold Potential – Definition and How to Determine THP

Thresh hold potential (THP) is a potential at which voltage gated Na-channels in membrane suddenly open, while in RMP that voltage gated Na-channels were closed.

After stimulation to the cell, voltage starts moving towards Thresh hold potential (THP) that is -70mV in ventricular myocardial cell. As soon as it touches THP, voltage gated Na-channel opens and lots of Na comes in and progressively neutralize the electronegativity. THP will become -60mV then -50mV, -40mV, and eventually becomes 0mV. This membrane potential is progressively going to positive value e.g.: +10mV. Now we can say that membrane lost its negative polarity and depolarized.

After depolarization of the cell, depolarization sensitive channels. i.e., Ca and K-channels, both are activated. K starts rushes outside because intracellular K is normally more as compared to extracellular K. When cell starts losing cations (K), voltage comes toward electronegativity but as soon as it becomes zero, Ca-channels also open. So for very short time, K keeps on moving out and Ca keeps on moving in, this loss and gain of the cations, no net change occurs in potential. As there is no voltage difference, we can say electrical potential is showing Plateau phase.

After sometime, Ca channels close and K-channels become more and more active. Due to heavy K-efflux, K is progressively lost until membrane potential becomes -90mV and now membrane is repolarized. ECG interpretation can be fun if you have clear concepts.

Gap Junction (Electrical Windows) Between Adjacent Myocardial Cells

These are very special connections between the adjacent myocardial cells, and also called electrical windows.

The cations (Na) from the first stimulated cell trickle into the next cell through these gap junctions, taking the voltage of the next cell towards the THP, then opening of the voltage gated Na-channels results in depolarization of the membrane. Then, the voltage gated Ca and K-channels open simultaneously for a short time, eventually Ca-channels are closed but K-channels keep on opened, taking the membrane potential to its resting potential, this is called repolarization of a membrane Thus, depolarization of one cell leads to the depolarization of the second cell and second cell is responsible for the depolarization of the third cell and so on.

This fluctuation of electrical potentials in membrane is called Action potential.

i) In atrial/ventricular myocardial cells, Na influx cause depolarization

ii) In SA/AV nodal cells, Ca influx cause depolarization.

How ECG Interpretation Machine (Galvanometer) Records Electrical Activity of the Heart?

Galvanometer – Introduction and Functions

ECG machine works on the principle of galvanometer. Galvanometer has positive and negative electrodes. Function of galvanometer is to detect the electrical activity in any object where its electrode are placed

i) When there is electrical activity in the myocardial cells, the needle of the galvanometer will fluctuate

ii) When there is no electrical activity in the myocardial cells, the needle of the galvanometer will remain at neutral position or at zero position.

When myocardial cells are stimulated, voltage goes towards THP; cells are depolarized and become electrically positive. The wave of depolarization moving from negative pole to the positive pole produces electromagnetic force represented by a vector

How Cardiac Vectors Represents the Electrical Activity of the Myocardium?

The vector represents the electrical activity of the myocardium.

Head of the vector points towards the direction in which charges are moving.

Length of the vector is directly proportional to the mass of the myocardium or number of myocardial cells

If we stimulate a thickened myocardium so the length of the vector increases.

Ventricular myocardium is thicker as compared to atrial myocardium

For example; when depolarization takes place in atrial myocardium (thin myocardium), small positive deflection will be produced and when depolarization takes place in ventricular myocardium (thick myocardium), large or strong positive deflection will be produced

During depolarization, positive charges move in and cells become electropositive, then positive charges (wave of depolarization) move towards the positive electrode, needle will deflect positively.

After complete depolarization of myocardial cells from one point to another, there is no further movement of the charges in the cells, vector disappears and needle will go back to its neutral position.

Whenever positive charges move away from the positive electrode, needle will deflect negatively.

During repolarization, K-efflux occurs and cells become electronegative. Now negative charges in the cells start moving towards the negative electrode, producing positive deflection of the needle.

Note: Similar charges moving towards similar electrode will produce positive deflection.

Electrical Activity of the Heart (Fast Conduction VS Slow Conduction)

Through some myocardial tissues current passes slowly or with moderate velocity and through other myocardial tissues current passes very rapidly.

Stimulation of Myocardial Tissues With Slow Electrical Conduction

a) These tissues consist of myocardial cells having gap junctions.

b) These cells don’t have any specialized conduct system.

c) One cell stimulates the second cell through gap junctions, and then this second cell stimulates the third one and so on.

d) Movement of depolarization takes place with moderate velocity.

e) If we stimulate this type of myocardial tissue, vector is produced from one point to another. Positive charges moving towards positive electrode produce positive deflection but with moderate speed.

Stimulation of Myocardial Tissues with Fast Electrical Conduction

These tissues consist of myocardial cells having also gap junctions, i.e. electrical windows.

There are special cells having following characteristics:

a) Wide diameter.

b) Large in size

c) Less resistance to conduction

d) More gap junctions

This fast conducting network of myocardial cell is called Purkinje cells

If we stimulate this type of myocardial tissue, vector is produced from one point to another. Positive charges moving towards positive electrode produce positive deflection but with high velocity.

Conversion of Electrical Events into Cardiac Vector and Then into ECG Pattern in One Cardiac Cycle

Let’s discuss how the electrical events in the heart translate into cardiac vectors and how these cardiac vectors are translated into ECG patterns?

At the beginning of cardiac cycle, SA nodes fires and generates the wave of depolarization then the cells adjacent to SA node are stimulated first and undergo depolarization. These cells lead to the depolarization of second group of the cells and second group of the cells lead to depolarization of third group of the cells and so on.

Wave of depolarization sweeps from SA node to within myocardium downward and leftward; hence we can say atria are depolarized downward and leftward because origin of depolarization i.e. SA node is present on the right upper side of the right atrium

The small vectors generated due to the wave of depolarization in atria move downward and leftward. All these vectors or depolarizing currents can be added together making a single vector. This vector represents the electromagnetic forces produced in the heart. This vector will be small with moderate velocity because atrial myocardium is less thick and do not have any specialized conduction system.

Atrial depolarization is the first electrical event in the heart during cardiac cycle

When both atria are completely depolarized, the depolarizing waves will hit the fibrous annulus which provided insulation between atria and ventricles. So most of the depolarizing current hits the fibrous tissue and terminates. The only single point from where depolarization can pass through to ventricles is AV node.

Those electrical depolarizing waves which hit the AV node, will be taking the current gradually to the ventricles. Hence, second electrical event is stimulation of AV node.

Function of AV Node (Atrioventricular Node) – A Modified Myocardium

AV node is modified myocardium i.e., specialized in slow conduction. When current passes through AV node, it takes about 0.01 seconds just to reach from atria to ventricles

When atrium is contracting, current is held in AV node for a while, so that atria can complete their contraction and fill the ventricles. After completing atrial contraction, ventricle should contract

AV Node (Slow Conduction System) VS Purkinje System (Fast Conduction System)

Why AV Node is Slow Conduction System?

I. Abundantly presence of small cells

II. These cells are arranged right angle to the direction of current flow

III. Less number of gap junctions

IV. Depolarization of AV nodal cells is Ca dependant

V. Less diameter of cells offer more resistance

VI. RMP is -60mV

Why Purkinje System is Fast Conduction System?

I. Large cells and less in number

II. More numbers of gap junctions.

III. Depolarization of Purkinje cells is Na dependent.

IV. Wide diameter, it offers less resistance within the cells.

V. RMP is -90mV (Cations move rapidly into the cells due to more electronegativity)

Current moving through AV node produces a very small vector, because AV node is a tiny myocardial tissue. As this vector is very small, ECG interpretation machine will not pick that up and we can say electrical silence in the heart.

Current from AV node passes to the Purkinje system by bundle of His. The bundle branches are made of specialized myocardial cells called Purkinje cells. When current is released from AV node into Purkinje system, ventricular depolarization takes place consisting of three steps:

1. Septal depolarization.

2. Major ventricular depolarization.

3. Basal depolarization

3 Steps of Ventricular Depolarization Occurrence

Septal Depolarization

Upper part of septum is fibrous and lower part is muscular. Covering of fibrous tissue around the bundle branches act as an insulator

Septal myocardium is stimulated by the left bundle branch (LBB), because there are some connections arising from the LBB which give stimulation to the septal myocardium

Right bundle branch (RBB) takes the current downward but it does not stimulate septum directly

First, there will be depolarization of left lower side of the interventricular septum. So the wave of depolarization moves from left and lower portion of the septum towards right and upper portion The vector representing electrical activity of septal depolarization moves rightward and upward. This is a small vector showing fast conduction

Ventricular Depolarization

The wave of depolarization comes from the bundle branches and their Purkinje fibers to the ventricular myocardium. Purkinje cells are present in the inner myocardium. Waves of depolarization move from inner myocardium to outer myocardium

As we know that left ventricle is three times thicker than right ventricle, so naturally depolarizing current produced in left ventricle is strong as compared to right ventricle. Hence the vector represented by the depolarization of left ventricle is strong i.e. larger as compared to the vector of right ventricular depolarization.

When multiple vectors are produced simultaneously they can be added together, so when left ventricle undergoes depolarization, we can make a single vector moving downward and leftward. On the other hand when right ventricle undergoes depolarization, a single vector is obtained that moves downward and leftward. By adding both vectors, we get a single vector that is called net vector representing the major part of both ventricles depolarization and this vector moves downward and leftward

Basal Depolarization

After septal and major ventricular depolarization, the wave of depolarization will reach to the basal area. In this area, current moves upward and leftward

There will be a small, fast vector representing basal part of the ventricles depolarization

Atrial Repolarization

When ventricular depolarization starts, atria undergo repolarization, so repolarizing current of the atria is masked by the depolarizing activity in the ventricles

Summary of Electrical Events in the Heart During One Cardiac Cycle

1.Atrial depolarization: Small vector moving downward and leftward with moderate speed

2.AV nodal silence

3.Ventricular Septal depolarization: Small vector moving rightward and upward with high speed

4.Major ventricular depolarization: Strong vector moving leftward and downward with high velocity

5.Basal ventricular depolarization: small vector moving rightward and upward with high velocity

6.Atrial repolarization: is masked by the depolarizing activity in the ventricles

We apply positive electrode on the left foot and negative electrode on right arm.

Applying the electrode to the left foot is equal to applying the electrode at the junction of left lower limb and the trunk. In the same way, applying the electrode to the right arm is equal to applying the electrode at the junction of right upper limb and the trunk

During one cardiac cycle, electrical vectors are produced due to electrical currents and these currents are very faithfully conducted to the body surface through body fluids. We hope you like this article on ECG interpretation.

Fluctuation of the Needle Due to Electrical Events in the Heart During One Cardiac Cycle

1.If there is no electrical activity in the heart, no vector is produced and needle will remain at the neutral position making straight line on ECG paper

2.After firing of SA node, atrial depolarization produces a small vector moving downward and leftward with moderate speed. As positive charges move towards positive electrode so needle will move upward (positive deflection) with moderate velocity making a small wave that is P-wave that represents atrial depolarization

3.After atrial depolarization, AV nodal depolarization produces very small vectors and electrodes don’t pick up any activity, so needle will remain at the neutral position making straight line on ECG paper. This straight line represents that current is passing through AV node

4.After nodal depolarization, septal ventricular depolarization takes place and produces a small vector moving rightward and upward with high speed. As positive charges move away from positive electrode so needle will move downward (negative deflection) with a high speed making a small wave i.e. Q-wave that represents ventricular septal depolarization

5.After ventricular septal depolarization, major ventricular depolarization takes place and produces a strong vector moving leftward and downward with high velocity. As positive charges move towards the positive electrodes so needle will move upward (positive deflection) with high speed making a strong wave (big wave, high amplitude) that is R-wave that represents major ventricular depolarization

6.After major ventricular depolarization, last event i.e., basal ventricular depolarization takes place and produces small vector moving upward and rightward with high speed. As positive charges move away from positive electrodes so needle will move downward (negative deflection) with high speed making a small wave that is S-wave that represents basal ventricular depolarization

7.These three waves; Q, S and R waves are collectively called QRS complex that represents the ventricular depolarization

Repolarization of Heart

After depolarization of ventricles there is onset of repolarization. Outer myocardial cells start losing K+ to extracellular fluid;

cells recover electro negativity and are repolarized. Rule of repolarization is, the part which depolarizes last repolarizes first.

Repolarization current is moving out to in, and net vector of repolarization is moving rightward and upward.

Why Outer Myocardium Repolarizes First?

During QRS and ST segment, ventricles are in systole. As blood flow is maintain better in outer myocardium during the systole so repolarization

start from outer myocardium. Repolarization is a slow process as compared to the depolarization because Na+ channels are fast channels but K+ channels are not as fast.

Repolarization Vector

When septum has started its repolarization, before it has repolarized completely, major ventricular repolarization and basal ventricular repolarization takes place. All repolarization currents are overlapped in the time and net current make a single important repolarization vector which is moving rightward and upward carrying negative current in its head.

This negative vector is directed towards the negative electrode, so deflection of the needle will be upward. Repolarization is the slow process; a positive but gradual wave is produced. This wave is called T wave and it represents ventricular repolarization.

Terminologies Used in ECG Interpretation


P wave represents atrial depolarization.


This straight line represents the conduction of cardiac impulse through the AV node.Here electrical activity is so small to be detected by the electrodes and ECG needle will not fluctuate, drawing a straight line.

QRS Complex

QRS complex represents the complete ventricular depolarization


Q wave represents the ventricular septum depolarization.

R wave represents the major ventricular depolarization.

S wave represents the basal ventricular depolarization.


This segment represents the plateau phase, i.e., no net electrical activity and ECG needle will not fluctuate drawing a straight line on the ECG paper.


T-wave represents the ventricular repolarization.

ECG Waves – Overview and its Types

Waves are the patterns drawn on the ECG paper due to fluctuation of the needle.

P Wave

In the normal ECG, first wave is P wave which represents the atrial depolarization. Atrial repolarization doesn’t make a wave because at that very moment ventricular depolarization is making QRS complex. So, atrial repolarization current is masked by ventricular depolarization current.

QRS Complex

The next is QRS complex or QRS interval of waves which represent the spread of depolarization in ventricles.

T Wave

Then there is a T wave representing the ventricular repolarization.

ECG Segments – Overview and its Types

The segments are the straight lines drawn on the ECG paper, when there is no fluctuation of the needle.

PR Segment

PR segment is the straight line that starts at the end of the P wave and ends at the beginning of QRS complex.

ST Segment

ST segment is the straight line between the end of the QRS complex and the beginning of the T wave. ST segment is drawn when spread of ventricular depolarization has terminated and yet rapid phase of repolarization has not started. Cations going in and out in this plateau phase without producing any significant electrical activity in the heart. So needle doesn’t fluctuate drawing the straight line. ECG interpretation is fun once you have crystal clear concepts.

ECG Intervals – Overview and its Types

Intervals can be defined in many ways as:

•The sum of wave and segment is called interval


•More than one wave together are called interval

PR Interval

PR interval is from beginning of the P wave up to the beginning of the QRS complex. It includes two electrical activities; atrial depolarization and conduction through AV node.

QRS Interval

QRS complex is also regarded as interval because there is more than one wave. QRS interval is a duration during which the current is spreading over the complete ventricular tissue.

QT Interval

QT interval is measured from the beginning of the QRS complex to the end of the T wave. It starts with the beginning of ventricular depolarization and terminates with the end of ventricular repolarization. During this time, action potential is generated and terminated in the ventricular tissue. This is the time when ventricle systole occurs.

U Wave

This is the wave in the ECG that follows the T wave in some individuals. U wave is produced due to the delayed repolarization of papillary muscles. Master ECG interpretation.

Calculation of Time and Duration of Waves, Segments and Intervals of ECG – EKG Patterns

ECG patterns are drawn by the machine on a special calibrated paper. ECG paper is calibrated with vertical and horizontal lines which divide the paper into big squares and small squares. During one minute, 300 big squares pass under the pointer’s needle. All the electrical activity of the heart during one minute will be drawn over 300 big squares.


300 big squares = 1 mint

300 big squares = 60 sec

1 big square = 0.2 sec

1 big square = 5 small squares

5 small squares = 0.2 sec

1 small square = 0.04 sec

“So, 1 small square on ECG paper is equal to 0.04 seconds.”

ECG Reading of Normal Heartbeat and Heart Rhythm of 72 beats per minute (bpm)

P Wave

P wave is drawn over two and half small squares. P wave duration is normally 0.1 seconds. It means current from SA node took 0.1 seconds to spread over the both atrias.

PR Segment

PR segment is drawn over two and half small squares. PR segment has duration of 0.1 seconds. It means current passes through the AV node in 0.1 seconds.

PR Interval

PR interval includes both P wave and PR segment. It is drawn over five small squares. PR interval duration is 0.2 seconds.

QRS Complex

QRS complex is drawn over two and half small squares, so duration of QRS complex is 0.1 seconds. It means spread of depolarization in ventricles takes 0.1 seconds. When was the last time you learned ECG interpretation like that? Knowing ECG interpretation is not enough you must master it. Your concepts can decide the future of your patient.

QT Interval

QT interval is drawn over 10-11 small squares, so duration of QT interval is 0.40-0.44 seconds. It means whole electrical activity of ventricles takes 0.40-0.44 seconds. We hope you liked our video on ECG interpretation. Please share this video on ECG interpretation with your friends. ECG interpretation Made Easy.

Students from Top Schools are Watching Our Videos!

ecg interpretation
We don’t teach just ECG Interpretation. We teach all Medical Sciences. Not a member? Sign up now to access 700+ videos on Basic Medical Sciences & Clinical Medicine. Free Access to new videos every week. Master Medical Sciences with crystal clear concepts.

About Dr. Najeeb Lectures.

Dr. Najeeb Lectures are the World’s Most Popular Medical Lectures. Our videos are being watched by millions of Medical, Dentistry, Nursing and Pharmacy students in over 100 countries.

We have 700+ videos on Anatomy, Neuroanatomy, Embryology, Histology, Physiology, Biochemistry, Genetics, Pharmacology, Microbiology, Immunology, Pathology and Clinical Medicine all recorded by Dr. Najeeb. This video on ECG interpretation has been watched over 1 Million times.

What makes us different?

We use hand-drawn illustrations to empower your learning process because we believe lasting knowledge results from clear concepts. Our videos are interactive and fun to watch so you don’t have to spend hours memorizing.

They say a picture is worth a thousand words, we have integrated that philosophy in our every lecture. We are Revolutionizing Medical Education through hand-drawn illustrations. One concept at a time. Just like we did in this video of ECG interpretation. This article on ECG interpretation is brought to you by Dr. Najeeb Lectures.


World's Most Popular Medical Lectures.

700+ videos on Basic Medical Sciences.

LIBRARY              SIGN UP