Essentials Of Electrocardiography (ECG)

Electrocardiography is the recording of the electrical impulses that are generated in the heart.

Electrodes are used to pick up these electrical impulses. There are altogether 12 electrodes, hence called as 12 lead ECG.

For a better understanding of ECG, it is important to learn about the conduction system of the heart. Click here to learn about the anatomy of the heart.

ECG Leads

There are 12 ECG leads.

  • Limb leads
    • Bipolar leads / Standard leads
      • Lead I
      • Lead II
      • Lead III
    • Unipolar leads /Augmented leads
      • aVR
      • aVL
      • AVF
  • Chest leads
    • V1 to V6

ECG lead placement

Recording Of The ECG

An ECG tracing is plotted on a ECG tracing paper.

ECG tracing paper

How A ECG Tracing Is Formed?

Each lead looks at the heart in different angles. Chest leads looks at the center in the coronal plane and the limb leads looks at the heart in the horizontal plane.

  • Lead I – Difference in voltage between the left arm and the right
    arm electrodes
  • Lead II – Difference in voltage between the left leg and the right arm
  • Lead III – Difference in voltage between the left leg and the left arm electrodes
  • Septal leads – V1, V2
  • Anterior leads – V3, V4
  • Lateral leads – V5, V6
  • High lateral leads – Lead I, aVL
  • Inferior leads – Lead II, Lead III, aVF

Check out this video to see how positive and negative deflections occur in an ECG.

Normal ECG Morphology

There are several components of an ECG record, that we have to know about.

  • P wave – Atrial depolarization
  • PR interval – Time duration of the depolarization wave to travel from the atria to the ventricles
  • QRS complex – Ventricular depolarization
  • ST segment – Time duration between ventricular depolarization and beginning of repolarization
  • T wave – Ventricular repolarization
  • QT interval – Total ventricular activity
  • U wave – Repolarization of Purkinje fibres


Normal ECG

ECG Tracing Analysis

An ECG tracing is a long strip, therefore it should be analyzed in a systematic manner. The sequence of analysis is mentioned below.

  • Look for standardization and aVR
  • Rhythm
  • Rate
  • Cardiac axis
  • P wave
  • PR interval
  • QRS complex
  • T wave & ST segment
  • QT interval
  • Evaluate other components
    • U wave
    • Ectopic beats
  • Interpretation of the ECG

Standardization And aVR

This represents proper calibration of the ECG machine and is included in every ECG tracing. A standard signal of 1mV is shown as 10 small squares.

aVR should always show inverted waves, except in dextrocardia or in faulty attachment of leads.

Standardization of an ECG


Assessed by measuring the R-R interval and P-P intervals

  • Regular
  • Irregular
    • Regularly irregular
    • Irregularly irregular


There are two formulas to determine the rate of an ECG tracing.

  • Rate = 1500 / No. of small boxes between R – R interval
  • Rate = 300 / No. of larges boxes between R – R interval

< 60 bpm – Bradycardia | < 100 bpm – Tachycardia

Calculation of rate in ECG

Cardiac Axis

Average direction of spread of the depolarization wave through the ventricles as seen from the front is called the cardiac axis.

First take a look at Lead I and aVF. In a normal cardiac axis both lead I and aVF have a QRS complex with a upward deflection.

Cardiac Axis

P  Wave

Look at lead II and identify the P wave.

Normal duration : 2 – 3 small squares | Normal amplitude : < 2.5 small squares

PR Interval

It is the time duration from the beginning of P wave to the beginning of R wave

Best seen in the rhythm strip

Normal duration : 3 – 5 small squares

QRS Complex

Normal duration : < 4 small squares | Normal amplitude : Variable

T Wave

Normal amplitude : 0.5 small squares in lead I, II. III and 1 small square in chest leads

QT interval 

It is the time duration between the beginning of QRS complex to the end of T wave. As it represents the total ventricular activity → Faster the heart rate shorter the QT interval and slower the heart rate longer the QT interval.

Normal duration : 9 – 11 small squares

U wave

U wave is not always present on an ECG tracing.

Abnormalities Of Cardiac Axis

The easy method that I use to remember this is by….

  • Right axis deviation – Like greeting with a handshake (Right hand) | Deflections are towards each other
  • Left axis deviation – Like saying bye bye | Deflections are opposite to each other

Causes of abnormal cardiac axis

  • Right axis deviation
    • Right ventricular hypertrophy
    • RBBB
    • Emphysema & co-pulmonale
    • Tetralogy of Fallot’s
  • Left axis deviation
    • Left ventricular hypertophy
    • LBBB
    • Wolf-Parkinson-White syndrome
    • Hypertrophic cardiomyopathy

Cardiac Axis

Abnormalities Of P Wave 

P mitrale
  • Criteria – Width of P wave in lead II is > 4 squares and notched
  • Occurs due to left atrial enlargement
  • Caused by mitral stenosis, mitral regurgitation, aortic stenosis
P Pulmonale
  • Criteria – Height of P wave in lead II is > 2.5 small squares (Tall P wave)
  • Occurs due to right atrial enlargement
  • Caused by co-pulmonale, emphysea, bronchial asthma, as a consequence of pulmonary hypertension
Inverted P waves
  • Sometimes in patients with pacemakers

Abnormalities Of PR Interval

Prolonged PR interval

  • Criteria – Duration of PR interval is > 5 small squares
  • Caused by heart blocks (Further discussed below)

Short PR interval

  • Criteria – Duration of PR interval is < 3 small squares
  • Caused by LGL syndrome and WPW syndrome

Heart Block

First Degree AV Block

  • Criteria – Prolonged PR interval remain constant from one beat to another

First degree heart block

Second Degree AV Block

  • Mobitz Type I AV Block (Wenckebach Phenomenon)
    • Criteria – Progressive lengthening of PR interval followed by a drop beat

2nd degree heart block - type 1

  • Mobitz Type II AV Block
    • Criteria – Constant PR interval followed by a drop beat

2nd degree heart block - type 2

Third Degree AV Block

  • Criteria – Presence of P waves which are not followed by QRS complex ( Atrial and ventricular rhythm is regular)

Complete heart block

Abnormalities Of QRS Complex & ST Segment

Abnormalities of QRS complex and ST segment is mainly associated with myocardial infarction.

Myocardial Infarction

In MI, there is lack of oxygen supply (ischemia) leading to muscle injury followed by death of the tissues. This is represented by ECG changes.

  • Ischemia – ST depression and/or T inversion
  • Injury – ST elevation (> 1 mm in limb leads and > 2 mm in chest leads)
  • Infarction – Pathological Q wave (Old infarct)

ECG changes of MI in chronological order is mentioned below.

ECG Changes in MI - Chronology

ST Elevated MI – According to the site of involvement

Anterior MI

  • Criteria – ST elevation in V3 and V4 which is taller than normal R waves in the same leads
  • In extension anterior MI ST elevations can be seen in V2 to V5
  • Occurs due to occlusion of left anterior descending artery, a branch of left coronary artery

Anterior Myocardial Infarction MI ECG

Lateral MI

  • Criteria – ST elevation in lead I, aVL, V5 and V6
  • Occurs due to occlusion of circumflex artery, a branch of left coronary artery

Lateral Myocardial Infarction MI ECG

Septal MI

  • Criteria – ST elevation in V2 and V3
  • Occurs due to occlusion of left anterior descending artery, a branch of left coronary artery

Septal Myocardial Infarction MI ECG

Inferior MI

  • Criteria – ST elevation in lead II, lead III and aVF
  • Occurs due to occlusion of posterior descending artery

Inferior Myocardial Infarction MI ECG

Posterior MI

  • Criteria
    • Dominant R wave in V1
    • Flattening of T wave or ST depression in V1, V2 and aVL
    • ST elevation in V8 and V9 (seen in a 15 lead ECG)
  • Occurs due to occlusion of posterior descending artery, a branch of right coronary artery or left circumflex artery

Posterior Myocardial Infarction MI ECG

Non ST Elevated MI & Unstable Angina

  • Criteria
    • ST depression
    • Flattening of T wave
    • T inversion

Bundle Branch Block

There are two types of bundle branch blocks

  • Right bundle branch block
  • Left bundle branch block

Right Bundle Branch Block

  • Criteria
    • Wide QRS complex (Duration is > 2.5 small squares)
    • RSR’ (R’ is taller than R) pattern in V1
    • Slurred S wave in lead 1 and V6
    • Right axis deviation may be present

Right Bundle Branch Block RBBB

Left Bundle Branch Block

  • Criteria
    • Wide QRS complex (Duration is > 2.5 small squares)
    • Deep and broad S wave in V1
    • Wide monophasic slurred R waves in lead I and V6 / RR’ pattern without a Q wave in lead I and V6
    • Left axis deviation

Left Bundle Branch Block LBBB

Ventricular Hypertrophy

Under ventricular hypertrophy we are going to discuss about….

  • Right ventricular hypertrophy
  • Left ventricular hypertrophy

Ventricular Hypertrophy (LVH & RVH)

Right ventricular hypertrophy

  • Criteria (Most commonly used)
    • Depth of S wave in V1 + Height of tallest R wave in V5 or V6 = > 35 mm  OR
    • R wave in V5 or V6 is > 26 mm

Left Ventricular Hypertrophy

  • Criteria (Most commonly used)
    • R wave in V1 / S wave in V1 < 1  OR
    • R wave in V1 + S wave in V4 or V5 = 11 mm

Left Ventricular Hypertrophy With Strain

  • Criteria
    • ST segment depression in V4, V5, V6
    • Reciprocal changes can be present in V1, V2, V3 such as ST segment elevation with concavity upwards

Right Ventricular Hypertrophy With Strain

  • Criteria
    • ST segment depression
    • T inversion


Simply it is described as an irregular heart beat. I will only discuss the most important arrhythmias that usually pops up frequently in the exams and  in patients that we find in daily basis.

Sinus Pause / Sinus Arrest

  • Criteria
    • Bradycardia or normal rate
    • Rhythm is regular except at pauses / arrests
    • Beats appear normal P wave, QRS complex, T wave and ST segment except at pauses / arrests

Sinus Arrest

Sinus Arrhythmia

  • Criteria
    • P waves follow QRS complexes
    • Duration of PR interval remains constant
    • RR interval changes present (As a response to respiration) – Irregular rhythm
  • These patients are asymptomatic and usually occur in children and young adults

Sinus Arrhythmia ECG

Sinus Bradycardia

  • Criteria
    • Sinus rhythm with normal P wave, QRS complex, ST segment and T waves
    • Bradycardia

Sinus Bradycardia

Sinus Tachycardia

  • Criteria
    • Normal ECG strip
    • Sometimes P wave can be buried
    • Tachycardia

Sinus Tachycardia

Supraventricular Tachycardia

  • Criteria
    • Tachycardia
    • Rhythm is regular
    • P waves can be embedded in the QRS complex
    • QRS complexes remain similar in every beat

Supraventricular Tachycardia

Atrial Fibrillation

  • Criteria
    • Tachycardia
    • Irregularly irregular rhythm
    • P waves are absent
    • Baseline variation

Atrial Fibrillation ECG AF

Atrial Flutter

  • Criteria
    • Tachycardia
    • Saw tooth appearance

Atrial Flutter

Idioventricular Rhythm

  • Criteria
    • Bradycardia
    • Regular rhythm
    • Absence of P waves
    • Broad, bizarre QRS complexes
  • In an accelerated idioventricular rhythm heart rate could be normal

Idioventricular rhythm ECG

Ventricular Tachycardia

  • Two types
    • Monomorphic – Have same shape and amplitude
    • Polymorphic – Have variable shapes and amplitude
  •  Criteria
    • Tachycardia
    • Regular rhythm
    • P waves are absent
    • Broad, abnormal QRS complexes
    • No identifiable T waves

Ventricular Tachycardia ECG

Ventricular Flutter

  • This occur during the transition of VT to VF

Ventricular Flutter ECG

Ventricular Fibrillation

  • Criteria
    • Tachycardia
    • Irregular rhythm
    • Broad, disorganized QRS complexes
    • Absence of P waves, T waves, ST segment

Ventricular Fibrillation

Wolff-Parkinson-White Syndrome

  • Criteria
    • Sinus rhythm
    • Short PR interval
    • Wide QRS complexes
    • Delta waves present

WPW syndrome ECG

Lown–Ganong–Levine syndrome

  • Criteria
    • Sinus rhythm
    • Short PR interval
    • Normal QRS complexes and T waves

Torsades De Pointes

  • Unusual variant of polymorphic VT with normal or long QT intervals
  •  Criteria
    • Tachycardia
    • Irregular rhythm
    • Absence of P waves
    • Wide QRS complex with a bizarre pattern

Torsades-de-pointes TDP

Abnormalities Of T Wave

Tall T wave

  • Hyperkalemia
  • Acute MI
  • Acute true posterior MI (in V1 and V2)

Small T wave

  • Hypokalemia
  • Hypothyroidism
  • Pericardial effusion

T inversion

  • MI
  • Myocardial ischemia
  • Ventricular ectopic
  • Ventricular hypertrophy with strain
  • Acute pericarditis

Acute Pericarditis

  • Criteria
    • ST segment elevated with capacity upwards

Acute pericarditis ECG

Abnormalities Of QT Interval

Causes Of Prolonged QT Interval

  • Bradycardia
  • Hypocalcemia
  • Acute MI
  • Acute chest syndrome
  • Hypothermia
  • Cerebrovascular accident
  • Hereditary syndrome
    • Jervell, Lange-Nielsen syndrome – Congenital deafness present
    • Romano-Ward syndrome

Causes Of Shortened QT Interval

  • Tachycardia
  • Hyperthermia
  • Hypercalcemia
  • Digoxin effect
  • Vagal stimulation

Other Abnormalities

Causes Of Prominent U Waves

  • Normally present
  • Hypokalemia
  • Bradycardia
  • Ventricular hypertrophy
  • Hypercalcemia
  • Hyperthyroidism


Changes In ECG In Metabolic Abnormalities


  • Tall T waves
  • Prolong PR interval
  • Broadening of QRS complex
  • ST elevation
  • Sine wave

ECG - Hyperkalaemia


  • Flattening of T wave
  • Prominent U wave
  • ST depression

ECG changes in Metabolic abnormalities

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Rojina rai
Rojina rai
1 year ago

Save me a lot of time. You are a life saver

4 months ago

“Chest leads looks at the center in the coronal plane and the limb leads looks at the heart in the horizontal plane.”

I think this should be corrected. It should be vice versa.
Limb leads- Coronal plane
Chest leads- Horizontal plane

Other than that your notes are amazing.