ECG- Exam Ready Notes

 

Electrocardiography (ECG/EKG)

Definition

Electrocardiography (ECG) is the graphic recording of the electrical activity of the heart during depolarization and repolarization using electrodes placed on the body surface. The recorded tracing is called an electrocardiogram (ECG or EKG).

Historical Background

Invented by Willem Einthoven, Introduced the string galvanometer, Developed the standard limb leads (I, II, III), Nobel Prize in Physiology/Medicine: 1924

Importance of ECG

ECG is:

• Non-invasive
• Rapid
• Inexpensive
• Highly informative

Clinical Uses

1. Diagnosis of arrhythmias
2. Myocardial infarction (MI)
3. Ischemic heart disease
4. Conduction abnormalities
5. Electrolyte disturbances
6. Chamber hypertrophy
7. Drug toxicity
8. Pericarditis
9. Monitoring ICU patients
10. Pre-operative assessment

Physiological Basis of ECG

The heart generates electrical impulses due to:

• Depolarization
• Repolarization

These currents spread through body tissues and are detected by surface electrodes.

Cardiac Conduction System

Components

1. SA node
2. Atrial pathways
3. AV node
4. Bundle of His
5. Right and left bundle branches
6. Purkinje fibers

Sequence of Activation

SA node → atria → AV node → Bundle of His → bundle branches → Purkinje fibers → ventricles

Principle of ECG Recording

When an electrical impulse:

• Moves towards a positive electrode → upward deflection
• Moves away from positive electrode → downward deflection
• Moves perpendicular → biphasic wave

ECG Paper

Standard ECG Paper

• Speed = 25 mm/sec
• 1 small square = 1 mm
• 1 large square = 5 mm

Time Calibration

Measurement	Value
1 small square	0.04 sec
1 large square	0.20 sec
5 large squares	1 sec

Voltage Calibration

Measurement	Voltage
10 mm	1 mV
1 mm	0.1 mV

Leads of ECG

Definition of Lead

A lead is a particular view of the heart’s electrical activity.

Types of ECG Leads

1. Bipolar Limb Leads (Standard Leads)

Introduced by Einthoven.

Lead	Connection
Lead I	Right arm (−) → Left arm (+)
Lead II	Right arm (−) → Left leg (+)
Lead III	Left arm (−) → Left leg (+)

Einthoven’s Triangle

The three limb leads form an imaginary triangle around the heart.

Einthoven’s Law

Lead II=Lead I+ Lead III

2. Unipolar Limb Leads (Augmented Leads)

Lead	Area Viewed
aVR	Right side of heart
aVL	Left upper heart
aVF	Inferior wall

3. Chest (Precordial) Leads

Lead	Position
V1	4th ICS right sternal border
V2	4th ICS left sternal border
V3	Between V2 and V4
V4	5th ICS mid-clavicular line
V5	Left anterior axillary line
V6	Left mid-axillary line

Areas Viewed by Chest Leads

Leads	Region Seen
V1–V2	Septal
V3–V4	Anterior
V5–V6	Lateral

Components of Normal ECG

Main Components

1. P wave
2. PR interval
3. QRS complex
4. ST segment
5. T wave
6. QT interval
7. U wave

P Wave

Represents: Atrial depolarization

Normal Features

Parameter	Normal Value
Duration	< 0.12 sec
Height	< 2.5 mm

Normal Characteristics

• Upright in I, II, aVF
• Inverted in aVR
• Biphasic in V1

Abnormal P Waves

P Mitrale

• Broad, notched P wave
• Left atrial enlargement

P Pulmonale

• Tall peaked P wave
• Right atrial enlargement

PR Interval

Represents: Time from atrial depolarization to ventricular depolarization

Includes:

• P wave
• PR segment

Normal PR Interval

PR interval=0.12 to 0.20 sec

Significance

Prolonged PR Interval

Seen in: First degree AV block

Short PR Interval

Seen in: Wolff-Parkinson-White syndrome (WPW)

QRS Complex

Represents: Ventricular depolarization

Components

• Q wave = first negative deflection
• R wave = first positive deflection
• S wave = negative deflection after R

Normal QRS Duration

QRS duration<0.12 sec

Normal Duration

0.06–0.10 sec

Significance of Wide QRS

Occurs in:

• Bundle branch block
• Ventricular tachycardia
• Hyperkalemia

Pathological Q Waves

Suggest: Myocardial infarction

Criteria

• Width > 0.04 sec
• Depth > 25% of R wave

ST Segment

Represents: Early ventricular repolarization

Normally:

• Isoelectric

ST Elevation

Seen in:

• Acute myocardial infarction
• Pericarditis

ST Depression

Seen in:

• Myocardial ischemia
• Digoxin effect

T Wave

Represents: Ventricular repolarization

Normal Features

• Upright in most leads
• Inverted in aVR

Tall T Waves

Seen in:

• Hyperkalemia

Inverted T Waves

Seen in:

• Ischemia
• Ventricular hypertrophy

QT Interval

Represents: Total ventricular electrical activity

Measured from: Beginning of QRS → end of T wave

Normal QT Interval

QTc < 440/ ms

Prolonged QT

May cause:

• Torsades de pointes

Causes:

• Hypocalcemia
• Drugs
• Congenital syndromes

U Wave

Small wave after T wave.

Seen in:

• Hypokalemia
• Bradycardia

Normal ECG Values

Parameter	Normal Value
Heart rate	60–100/min
P wave duration	<0.12 sec
PR interval	0.12–0.20 sec
QRS duration	<0.12 sec
QT interval	<0.44 sec
ST segment	Isoelectric

 

Determination of Heart Rate

Method 1: Large Box Method

Example

If RR interval = 4 large boxes

Method 2: Small Box Method

Cardiac Axis

Definition

Average direction of ventricular depolarization.

Normal Axis: -30° to +90°

Axis Deviation

Type	Causes
Left axis deviation	LVH, LBBB
Right axis deviation	RVH, pulmonary disease

 

Rhythm Analysis

Normal Sinus Rhythm Criteria

1. HR = 60–100/min
2. Regular rhythm
3. Each P followed by QRS
4. Normal PR interval

Systematic ECG Interpretation

Stepwise Approach

Step 1: Check Calibration

• Paper speed
• Voltage

Step 2: Determine Heart Rate

Step 3: Determine Rhythm

Step 4: Examine P Waves

Step 5: Measure PR Interval

Step 6: Assess QRS Complex

Step 7: Assess ST Segment

Step 8: Examine T Waves

Step 9: Measure QT Interval

Step 10: Determine Axis

ECG Changes in Common Conditions

1. Sinus Bradycardia

ECG Features

• HR < 60/min
• Normal rhythm

Causes

• Athletes
• Hypothyroidism
• Increased vagal tone

2. Sinus Tachycardia

ECG Features

• HR >100/min

Causes

• Fever
• Shock
• Hyperthyroidism

3. Atrial Fibrillation

ECG Features

• No P waves
• Irregularly irregular rhythm

4. Atrial Flutter

ECG Features

• Saw-tooth flutter waves

5. Ventricular Tachycardia

ECG Features

• Wide QRS complexes
• Rapid ventricular rhythm

6. Ventricular Fibrillation

ECG Features

• Chaotic irregular waves
• No organized complexes

Medical emergency.

7. Myocardial Infarction

ECG Changes

Early: Hyperacute T waves

Acute: ST elevation

Later: Pathological Q waves

Chronic: T wave inversion

Localization of MI

Leads	Region
II, III, aVF	Inferior wall
V1–V4	Anterior wall
I, aVL, V5, V6	Lateral wall

Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

ECG Features

• Wide QRS
• rSR′ pattern in V1

Left Bundle Branch Block (LBBB)

ECG Features

• Broad notched R waves in V5/V6

Electrolyte Abnormalities on ECG

Electrolyte Disorder	ECG Finding
Hyperkalemia	Tall T waves
Hypokalemia	U waves
Hypercalcemia	Short QT
Hypocalcemia	Long QT

 

Clinical Significance of ECG

ECG helps in:

• Emergency diagnosis
• Monitoring therapy
• Detecting silent ischemia
• Assessing pacemakers
• Evaluating electrolyte imbalance

Advantages of ECG

• Simple
• Cheap
• Bedside procedure
• Rapid diagnosis

Limitations of ECG

• May be normal despite disease
• Interpretation requires expertise
• Transient abnormalities may be missed

High-Yield Points

• P wave = atrial depolarization
• QRS = ventricular depolarization
• T wave = ventricular repolarization
• PR interval normal = 0.12–0.20 sec
• QRS duration <0.12 sec
• ST elevation = acute MI
• Hyperkalemia = tall T waves
• Hypokalemia = U waves
• Atrial fibrillation = absent P waves
• ECG paper speed = 25 mm/sec

Summary

Electrocardiography is one of the most essential diagnostic tools in medicine. Understanding ECG fundamentals—including waves, intervals, leads, axis, and interpretation—is critical for MBBS and NEET students. Mastery of normal ECG patterns forms the basis for diagnosing arrhythmias, myocardial infarction, electrolyte disturbances, and conduction defects.

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