An electrocardiogram (ECG) is a non-invasive test that records the electrical activity of the heart over time to assess heart rhythm, rate, conduction abnormalities, ischemia, and structural heart disease.

What does a normal ECG show?

A normal ECG shows sinus rhythm with a rate of 60–100 bpm, normal P waves before each QRS complex, PR interval 120–200 ms, QRS duration <120 ms, and no significant ST-T abnormalities.

What are the main components of an ECG waveform?

The main components are the P wave (atrial depolarization), PR interval (AV conduction), QRS complex (ventricular depolarization), ST segment, T wave (ventricular repolarization), and QT interval.

How many leads are used in a standard ECG?

A standard ECG uses 12 leads: 6 limb leads (I, II, III, aVR, aVL, aVF) and 6 precordial chest leads (V1–V6).

What does ST elevation on ECG indicate?

ST elevation usually indicates acute myocardial injury, most commonly ST-elevation myocardial infarction (STEMI), but can also be seen in pericarditis, early repolarization, and ventricular aneurysm.

What ECG changes are seen in myocardial infarction?

Typical changes include hyperacute T waves, ST elevation or depression, pathological Q waves, and T wave inversion depending on the stage and location of infarction.

What is the significance of QT interval prolongation?

Prolonged QT interval increases the risk of torsades de pointes, a potentially life-threatening polymorphic ventricular tachycardia.

How is heart rate calculated on ECG?

In regular rhythm, heart rate is calculated as 300 divided by the number of large squares between two R waves. In irregular rhythm, QRS complexes are counted in 10 seconds and multiplied by 6.

What ECG findings suggest atrial fibrillation?

Atrial fibrillation shows an irregularly irregular rhythm with absent P waves and variable R–R intervals.

What is a wide QRS complex and what does it indicate?

A wide QRS complex (>120 ms) suggests abnormal ventricular conduction such as bundle branch block, ventricular rhythm, hyperkalemia, or drug toxicity.

What are common ECG changes in electrolyte abnormalities?

Hyperkalemia causes tall peaked T waves and wide QRS, hypokalemia causes U waves and flat T waves, hypercalcemia shortens QT interval, and hypocalcemia prolongs QT interval.

What is axis deviation on ECG?

Axis deviation refers to abnormal direction of ventricular depolarization. Left axis deviation and right axis deviation are associated with specific cardiac and pulmonary conditions.

What ECG features indicate pericarditis?

Acute pericarditis typically shows diffuse ST elevation with PR segment depression across multiple leads.

Can ECG be normal in heart disease?

Yes, ECG can be normal in early ischemia, stable angina, or some structural heart diseases, so clinical correlation is always required.

Why is ECG important in emergency medicine?

ECG provides rapid diagnosis of life-threatening conditions such as myocardial infarction, ventricular arrhythmias, heart block, and electrolyte disturbances, guiding immediate management.

Electrocardiogram (ECG)

Q: What are the main components of an ECG waveform?

The main components are the P wave (atrial depolarization), PR interval (AV conduction), QRS complex (ventricular depolarization), ST segment, T wave (ventricular repolarization), and QT interval.

Q: How many leads are used in a standard ECG?

A standard ECG uses 12 leads: 6 limb leads (I, II, III, aVR, aVL, aVF) and 6 precordial chest leads (V1–V6).

Q: What does ST elevation on ECG indicate?

ST elevation usually indicates acute myocardial injury, most commonly ST-elevation myocardial infarction (STEMI), but can also be seen in pericarditis, early repolarization, and ventricular aneurysm.

Q: What ECG changes are seen in myocardial infarction?

Typical changes include hyperacute T waves, ST elevation or depression, pathological Q waves, and T wave inversion depending on the stage and location of infarction.

Q: What is the significance of QT interval prolongation?

Prolonged QT interval increases the risk of torsades de pointes, a potentially life-threatening polymorphic ventricular tachycardia.

Q: How is heart rate calculated on ECG?

In regular rhythm, heart rate is calculated as 300 divided by the number of large squares between two R waves. In irregular rhythm, QRS complexes are counted in 10 seconds and multiplied by 6.

Q: What ECG findings suggest atrial fibrillation?

Atrial fibrillation shows an irregularly irregular rhythm with absent P waves and variable R–R intervals.

Q: What is a wide QRS complex and what does it indicate?

A wide QRS complex (>120 ms) suggests abnormal ventricular conduction such as bundle branch block, ventricular rhythm, hyperkalemia, or drug toxicity.

Definition

An Electrocardiogram (ECG) is a non-invasive diagnostic test that records the electrical activity of the heart over time using surface electrodes placed on the body. It reflects depolarization and repolarization of atrial and ventricular myocardium.

Key external reference:

https://radiopaedia.org/articles/electrocardiogram

https://www.ecgpedia.org/wiki/Main_Page

Indications

Chest pain / suspected acute coronary syndrome
Palpitations, syncope, presyncope
Arrhythmia detection and monitoring
Electrolyte abnormalities
Drug toxicity (digoxin, antiarrhythmics)
Structural heart disease screening
Pre-operative assessment

ECG Leads and Views

Standard 12-Lead ECG

Limb Leads

Lead I, II, III (Bipolar)
aVR, aVL, aVF (Augmented unipolar)

Chest (Precordial) Leads

V1–V6

Anatomical Correlation

| Leads | Area of Heart |

| ------------- | ----------------- |

| II, III, aVF | Inferior wall |

| I, aVL, V5–V6 | Lateral wall |

| V1–V4 | Anterior / Septal |

| V7–V9 | Posterior wall |

| V3R–V4R | Right ventricle |

Reference:

https://litfl.com/ecg-lead-positioning/

ECG Paper and Calibration

Speed: 25 mm/sec
Voltage: 10 mm = 1 mV
1 small square = 0.04 sec
1 large square = 0.20 sec

Normal ECG Waves and Intervals

P Wave

Atrial depolarization
Duration: <120 ms
Height: <2.5 mm (lead II)

Abnormalities:

Tall P → Right atrial enlargement
Broad/notched P → Left atrial enlargement

Reference:

https://www.ecgpedia.org/wiki/P_wave

PR Interval

AV nodal conduction
Normal: 120–200 ms

Abnormal:

Short PR → WPW syndrome
Prolonged PR → First-degree AV block

QRS Complex

Ventricular depolarization
Normal duration: <120 ms

Wide QRS causes:

Bundle branch block
Ventricular rhythm
Hyperkalemia
Drug toxicity

ST Segment

Normally isoelectric
Elevation or depression indicates ischemia or injury

Reference:

https://litfl.com/st-segment-ecg-library/

T Wave

Ventricular repolarization
Inversion indicates ischemia, strain, CNS pathology

QT Interval

Total ventricular depolarization + repolarization
QTc (Bazett formula):

QTc = QT / √RR

Normal QTc:

Male: <440 ms
Female: <460 ms

Prolonged QT → risk of Torsades de Pointes

Reference:

https://litfl.com/qt-interval-ecg-library/

Systematic ECG Interpretation (STEPWISE)

1. Rate

Regular rhythm: 300 ÷ large squares
Irregular rhythm: count QRS in 10 sec × 6

2. Rhythm

Sinus rhythm: P before every QRS, upright P in II
Irregularly irregular → Atrial fibrillation

3. Axis

Normal: −30° to +90°
Left axis deviation → LBBB, LVH
Right axis deviation → RBBB, PE

Reference:

https://litfl.com/ecg-axis-interpretation/

4. Intervals

PR, QRS, QTc evaluation

5. Hypertrophy Patterns

Left Ventricular Hypertrophy

S(V1) + R(V5/V6) > 35 mm

Right Ventricular Hypertrophy

R/S > 1 in V1

6. Ischemia and Infarction

STEMI Criteria

ST elevation ≥1 mm in ≥2 contiguous leads
New LBBB with symptoms

NSTEMI

ST depression, T inversion, positive troponin

Reference:

https://www.escardio.org/Guidelines

Common ECG Arrhythmias

Atrial Fibrillation

No P waves
Irregularly irregular rhythm

Atrial Flutter

Saw-tooth flutter waves
Ventricular rate often 150 bpm

Ventricular Tachycardia

Wide QRS tachycardia
AV dissociation

Ventricular Fibrillation

Chaotic waveform
Cardiac arrest rhythm

Reference:

https://litfl.com/ecg-library/

ECG in Electrolyte Abnormalities

| Electrolyte | ECG Changes |

| ------------- | ---------------------------------- |

| Hyperkalemia | Tall tented T, wide QRS, sine wave |

| Hypokalemia | U waves, flat T |

| Hypercalcemia | Short QT |

| Hypocalcemia | Prolonged QT |

Reference:

https://radiopaedia.org/articles/electrolyte-disturbances-on-ecg

ECG in Drug Toxicity

Digoxin

Scooped ST depression
Atrial tachycardia with block

Tricyclic Antidepressants

Wide QRS
Right axis deviation

Special ECG Patterns

Brugada syndrome: Coved ST elevation V1–V3
WPW: Short PR, delta wave
Pericarditis: Diffuse ST elevation, PR depression
Pulmonary embolism: S1Q3T3 pattern

Reference:

https://radiopaedia.org/articles/brugada-syndrome

Limitations of ECG

Can be normal in early ischemia
Poor sensitivity for posterior MI without additional leads
Electrical activity does not always equal mechanical function

Electrocardiogram ECG Interpretation Guide for Medical and Clinical Practice

Electrocardiogram (ECG)

Definition

Indications