ECG Anatomy Explained: Key Components and Functions

1. What is an ECG?

• Electrocardiogram (ECG) is a graphical recording of the electrical activity of the heart.
• It helps assess the heart's rhythm, conduction, and function.

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2. Principle

• The depolarization and repolarization of heart muscles generate electrical currents.
• These currents are detected by electrodes placed on the body surface.

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3. ECG Machine & Leads

• Electrodes: Placed on limbs and chest
• Leads: Record potential differences
• Limb Leads:
• Bipolar: I, II, III
• Unipolar: aVR, aVL, aVF
• Chest Leads (Precordial): V1 to V6

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4. Standard ECG Paper

• Paper moves at 25 mm/sec
• 1 small square = 0.04 sec (horizontal), 1 mm = 0.1 mV (vertical)

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5. Waves of Normal ECG

Wave	Represents	Normal Duration
P wave	Atrial depolarization	~0.08–0.10 sec
QRS complex	Ventricular depolarization	~0.06–0.10 sec
T wave	Ventricular repolarization	~0.10–0.25 sec
U wave (optional)	Late repolarization (Purkinje)	Very small

6. Intervals & Segments

Interval/Segment	Meaning	Normal Duration
PR interval	Atrial depol. → Ventricular depol.	0.12 – 0.20 sec
ST segment	Ventricles are depolarized	Flat (isoelectric)
QT interval	Entire ventricle activity	~0.35 – 0.44 sec
RR interval	Time between 2 R waves	Heart rate indicator

7. Cardiac Electrical Conduction Pathway

Mnemonic: "SA-AV-Bundle-Purkinje"

1. SA Node – Pacemaker of the heart
2. AV Node
3. Bundle of His
4. Right & Left Bundle Branches
5. Purkinje Fibers → Ventricular contraction

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8. Clinical Importance of ECG

Finding	Indication
↑ PR Interval	First-degree heart block
Wide QRS complex	Bundle branch block
ST segment elevation	Myocardial infarction (MI)
T wave inversion	Ischemia or strain
Irregular RR interval	Arrhythmia (e.g., AF)
No P wave	Atrial fibrillation (AF)

9. Mnemonics for ECG Interpretation

“Rate, Rhythm, PQRST, ST”
→ Check:

• Rate: via RR interval
• Rhythm: Regular or irregular
• P wave: Present? Normal?
• QRS: Width & shape
• ST segment: Elevated or depressed?