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ECG - Easy explained



Electrocardiography (ECG) – Detailed Overview

Definition

Electrocardiography is a non-invasive diagnostic procedure that records the electrical activity of the heart over time using surface electrodes. The graphical representation obtained is called an electrocardiogram.

Historical Background

  • Late 19th century – Augustus Waller first recorded the human ECG (1887).

  • 1903 – Willem Einthoven developed the string galvanometer and introduced the standard limb leads (Lead I, II, III), earning the Nobel Prize (1924).

  • Since then, ECG has evolved with 12-lead systems, portable Holter monitors, and computer-assisted interpretations.

Physiological Basis

  • The heart’s rhythmic contraction is controlled by electrical impulses generated and conducted through:

    • SA node → Atria → AV node → Bundle of His → Purkinje fibers → Ventricular muscle.

  • Depolarization and repolarization of myocardial cells create electrical potentials detected by electrodes.

ECG Leads

  1. Standard Limb Leads – I, II, III (Einthoven's triangle).

  2. Augmented Limb Leads – aVR, aVL, aVF.

  3. Precordial Leads – V1 to V6.

  • Together, these form the 12-lead ECG.

ECG Paper & Calibration

  • Paper speed: 25 mm/sec.

  • Vertical axis: 10 mm = 1 mV.

  • Small square: 0.04 sec horizontally, 0.1 mV vertically.

Waves, Intervals, and Segments

  • P wave – Atrial depolarization.

  • PR interval – 0.12–0.20 sec; time for impulse from SA node to ventricles.

  • QRS complex – Ventricular depolarization (normal <0.12 sec).

  • T wave – Ventricular repolarization.

  • ST segment – Time between depolarization and repolarization; isoelectric normally.

  • QT interval – Ventricular depolarization + repolarization.

Normal ECG Values

Parameter Normal Range
Heart rate 60–100 bpm
PR interval 0.12–0.20 s
QRS duration <0.12 s
QT interval 0.35–0.44 s

Clinical Uses

  1. Diagnosis of cardiac arrhythmias – e.g., atrial fibrillation, ventricular tachycardia.

  2. Detection of myocardial ischemia and infarction – ST elevation, T wave inversion, Q waves.

  3. Conduction defects – heart blocks, bundle branch blocks.

  4. Electrolyte imbalance – hyperkalemia (tall T waves), hypokalemia (U waves).

  5. Monitoring effects of drugs – e.g., digoxin, antiarrhythmics.

  6. Assessment of pacemaker function.

Advantages

  • Non-invasive, inexpensive, and quick.

  • Widely available and can be repeated frequently.

Limitations

  • Provides only electrical information, not mechanical function.

  • May miss intermittent or transient arrhythmias (need Holter monitoring).

  • Requires correct electrode placement and patient cooperation.

Recent Advances

  • Computerized ECG interpretation.

  • Wearable continuous ECG devices.

  • Tele-ECG for remote diagnosis.


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