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what is an ECG electrocardiogram overview

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ECG electrocardiogram normal waveform PQRST

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Here is a comprehensive overview of the ECG (Electrocardiogram):

ECG (Electrocardiogram)

An electrocardiogram (ECG, or EKG) is a graphical record of the electrical activity of the heart over time. Because body fluids are good conductors (the body acts as a volume conductor), fluctuations in electrical potential generated by the heart's action potentials can be detected at the body surface using electrodes and recorded as a waveform.

How It Works

The heart generates electrical impulses that spread in a predictable sequence:
  1. SA node fires → spreads to both atria → atrial contraction → produces the P wave
  2. Impulse reaches the AV node → deliberate slow conduction (creates a pause) → represented by the PR interval
  3. Impulse travels down the His bundle → left/right bundle branches → Purkinje fibers → depolarizes ventricular muscle → produces the QRS complex
  4. Ventricles then repolarize → produces the T wave

The ECG Waveform (PQRSTU)

ECG waveform showing P, QRS, T, and U waves with labeled intervals
Wave / IntervalMeaning
P waveAtrial depolarization (atria contracting)
PR intervalAV conduction time (normal: 0.12–0.20 s)
QRS complexVentricular depolarization (ventricles contracting); normal duration ≤ 0.10 s
ST segmentPlateau of ventricular action potential; isoelectric in normal ECG
T waveVentricular repolarization
QT intervalTotal ventricular electrical activity (normal: ~0.40–0.43 s)
U waveInconstant; may reflect slow repolarization in some ventricular myocytes

The 12-Lead ECG

A standard clinical ECG records from 12 different viewpoints (leads) of the heart:

Bipolar Limb Leads (standard leads)

  • Lead I — Left arm (+) vs Right arm (−)
  • Lead II — Left leg (+) vs Right arm (−)
  • Lead III — Left leg (+) vs Left arm (−)

Augmented Unipolar Limb Leads

  • aVR — Right arm
  • aVL — Left arm
  • aVF — Left foot (inferior view)

Precordial (Chest) Leads

  • V1–V6 — Placed across the chest wall; view the heart from anterior and lateral perspectives
12-lead ECG example showing PQRST across all leads

What the ECG Is Used For

  • Detecting arrhythmias (abnormal heart rhythms)
  • Diagnosing myocardial infarction (heart attack) — ST elevation/depression, Q waves
  • Identifying conduction blocks — prolonged PR, widened QRS
  • Assessing electrolyte abnormalities — e.g., hypokalemia causes flat T waves and U waves
  • Monitoring drug effects — e.g., prolonged QT from certain medications
  • Evaluating ventricular hypertrophy, pericarditis, pulmonary embolism, and more

Key Principles

  • Upward deflection = depolarization moving toward the active electrode
  • Downward deflection = depolarization moving away from the active electrode
  • Einthoven's triangle: electrodes on both arms and the left leg form an equilateral triangle with the heart at center — the basis of the standard limb leads
Sources: Ganong's Review of Medical Physiology, 26th ed.; Miller's Anesthesia, 10th ed.
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