3- Electrocardiogram (ECG)

Q1: What is the relationship between electrical events of cardiac excitation and ECG waves/intervals?

• P wave: Represents atrial depolarization, starting from SA node

• PR interval: Time between start of atrial depolarization and start of ventricular depolarization (AV node delay)

• QRS complex: Represents ventricular depolarization

• ST segment: Period when ventricles are fully depolarized (plateau phase)

• T wave: Represents ventricular repolarization

• Key timing:

  • PR interval: ~0.2 seconds

  • QRS duration: ~0.1 seconds

  • QT interval: Represents total ventricular depolarization and repolarization time

Q2: What are Einthoven's basic electrocardiographic conventions and how is mean electrical axis determined?

• Einthoven's Triangle Conventions:

  • Three bipolar limb leads (I, II, III) form an equilateral triangle

  • Lead I: Right arm (-) to left arm (+)

  • Lead II: Right arm (-) to left leg (+)

  • Lead III: Left arm (-) to left leg (+)

• Mean Electrical Axis:

  • Normal axis: 0° to +90°

  • Left axis deviation: < 0°

  • Right axis deviation: > +90°

  • Determined by looking at net QRS deflection in leads I and II

  • Axis points toward positive deflections and away from negative deflections

Q3: What comprises the standard 12-lead ECG?

• 12 leads provide different views of heart's electrical activity:

  • 3 bipolar limb leads (I, II, III)

  • 3 augmented limb leads (aVR, aVL, aVF)

  • 6 precordial (chest) leads (V1-V6)

• Lead Placement:

  • Limb leads: Placed on extremities

  • Precordial leads: Placed across chest wall in specific anatomical locations

• Recording Conventions:

  • Paper speed: 25 mm/second

  • Voltage calibration: 10 mm/mV

  • Each small box = 0.04 seconds

  • Each large box = 0.2 seconds

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