Heart Rate

Depolarization and Repolarization of Myocardium

• Depolarization and repolarization waves have opposite directions

  • However, the electric vector always has the same direction

• Depolarization occurs rapidly

  • Resulting in a narrow R wave

• Repolarization occurs more slowly

  • Resulting in a broad T wave

• The ventricles depolarize sequentially

  1. Ventricular septum (Q wave)
  2. Ventricles (R wave)
  3. Base of the left ventricle (S wave)

• The ventricles are activated sequentially:
  1. Ventricular septum (VS - septal vector)
  2. Left and right ventricles (VM - main vector)
  3. Base of the left ventricle (VT - terminal vector)
• QRS complex appears differently in each lead
  • Because each lead "views" the vectors from a different angle
  • If the vector is directed
    • Towards the lead - creates a positive deflection
    • Away from the lead - creates a negative deflection

Ventricular depolarization (QRS complex) is shown in the images

Limb Leads

• "View" the vectors in the frontal plane

Chest Leads

• "View" the vectors in the horizontal plane

Wilson Central Terminal (WCT)

Einthoven ECG Leads

• Leads (I, II, III) can be shifted in parallel towards the central terminal
  • The Wilson central terminal serves as a negative electrode calculated by the ECG device
• The electrical axis of each lead (I, II, III)
  • directs from the terminal towards the positive electrode