Cardiac Conduction and ECG

Question 1

Describe the normal sequence of cardiac depolarization and repolarization.

Answer 1

Depolarization
-Electric signal originates within the heart itself
*Neural input not required but can modulate activity
-Electrical impulse is conducted from cell-to-cell by direct coupling
*Gap junctions connect cells
-Cardiac AP originates at SA node: modified myocytes w/ pacemaker fxn
*Rapid conduction due to fast Na+ channels
-AP exits SA node and spreads to atrial cells causing contraction
*provides kick to fill ventricle w/ blood
-Potential conducted to ventricles via the AV node
*Conduction is slow allowing for complete emptying of atria and filling of ventricle before next step
*Slow conduction due to slow Ca2+ channels
-Conducted to His-purkinje system
*Purkinje fibers blend w/ ventricular muscle
*AP conducted from Apex to base and endo to epicardium
Repolarization
-Epicardium to endocardium
-AP shorter in epicardium

Question 2

Define the standard bipolar limb leads, augmented unipolar limb leads and discuss how the mean cardiac vector is assessed in each lead.

Answer 2

• Standard bipolar limb leads
  
  • Lead 1: LT arm-RT arm
  • Lead 2: LT leg-RT arm
  • Lead 2: LT leg-LT arm
  • Einthoven’s triangle: able to measure potential difference b/t leads
    
    • RT arm: -/-
    • LT leg: +/+
    • LT arm: +/-
• Augmented unipolar limb leads:
  
  • central terminus (Wilson’s central terminal, aka “V”) is constructed by connecting the three limb leads.
    
    • Lead aVR: diff b/t V and RT arm
    • Lead aVL: diff b/t V and LT arm
    • Lead aVF: diff b/t V and LT leg
• Unipolar Precordial Leads
  
  • 6 leads placed on chest
  • Measure potential difference b/t V and the surface chest leads

Question 3

Explain the directions of deflections in a bipolar lead system in general, and discuss how the normal sequence of cardiac depolarization and repolarization is reflected in the normal ECG.

Answer 3

• SA node depolarization too small to be detected
• Atrial depolarization causes p wave of ECG
• Atrial repolarization coincides w/ ventricular depolarization (gets buried)
• AV node depolarization too small to be detected
• Ventricular depolarization causes QRS complex
  
  • abnormal PR intervals may indicate pathology
• Brief period after is ST: all regions in plateau (depolarized)
• Repolarization:
  
  • Ventricle repolarization generates T wave: b/t phase 2 and 3
  • U wave after T: repolarization of M cells
    
    • Cells b/t the endocardium and epicardium