Cardiac Conduction System and ECG

Components of the Cardiac Conduction System

• Sinoatrial (SA) Node:
  • Location: Right atrium near the superior vena cava
  • Function: Natural pacemaker, initiates heartbeat, determines heart rate
  • Activity: Electrical impulses spread throughout both atria, causing them to contract
• Atrioventricular (AV) Node:
  • Location: Other side of the right atrium near the AV valve
  • Function: Electrical gateway to the ventricles, delays electrical impulses to ensure complete atrial ejection before ventricular contraction
  • Activity: Receives signals from the SA node and passes them onto the atrioventricular (AV) bundle (Bundle of His)
• Atrioventricular Bundle (Bundle of His):
  • Division: Right and left bundle branches
  • Function: Conduct impulses toward the apex of the heart
• Purkinje Fibers:
  • Function: Spread impulses throughout the ventricular myocardium, causing ventricles to contract

Electrocardiogram (ECG or EKG)

• Purpose: Composite recording of all action potentials produced by the nodes and myocardial cells
• Key Waves and Segments:
  • P Wave: Depolarization of atria; occurs when SA node fires
    • Atrial contraction (systole) starts ~100 ms after P wave begins
  • P-Q Segment: Time signals travel from the SA node to the AV node
  • QRS Complex: Ventricular depolarization and firing of AV node
    • Q Wave: Depolarization of interventricular septum
    • R Wave: Depolarization of the main mass of ventricles
    • S Wave: Last phase of ventricular depolarization at the base of the heart
    • Note: Atrial repolarization occurs during this time but is obscured by QRS complex
  • S-T Segment: Plateau in myocardial action potential; ventricles contract and pump blood
  • T Wave: Ventricular repolarization before ventricular relaxation (diastole)

Cardiac Cycle

• The electrical cycle represented in the ECG corresponds to cardiac events, repeating with every heartbeat.