Cardiac Conduction System

Main Components

Sinoatrial Node (SA Node)
- Located in the right atrium near the entrance of the superior vena cava.
- Acts as the natural pacemaker of the heart.
- Initiates heartbeat and determines heart rate.
- Spreads electrical impulses throughout both atria, causing them to contract.
Atrioventricular Node (AV Node)
- Located on the other side of the right atrium, near the AV valve.
- Serves as an electrical gateway to the ventricles.
- Delays the passage of electrical impulses to ensure atria have ejected all blood into the ventricles before contraction.
Atrioventricular Bundle (AV Bundle or Bundle of His)
- Receives signals from the AV node.
- Divides into right and left bundle branches conducting impulses toward the heart's apex.
Purkinje Fibers
- Spread throughout the ventricular myocardium.

A composite recording of all action potentials produced by nodes and myocardial cells.
P Wave:
- Represents depolarization of the atria.
- Occurs when the atria are full of blood and the SA node fires.
- Atrial contraction (atrial systole) starts ~100 ms after P wave begins.
P-Q Segment:
- Represents the time signals travel from the SA node to the AV node.
QRS Complex:
- Marks the firing of the AV node and represents ventricular depolarization.
- Q Wave: Depolarization of the interventricular septum.
- R Wave: Depolarization of the main mass of the ventricles.
- S Wave: Last phase of ventricular depolarization at the heart's base.
- Atrial repolarization occurs but is obscured by the QRS complex.
S-T Segment:
- Reflects the plateau in myocardial action potential.
- When ventricles contract and pump blood.
T Wave:
- Represents ventricular repolarization before ventricular relaxation (ventricular diastole).