The Cardiac Conduction System

Components of the System

Sinoatrial Node (SA Node)
- Located in the right atrium near the entrance of the superior vena cava.
- Natural pacemaker of the heart.
- Initiates all heartbeats and determines heart rate.
- Electrical impulses spread throughout both atria, causing them to contract.
Atrioventricular Node (AV Node)
- Located on the other side of the right atrium, near the AV valve.
- Electrical gateway to the ventricles.
- Delays electrical impulses to ensure atria ejection before ventricular contraction.
- Passes signals to the atrioventricular bundle (AV bundle or Bundle of His).
Atrioventricular Bundle (AV Bundle or Bundle of His)
- Divided into right and left bundle branches.
- Conducts impulses toward the apex of the heart.
- Impulses then passed to Purkinje fibers, which spread throughout the ventricular myocardium.

Composite recording of all action potentials produced by the nodes and myocardial cells.
Each wave or segment corresponds to an event in the cardiac electrical cycle.

P Wave
- Occurs when the atria are full of blood and the SA node fires.
- Represents atrial depolarization.
- Atrial systole (contraction) starts about 100 milliseconds after the P wave begins.
P-Q Segment
- Represents the travel time of signals from SA node to AV node.
QRS Complex
- Marks the firing of the AV node and ventricular depolarization.
- Q Wave: Depolarization of the interventricular septum.
- R Wave: Depolarization of main mass of ventricles.
- S Wave: Last phase of ventricular depolarization at the base of the heart.
- Atrial Repolarization occurs during this time but is obscured by the QRS complex.
S-T Segment
- Reflects the plateau in the myocardial action potential.
- Represents ventricular contraction and blood pumping.
T Wave
- Represents ventricular repolarization immediately before ventricular relaxation (ventricular diastole).