NARRATOR: The cardiac conduction system consists of the following components:

the sinoatrial node, or SA node,

located in the right atrium near the entrance of the superior vena cava.

This is the natural pacemaker of the heart

that initiates all heartbeat and determines heart rate.

Electrical impulses from the SA node spread

throughout both atria and stimulate them to contract.

The atrial ventricular node, or AV node,

located on the other side of the right atrium,

near the AV valve.

The AV node serves as electrical gateway to the ventricles.

It delays the passage of electrical impulses to the ventricles.

This delay is to ensure that the atria have ejected

all the blood into the ventricles before the ventricles contract.

The AV node receives signals from

the SA node and passes them on to the atrioventricular bundle,

AV bundle, or bundle of His.

This bundle is then divided into right and left

bundle branches which conduct the impulses toward the apex of the heart.

The signals are then passed onto Purkinje fibers,

turning upward and spreading throughout the ventricular myocardium.

Electrical activities of the heart can be recorded in the form of electrocardiogram,

ECG, or EKG.

An ECG is a composite recording of

all the action potentials produced by the nodes and the cells of the myocardium.

Each wave or segment of the ECG corresponds

to a certain event of the cardiac electrical cycle.

When the atria are full of blood,

the SA node fires,

electrical signals spread throughout the atria and cause them to depolarize.

This is represented by the P wave on the ECG.

Atrial contraction or atrial systole,

starts about 100 milliseconds after the P wave begins.

The P-Q segments represent the time the signals travels from the SA node to the AV node.

The QRS complex marks the firing of

the AV node and represents ventricular depolarization.

Q wave corresponds to depolarization of the interventricular septum.

R wave is produced by depolarization of the main mass of the ventricles.

S wave represents the last phase of ventricular depolarization at the base of the heart.

Atrial repolarization also occurs during this time,

but the signal is obscured by the large QRS complex.

The S-T segment reflects the plateau in the myocardial action potential;

this is when the ventricles contract and pump blood.

The T wave represents ventricular repolarization

immediately before ventricular relaxation or ventricular diastole.

The cycle repeats itself with every heartbeat.