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

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the sinoatrial node, or SA node,

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

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This is the natural pacemaker of the heart

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that initiates all heartbeat and determines heart rate.

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Electrical impulses from the SA node spread

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throughout both atria and stimulate them to contract.

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The atrial ventricular node, or AV node,

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located on the other side of the right atrium,

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near the AV valve.

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The AV node serves as electrical gateway to the ventricles.

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It delays the passage of electrical impulses to the ventricles.

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This delay is to ensure that the atria have ejected

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all the blood into the ventricles before the ventricles contract.

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The AV node receives signals from

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

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AV bundle, or bundle of His.

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This bundle is then divided into right and left

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

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The signals are then passed onto Purkinje fibers,

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turning upward and spreading throughout the ventricular myocardium.

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

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ECG, or EKG.

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An ECG is a composite recording of

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

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Each wave or segment of the ECG corresponds

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to a certain event of the cardiac electrical cycle.

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When the atria are full of blood,

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the SA node fires,

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

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This is represented by the P wave on the ECG.

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Atrial contraction or atrial systole,

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starts about 100 milliseconds after the P wave begins.

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

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The QRS complex marks the firing of

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the AV node and represents ventricular depolarization.

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Q wave corresponds to depolarization of the interventricular septum.

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

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

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Atrial repolarization also occurs during this time,

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but the signal is obscured by the large QRS complex.

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

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this is when the ventricles contract and pump blood.

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The T wave represents ventricular repolarization

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immediately before ventricular relaxation or ventricular diastole.

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The cycle repeats itself with every heartbeat.