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  • 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.

  • It initiates all heartbeat and determines heart rate.

  • Electrical impulses from the SA node spread throughout both atria and stimulate them to

  • contract.

  • - The atrioventricular 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 onto 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 ms after the P wave begins.

  • The P-Q segment represents the time the signals travel 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.

The cardiac conduction system consists of the following components:

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