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Cardiac Conduction System

Normal cardiac impulses originate in the sinus node or SA node, a structure located in the long, superior portion of the right atrium at the juncture with the Superior Vena Cava.

From their conduction from the sinus node moves via the three internodal pathways:

• The anterior internodal pathway arises at the cranial end of the sinus node. It divides into branches, one to the left atrium called the Bachmann’s bundle and the other along the right side of the interatrial septum to the AV node.

• The middle internodal pathway arises along the endocardial surface of the sinus node and descends through the interatrial septum to the AV node.

• The posterior internodal pathway arises from the caudal end of the sinus node and approaches the AV node at its posterior aspect.

The speed of Conduction through the atria via these pathways is approximately 1000 mm/s.

From the Internodal pathways, it reaches the Atrioventricular or AV node. The AV node is inferiorly in the right atrium, anterior to the ostium of the coronary sinus, and above the tricuspid valve. The speed of Conduction is slowed (about 200 mm/s) through the AV node. The AV node is anatomically a complicated network of fibers. These fibers converge at their lower margin to form a discrete bundle of fibers, the bundle of His (or AV bundle). This structure penetrates the annulus fibrosis and arrives at the upper margin of the muscular intraventricular septum. There the bundle of His gives origin to the bundle branches.

The left bundle branch arises as a series of radiations, or fascicles, at right angles to the bundle of His. A superior anterior fascicle courses down the anterior aspect of the interventricular septum to the anterolateral papillary muscle, where it breaks into a Purkinje network. The inferior posterior fascicle is shorter and thicker, passing posteriorly to the base of the posteromedial papillary muscle, where it branches into the Purkinje network. Purkinje fibers to the interventricular septum may arise as separate radiation or as fibers from the anterior or posterior fascicles.

The right bundle branch courses down the interventricular septum on the right side. It contributes Purkinje fibers to the septum only near the apex of the right ventricle. At the lower end of the septum, it passes into the right ventricular wall, where it branches into a Purkinje network.

As the electrical impulse leaves the AV node, it passes into the bundle of His and then down the bundle branches simultaneously. The first section of the ventricle to begin depolarization is the midportion of the interventricular septum from the left side, giving rise to the normal Q wave on the 12-lead ECG. The walls of the left and right ventricles are depolarized simultaneously. The speed of Conduction through the ventricular Purkinje network is rapid, about 4000 mm/s. Conduction in the ventricular muscle is considerably slower (about 400 mm/s).

Disclaimer: This video is for educational purposes and is not intended as medical advice. While we strive for 100% accuracy, errors may occur, and medications or protocols may change over time.


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