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Electrophysiology
The heart is designed to pump blood to the central and peripheral circulation. To accomplish this, there are two types of cells in the heart, electrical and mechanical. The electrical cells are part of the conduction system and have unique properties including automaticity, conductivity, rhythmaticity, and excitability. Automaticity means that the cells can automatically generate electrical impulses. The property of conductivity allows the impulse to be conducted from cell to cell. Rhythmaticity refers to the ability to rhythmically generate the impulses. On this page, we will review the role of electrolytes and the electrical conduction system of the heart.
Electrolytes play an important role in the hearts electrical activity. There are three major electrolytes - sodium, potassium, and calcium. In a resting cell, potassium is inside of the cell and sodium is outside the cell, making the electrical charge is more positive on the outside than on the inside. At this point, the positive and negative charges are balanced and the cell is polarized. Polarization is a ready state, where the cells are ready for action. The chemicals then switch places, depolarizing the cells. The atria typically depolarize first, followed by the ventricles. It is important to understand that depolarization is an electrical activity. It a healthy heart depolarization should cause a coordinated contraction. Repolarization is the return of electrical charges to their original state of readiness. |
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From the AV node, the impulse travels through the Bundle of His to the left and right bundle branches, and finally to the Purkinje fibers. Each portion of the heart has a built in or inherent rate. In general, the site with the fastest rate will become the pacemaker site, with the SA node being the normal pacing site. When irritability occurs, a site may speed up and take over as the pacing site. When a site fails, a lower site takes over resulting in an escape rhythm. |