Bradycardias

When the heart beats too slowly (less than 60 beats per minute at rest), the condition is known as a bradycardia. This isn't always a medical problem; people who are aerobically fit tend to have lower heart rates, sometimes below 60 beats per minute. Yet their hearts efficiently pump enough blood throughout the body. When the heart is not pumping sufficient blood because of a slow heartbeat, it may fall into one of the categories of bradycardia.

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• Sick sinus: Normally the electrical messages in the heart begin in a cluster of cells in the right atrium, called the sinus node (SA node). The sinus node is your heart's natural pacemaker. If it is malfunctioning, it can cause the heart to beat too slowly or to alternate between fast and slow beating. This problem may be caused by a blocking of the electrical impulse near the SA node.
• Conduction block: The electrical pathway may also be blocked near the AV node, in between the atria and ventricles. A conduction block doesn't completely stop the heartbeat because another set of cells in the AV node or ventricles act as a secondary pacemaker. When this "backup" system goes into effect, the heartbeat is slower than when it is regulated by the SA node, which serves as the atrial pacemaker. Occasionally, the "backup" system fails to activate, and the heart temporarily stops beating, resulting in loss of consciousness since blood is no longer reaching the brain. The block can be caused by heart disease, an injury to the area during heart surgery, a congenital defect, or the side effects of certain drugs.