When the heart beats too slowly

Known as bradycardia, this heart rhythm disorder can cause fatigue, dizziness, and fainting.

The steady beat of your heart depends on a crescent-shaped cluster of cells in the upper right portion of your heart. Called the sinoatrial or sinus node, it emits a tiny jolt of electrical current that triggers the heart to contract and pump blood throughout the body (see illustration). Because the sinus node determines the heart's pace and rhythm, it is sometimes called the body's natural pacemaker.

Just as your skin, joints, and other parts of your body reveal signs of normal wear and tear as you age, so too can the structures inside your heart. "This age-related degeneration can affect the sinoatrial node and other parts of the heart's conduction system," says Dr. Peter Zimetbaum, director of clinical cardiology at Harvard-affiliated Beth Israel Deaconess Medical Center.