Harvard Heart Letter

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 heartís pace and rhythm, it is…

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