Tachycardia, sinus: A fast heartbeat (tachycardia) because of rapid firing of the sinoatrial (sinus) node.
The sinoatrial (or sinus) node is the natural pacemaker of the heart. It is located in the wall of the right atrium (the right upper chamber of the heart). Normal cardiac impulses start there and are transmitted to the atria and down to the ventricles (the lower chambers of the heart).
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