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).
Sinus tachycardia occurs in response to exercise, exertion,
excitement, pain, fever, excessive thyroid hormone, low blood oxygen
(hypoxia), stimulant drugs (such as caffeine and amphetamines), etc.
Although sinus tachycardia is usually a response to one or another
fleeting stimulus, it can upon occasion be a sign of heart failure,
heart valve disease, or another significant illness.
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