Sinus bradycardia: A regular but unusually slow heart beat (50 beats/minute or less at rest). Sinus bradycardia can be the result of many things including good physical fitness, medications, and some forms of heart block. "Sinus" refers to the sinus node, the heart's natural pacemaker which creates the normal regular heartbeat. "Bradycardia" means that the heart rate is slower than normal.
Fainting can occur with sinus bradycardia if the heart slows down even more. This can occur, for example, when the vagus nerve sends a signal to the heart that suddenly drops the heart rate. Choking or gagging can trigger this response which is called the vasovagal reflex. US President George W. Bush has sinus bradycardia and had an episode of fainting due to a vasovagal response after choking on a pretzel.
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