Syncope: what is the trigger?
Although a syncopal attack is frequently preceded by prodromal symptoms, sometimes the onset can be so abrupt that there is no warning at all. The switch in autonomic responses responsible for such an attack is quite rapid and dramatic, but the trigger for this remains one of the unresolved mysteries in cardiovascular physiology.
Hainsworth
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D001341:Autonomic Nervous System / Q000502:physiology; D006801:Humans; D013575:Syncope / Q000209:etiology*; D014664:Vasodilation / Q000502:physiology