Transient loss of consciousness, typically lasting no longer than 1 to 2 minutes, due to a transient drop in cerebral perfusion pressure
Most common mechanism is vasovagal or neurocardiogenic, in which a variety of stimuli and conditions—pain, heat, dehydrated state, emotional upset, carotid pressure—trigger increased vagal tone, leading to slowed heart rate, peripheral vasodilation, and decreased cerebral perfusion.
There's more to see -- the rest of this topic is available only to subscribers.