[efattah]

I have had periods where I could black out from just taking a deep breath.

Adrenaline controls constriction of the abdominal vessels to push up BP in the brain as soon as you stand up. When sitting or laying for long periods, the adrenals shut off and can be delayed when standing up, causing a near BO or BO.

Raw food diets, vegetarian or low salt diets are a disaster for blackouts. The first thing I tell new divers is to get plenty of salt and stay hydrated. Low carb is also a big factor since low glycogen = low hydration.

Stress affects the adrenals, which affects your adrenaline and vagus nerve. The condition of the vagus nerve is important. During times of bad adrenal & vagus function, I could not do an inhale static without blackout out around 1'10" from vasovagal miscommunication.

Another lower probability could be something like sickle cell anemia.