[lungfish]

Shallow Water Blackout:
Usually occurs in 10meters of water or less and primary contributing factors are: hyperventilation, exercise, a competitive personality and youth. In otherwords, you can black out in shallow water for a number of reasons so plan and prepare for your dive carefully. Get trained, take a course.

Vacuum Effect:
While a diver descends lung volume decreases and 02 concentration increases due to hydrostatic pressure increasing. On ascent, the lungs increase in volume as hydrostatic pressure decreases which rapidly reduces 02 concentration to critical levels. The Vacuum effect is really a net flow of 02 back to the lungs from all the organs including the brain. It is most pronounced in the last 33ft of ascent where the pressure differential is greatest. It is instantaneous and without warning and is the result of critically low 02 level effectively switching off the brain. (paraphrasing from the manual).

Deepwater Blackout:
1)During descent bradycardia occurs and heartrate and pulse fall to very low levels. However, we are head down and that makes it easier for oxygenated blood to reach our brains. When we turn upright at the bottom of the dive we momentarily interupt blood flow to our head and we may black out. One should be very careful at this phase and spend a moment in the horizantal position before one slowly inverts oneself.

2)Or one can over-purge and reduce ones C02 levels too low and not notice when 02 levels are too low to sustain consciousness.

Primary contributing factors include longer breath-holds at deeper depths, hyperventilating or excessive purging, insufficient rest periods between dives and aggressive turnarounds.