...digressing back to shallow-water blackout...
There recently was a seminar given at the Univ of Hawaii Med School concerning shallow-water blackout (as related to freediving), probably in response to the deaths that Searat mentioned. Basically, a hypoxic condition is any in which your ppO2 (partial pressure of O2) falls below 0.21 (21%). Unconciousness occurs generally at a ppO2 of 0.10. On the other hand, the breathing trigger is the high amount of CO2 detected by peripheral receptors principally located along the carotid artery and aortic arch. What happens in shallow-water blackout is that the ppO2 falls into the 0.10 range giving unconciousness but the CO2 levels are not high enough to provoke spontaneous respiration. In freediving, this typically occurs within 15ft of the surface. It also means that if the diver manages to break into air before spontaneous respiration, there's a reasonable chance of survival. People react differently to hypoxia; some become euphoric, others may feel dread. Rob White, owner of BlueWater Hunters in Kailua-Kona, advocates extensive pool practice: 1) so that you know your signs of impending blackout and 2) to rehearse rescue and recovery of a fellow diver.
(Note: the physiology here is based on my recollection of the presentation and should not be relied upon without verification from other sources.)
