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Shallow-water blackout

Shallow-water blackout refers to loss of consciousness during a dive associated with blackout at a shallow depth. The term is used for several different mechanisms, depending on context; therefore, this term may often lead to confusion.

  • Freediving blackout#Shallow water blackout. Blackout which occurs when all phases of a breathhold dive have taken place in shallow water (i.e., where depressurisation is not a significant factor). The mechanism for this type of shallow water blackout is hypoxia expedited by hypocapnia caused by voluntary hyperventilation before the dive. Blackouts which occur in swimming pools are probably driven only by excessive hyperventilation, with no significant influence of pressure change.
  • Freediving blackout#Ascent blackout. Loss of consciousness caused by cerebral hypoxia at the end of a deep breath-hold dive during the latter part of the ascent or immediately after surfacing due to lowered oxygen partial pressure caused by reduction in ambient pressure. Blackout in the shallow stage of ascent from deep free dives is also sometimes called deep water blackout, which can be confusing.
  • Rebreather diving#Hazards. Also used scuba diving for hypoxic loss of consciousness while ascending on a rebreather due to sudden uncompensated drop of oxygen partial pressure in the breathing loop due to the pressure reduction of ascent, usually associated with manual closed circuit and semi-closed circuit rebreathers.