Freediving blackout, breath-hold blackout, or apnea blackout is a class of hypoxic blackout, a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold (freedive or dynamic apnea) dive, when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it. It can be provoked by hyperventilating just before a dive, be a consequence of the pressure reduction on ascent, or a combination of the two. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers and have not experienced problems before. Blackout may also be referred to as a syncope or fainting. Divers and swimmers who black out or grey out underwater during a dive will usually drown unless rescued and resuscitated within a short time. Freediving blackout has a high fatality rate, and mostly involves males younger than 40 years, but is generally avoidable. Risk cannot be quantified, but is clearly increased by any level of hyperventilation. Freediving blackout can occur on any dive profile - at constant depth, on an ascent from depth, or at the surface following ascent from depth - and may be described by a number of terms, depending on the dive profile and depth at which consciousness is lost. Blackout during a shallow dive differs from blackout during ascent from a deep dive, in that blackout during ascent is precipitated by depressurisation on ascent from depth, while blackout in consistently shallow water is a consequence of hypocapnia following hyperventilation.
Terminology Different types of freediving blackout have become known under a variety of names. These include:
ascent blackout blackout on ascent hypoxia of ascent Loss of consciousness which occurs as the surface is approached, or even at the surface, following a deep breath-hold dive (generally over ten metres) and typically involving free-divers practicing dynamic apnea depth diving, usually at sea. The mechanism of ascent blackout is hypoxia, arising from the rapid drop in the partial pressure of oxygen in the lungs on ascent as the ambient pressure drops and the gas in the lungs expands to surface volume. See latent hypoxia.
constant depth blackout constant pressure blackout isobaric blackout A specific form of hypoxic blackout which occurs where all phases of the dive have taken place in shallow water; hence, depressurisation is not a significant factor. This is often referred to by breath-hold divers as shallow water blackout. The mechanism for this type of blackout is hypoxia expedited by hypocapnia caused by voluntary hyperventilation before the dive. These blackouts typically occur in swimming pools and are probably driven only by excessive hyperventilation, with no significant influence of pressure change.
deep water blackout An alternative term for blackout on ascent when used by free divers. As it is also used for other purposes, ascent blackout is the less ambiguous option.
hyperventilation-induced blackout The recommended term for cases where hyperventilation is known, or suspected, to have been a contributing factor in either shallow water blackout or deep water blackout.
latent hypoxia The precursor to blackout on ascent, where the partial pressure of oxygen remains sufficient to maintain consciousness, but only at depth, under pressure, and is already insufficient to maintain consciousness at the shallower depths that must be encountered on ascent.
shallow water blackout A 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. 1. Blackout which occurs when all phases of the dive have taken place in shallow water (i.e., where depressurisation is not a significant factor) and typically involves dynamic apnea distance swimmers, usually in a swimming pool. 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. This can also be described as a constant pressure blackout or isobaric blackout. 2. A 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 and ascent blackout, which can be confusing. 3. Used in diving but not in free diving, loss of consciousness while ascending on a rebreather, due to sudden drop of oxygen partial pressure in the breathing loop, usually associated with manual CCR and SCR. As there is a large overlap between the research communities studying the physiology of freediving and other modes of underwater diving, this usage can also lead to confusion.
surface blackout A possible final stage of blackout on ascent that occurs when a diver with low levels of circulating oxygen has surfaced and begun breathing, yet blacks out before the inhaled oxygen has had time to reach the brain.
underwater blackout syndrome hypoxic blackout A loss of consciousness during a breath-hold submersion preceded by hyperventilation, where alternative causes of blackout have been excluded.
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