page_title: Freediving blackout
section_title: Ascent blackout
hierarchical_section_title: Freediving blackout / Mechanisms / Ascent blackout
caption_attribution_description: Photo by James Frazeel. Posted here for his approval. Previously contacted him for approval, now recontacting to view in place. Will advise on approval.
context_page_description: 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 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, or as a consequence of the pressure reduction on ascent, or a combination of these. 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.
context_section_description: An ascent blackout, or deep water blackout, is a loss of consciousness caused by cerebral hypoxia on ascending from a deep freedive or breath-hold dive, typically of ten metres or more when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it.  Victims typically black out close to the surface, usually within the top three metres, sometimes even as they break surface and have often been seen to approach the surface without apparent distress only to sink away.  It is quite rare for blackouts to occur while at the bottom or in the early stages of ascent; divers who drown in these stages are usually found to have inhaled water, indicating that they were conscious and succumbed to an uncontrollable urge to breathe rather than blacking out.  Victims are usually established practitioners of deep breath-hold diving, are fit, strong swimmers and have not experienced problems before. Blackout by this mechanism may occur even after surfacing from depth and breathing has commenced if the inhaled oxygen has not yet reached the brain and may be referred to as a surface blackout.
  The partial pressure of oxygen in the air in the lungs controls the oxygen loading of blood. A critical pO₂ of 30 millimetres of mercury (40 mbar) in the lungs will sustain consciousness when breathing is resumed after a breath-hold dive. This is about 4% oxygen in the lungs and 45% oxygen saturation of the arterial blood. At 30 msw (4 bar), 2% by volume oxygen in the lung gas gives a pO₂ of 60 millimetres of mercury (80 mbar). At 10 msw (2 bar), for the same 2% oxygen, the pO₂ would be 30 millimetres of mercury (40 mbar), i.e. marginal. At the surface the same 2% oxygen drops to 15 millimetres of mercury (20 mbar), ignoring metabolic use.
  Three factors are thought to be involved: Voluntary suppression of breathing and rapid depressurisation are necessarily present, and self-induced hypocapnia by hyperventilation is known to be present in many cases. Depressurisation on ascent is an explanation for the shallow depth of ascent blackouts but does not fully explain all cases unless accompanied by an underlying suppression of the urge to breathe through self-induced hypocapnia via hyperventilation.
  Voluntary suppression of breathing.  Deep water blackout is sometimes attributed simply to the practiced diver's ability through training to suppress the urge to breathe.  If surviving divers are aware that they have heavily suppressed the urge to breathe towards the end of the dive there is a tendency to look no further for an explanation.  However, there are two problems with this as an explanation:
  Even with a high level of training the hypercapnic urge to breathe is almost impossible to overcome; swimmers typically suffer an uncontrollable, violent, deep inhalation of water even when, intellectually, they know that to do so is fatal. This is a simple case of running out of air and drowning.  Victims of ascent blackout, if they have any water in the lungs at all will have a limited amount in the bronchi consistent with natural ingress after death.
  Victims of deep water blackout closely observed from both below and above water do not exhibit the signs of distress associated with an uncontrollable urge to breathe and those that have survived a blackout report no such distress.  Many blackout events have been closely observed and even filmed because deep dynamic apnoea dives are a competitive event and very deep dives require a considerable support crew both above and below water.  Anecdotal accounts of healthy divers holding their breath to the point of unconsciousness without hyperventilation are difficult to substantiate and the ability, if it exists, is certainly extremely rare.
  Rapid depressurisation.  Because ascent blackout occurs as the diver approaches the surface from a deep dive, depressurisation is clearly present. Consciousness depends on a minimum partial pressure of oxygen in the brain not on the absolute quantity of the gas in the system. At the surface, the air in the 
caption: Latent hypoxia hits on ascent
exif: {}