Shallow-water blackout

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Shallow-water blackout is loss of consciousness at a shallow depth due to hypoxia during a dive, which could be the result of any one of significantly differing causative circumstances. The term is ambiguous, and the depth range in which it may occur is generally shallow relative to the preceding part of the dive, but also occurring when the entire dive takes place at an almost constant depth within a few metres of the surface. various situations may be referred to as shallow water blackout, but differ in how the hypoxia is induced: Some occur in a context of freediving, others occur during ascent while scuba diving, usually when using a rebreather, and occasionally while surface-supplied diving.

Contents

Freediving

Two very different breathhold dive profiles can lead to hypoxic blackout at shallow depth.

At constant depth

Blackout may occur when all phases of a breathhold dive have taken place in shallow water, where depressurisation during ascent is not a significant factor, and the blackout may occur without warning before the diver attempts to surface. The mechanism for this type of shallow water blackout is lack of arterial oxygen expedited by low carbon dioxide levels, as a consequence of voluntary hyperventilation before the dive. Blackouts which occur in swimming pools are probably driven only by excessive hyperventilation, with no significant influence from pressure change. There is broad agreement among diving physiologists to call this shallow water blackout or constant pressure blackout. [1] [2] [3] [4] [5]

During ascent

Blackout can occur during ascent from a deep freedive or immediately after surfacing. This is due to a relatively rapidly lowered oxygen partial pressure caused by a reduction in ambient pressure after much of the available arterial oxygen has been used up at the higher partial pressures induced by depth, leaving the diver in a state of latent hypoxia, with actual cerebral hypoxia inevitable during ascent. Blackout in the shallow stage of ascent from deep free-dives is less ambiguously called "ascent blackout", or unambiguously "freediving blackout of ascent", and has also sometimes been called "deep water blackout", which is also an ambiguous term, being used by some authors for loss of consciousness that occurs at depths of greater than about 60 metres (200 ft) when breathing air, hypothetically as a consequence of nitrogen narcosis, oxygen toxicity, or both. [1] [2] [6] [5] [7] [8]

Scuba and surface-supplied diving

One of the hazards of rebreather diving is a hypoxic loss of consciousness while ascending because of a sudden uncompensated drop of oxygen partial pressure in the breathing loop. This occurs as a result of the pressure reduction during ascent, usually associated with manually controlled closed circuit rebreathers and semi-closed circuit rebreathers, (also known as gas extenders), which do not use automatic feedback from the measured oxygen partial pressure to control the mixture in the loop. A similar effect can occur in open circuit scuba and surface-supplied dicing if a diver continues to breathe a hypoxic gas intended for avoiding oxygen toxicity in the deep sector, at a depth shallower than the minimum operating depth for the gas, but this is usually just called hypoxia. [9]

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<span class="mw-page-title-main">Trimix (breathing gas)</span> Breathing gas consisting of oxygen, helium and nitrogen

Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.

<span class="mw-page-title-main">Technical diving</span> Extended scope recreational diving

Technical diving is scuba diving that exceeds the agency-specified limits of recreational diving for non-professional purposes. Technical diving may expose the diver to hazards beyond those normally associated with recreational diving, and to a greater risk of serious injury or death. Risk may be reduced via appropriate skills, knowledge, and experience. Risk can also be managed by using suitable equipment and procedures. The skills may be developed through specialized training and experience. The equipment involves breathing gases other than air or standard nitrox mixtures, and multiple gas sources.

<span class="mw-page-title-main">Deep diving</span> Underwater diving to a depth beyond the norm accepted by the associated community

Deep diving is underwater diving to a depth beyond the norm accepted by the associated community. In some cases this is a prescribed limit established by an authority, while in others it is associated with a level of certification or training, and it may vary depending on whether the diving is recreational, technical or commercial. Nitrogen narcosis becomes a hazard below 30 metres (98 ft) and hypoxic breathing gas is required below 60 metres (200 ft) to lessen the risk of oxygen toxicity.

<span class="mw-page-title-main">Generalized hypoxia</span> Medical condition of oxygen deprivation

Generalized hypoxia is a medical condition in which the tissues of the body are deprived of the necessary levels of oxygen due to an insufficient supply of oxygen, which may be due to the composition or pressure of the breathing gas, decreased lung ventilation, or respiratory disease, any of which may cause a lower than normal oxygen content in the arterial blood, and consequently a reduced supply of oxygen to all tissues perfused by the arterial blood. This usage is in contradistinction to localized hypoxia, in which only an associated group of tissues, usually with a common blood supply, are affected, usually due to an insufficient or reduced blood supply to those tissues. Generalized hypoxia is also used as a synonym for hypoxic hypoxia This is not to be confused with hypoxemia, which refers to low levels of oxygen in the blood, although the two conditions often occur simultaneously, since a decrease in blood oxygen typically corresponds to a decrease in oxygen in the surrounding tissue. However, hypoxia may be present without hypoxemia, and vice versa, as in the case of infarction. Several other classes of medical hypoxia exist.

<span class="mw-page-title-main">Scuba diving</span> Swimming underwater, breathing gas carried by the diver

Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The name scuba is an anacronym for "Self-Contained Underwater Breathing Apparatus" and was coined by Christian J. Lambertsen in a patent submitted in 1952. Scuba divers carry their own source of breathing gas, usually compressed air, affording them greater independence and movement than surface-supplied divers, and more time underwater than free divers. Although the use of compressed air is common, a gas blend with a higher oxygen content, known as enriched air or nitrox, has become popular due to the reduced nitrogen intake during long or repetitive dives. Also, breathing gas diluted with helium may be used to reduce the effects of nitrogen narcosis during deeper dives.

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.

<span class="mw-page-title-main">Latent hypoxia</span> Lung gas and blood oxygen concentration sufficient to support consciousness only at depth

Latent hypoxia is a condition where the oxygen content of the lungs and arterial blood is sufficient to maintain consciousness at a raised ambient pressure, but not when the pressure is reduced to normal atmospheric pressure. It usually occurs when a diver at depth has a lung gas and blood oxygen concentration that is sufficient to support consciousness at the pressure at that depth, but would be insufficient at surface pressure. This problem is associated with freediving blackout and the presence of hypoxic breathing gas mixtures in underwater breathing apparatus, particularly in diving rebreathers.

<span class="mw-page-title-main">Bailout bottle</span> Emergency gas supply cylinder carried by a diver

A bailout bottle (BoB) or, more formally, bailout cylinder is a scuba cylinder carried by an underwater diver for use as an emergency supply of breathing gas in the event of a primary gas supply failure. A bailout cylinder may be carried by a scuba diver in addition to the primary scuba set, or by a surface supplied diver using either free-flow or demand systems. The bailout gas is not intended for use during the dive except in an emergency, and would be considered a fully redundant breathing gas supply if used correctly. The term may refer to just the cylinder, or the bailout set or emergency gas supply (EGS), which is the cylinder with the gas delivery system attached. The bailout set or bailout system is the combination of the emergency gas cylinder with the gas delivery system to the diver, which includes a diving regulator with either a demand valve, a bailout block, or a bailout valve (BOV).

<span class="mw-page-title-main">Scuba gas planning</span> Estimation of breathing gas mixtures and quantities required for a planned dive profile

Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive. It may assume that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.

<span class="mw-page-title-main">Decompression practice</span> Techniques and procedures for safe decompression of divers

To prevent or minimize decompression sickness, divers must properly plan and monitor decompression. Divers follow a decompression model to safely allow the release of excess inert gases dissolved in their body tissues, which accommodated as a result of breathing at ambient pressures greater than surface atmospheric pressure. Decompression models take into account variables such as depth and time of dive, breathing gasses, altitude, and equipment to develop appropriate procedures for safe ascent.

<span class="mw-page-title-main">Scuba gas management</span> Logistical aspects of scuba breathing gas

Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.

<span class="mw-page-title-main">Rebreather diving</span> Underwater diving using self contained breathing gas recycling apparatus

Rebreather diving is underwater diving using diving rebreathers, a class of underwater breathing apparatus which recirculate the breathing gas exhaled by the diver after replacing the oxygen used and removing the carbon dioxide metabolic product. Rebreather diving is practiced by recreational, military and scientific divers in applications where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable. The main advantages of rebreather diving are extended gas endurance, low noise levels, and lack of bubbles.

Human physiology of underwater diving is the physiological influences of the underwater environment on the human diver, and adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply. It, therefore, includes the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus. Several factors influence the diver, including immersion, exposure to the water, the limitations of breath-hold endurance, variations in ambient pressure, the effects of breathing gases at raised ambient pressure, effects caused by the use of breathing apparatus, and sensory impairment. All of these may affect diver performance and safety.

<span class="mw-page-title-main">Outline of underwater diving</span> List of articles related to underwater diving grouped by topical relevance

The following outline is provided as an overview of and topical guide to underwater diving:

<span class="mw-page-title-main">Index of underwater diving: N–Z</span> Alphabetical listing of underwater diving related topics

The following index is provided as an overview of and topical guide to underwater diving: Links to articles and redirects to sections of articles which provide information on each topic are listed with a short description of the topic. When there is more than one article with information on a topic, the most relevant is usually listed, and it may be cross-linked to further information from the linked page or section.

The science of underwater diving includes those concepts which are useful for understanding the underwater environment in which diving takes place, and its influence on the diver. It includes aspects of physics, physiology and oceanography. The practice of scientific work while diving is known as Scientific diving. These topics are covered to a greater or lesser extent in diver training programs, on the principle that understanding the concepts may allow the diver to avoid problems and deal with them more effectively when they cannot be avoided.

<span class="mw-page-title-main">Index of underwater diving: A–M</span> Alphabetical listing of underwater diving related topics

The following index is provided as an overview of and topical guide to underwater diving:

References

  1. 1 2 Brubakk, A. O.; Neuman, T. S. (2003). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders Ltd. p. 800. ISBN   978-0-7020-2571-6.
  2. 1 2 Lindholm, P; Pollock, N.W.; Lundgren, C.E.G., eds. (2006). Breath-hold diving. Proceedings of the Undersea and Hyperbaric Medical Society/Divers Alert Network 2006 June 20–21 Workshop. Durham, NC: Divers Alert Network. ISBN   978-1-930536-36-4.
  3. ↑ Edmonds, C. (1968). Shallow Water Blackout. RANSUM-8-68 (Report). Royal Australian Navy, School of Underwater Medicine.
  4. ↑ Lindholm P, Lundgren CE (2006). "Alveolar gas composition before and after maximal breath-holds in competitive divers". Undersea Hyperb Med. 33 (6): 463–7. PMID   17274316.
  5. 1 2 Campbell, Ernest (1996). "Free Diving and Shallow Water Blackout". Diving Medicine Online. scuba-doc.com. Retrieved 24 January 2017.
  6. ↑ Elliott, D. (1996). "Deep Water Blackout". South Pacific Underwater Medicine Society Journal. 26 (3). ISSN   0813-1988. OCLC   16986801.
  7. ↑ Exley, Sheck (1986). Basic Cave Diving: A Blueprint for Survival (PDF) (5th ed.). Lake City, Florida: National Speleological Society Cave Diving Section.
  8. ↑ Eliott, David (1996). "Deep water blackout". SPUMS Journal. 26 (3): 205–208.
  9. ↑ Pridmore, Simon (22 April 2012). "The Insidious Threat of Hypoxic Blackout in Rebreather Diving". X-Ray Mag. AquaScope Media ApS. Archived from the original on 22 March 2018. Retrieved 21 March 2018.
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