A is used to treat some diving disorders and for training divers to recognise the symptoms.are specifically arising from. The and of these may present during a dive, on surfacing, or up to several hours after a dive. Have to breathe a gas which is at the same pressure as their surroundings , which can be much greater than on the surface. The ambient pressure underwater increases by 1 (100 kPa) for every 10 metres (33 ft) of depth.The principal conditions are: (which covers and );;;; and (burst lung). Although some of these may occur in other settings, they are of particular concern during diving activities.The disorders are caused by at the high pressures encountered at depth, and divers will often breathe a gas mixture different from air to mitigate these effects., which contains more and less, is commonly used as a breathing gas to reduce the risk of decompression sickness at (up to about 40 metres (130 ft)).

May be added to reduce the amount of nitrogen and oxygen in the gas mixture when diving deeper, to reduce the effects of narcosis and to avoid the risk of oxygen toxicity. This is complicated at depths beyond about 150 metres (500 ft), because a helium–oxygen mixture then causes high pressure nervous syndrome. More exotic mixtures such as, a hydrogen–helium–oxygen mixture, are used at extreme depths to counteract this. The recompression chamber at the used for treating DCS and training(DCS) occurs when gas, which has been breathed under high pressure and dissolved into the, forms bubbles as the pressure is reduced on ascent from a dive.

The results may range from pain in the joints where the bubbles form to blockage of an leading to damage to the, or death. While bubbles can form anywhere in the body, DCS is most frequently observed in the shoulders, elbows, knees, and ankles. Joint pain occurs in about 90% of DCS cases reported to the, with symptoms and skin manifestations each present in 10% to 15% of cases. DCS is very rare in divers. The table below classifies the effects by affected organ and bubble location. The pulmonary circulationIf the compressed air in a diver's cannot freely escape during an ascent, particularly a rapid one, then the lung tissues may rupture, causing (PBT). The air may then enter the producing (AGE), with effects similar to severe.

Hydrogen narcosis (also known as the hydrogen effect) is the psychotropic state induced by breathing hydrogen at high pressures. Hydrogen narcosis produces symptoms such as hallucinations, disorientation, and confusion, which are similar to hallucinogenic drugs.It can be experienced by deep-sea divers who dive to 300 m (1,000 ft) below sea level breathing hydrogen mixtures. Helpful SCUBA diving links. Narcosis Dive Company has daily charter trips on one of the most spacious dive boats in town.

Although AGE may occur as a result of other causes, it is most often secondary to PBT. AGE is the second most common cause of death while diving ( being the most common stated cause of death).

Gas bubbles within the arterial circulation can block the supply of blood to any part of the body, including the brain, and can therefore manifest a vast variety of symptoms. The following table presents those signs and symptoms which have been observed in more than ten percent of cases diagnosed as AGE, with approximate estimates of frequency.Other conditions that can be caused by pulmonary barotrauma include,. Narcosis can produce, making it difficult to read multiple gauges.is caused by the pressure of dissolved gas in the body and produces to the. This results in alteration to thought processes and a decrease in the diver's ability to make judgements or calculations. It can also decrease, and worsen performance in tasks requiring.

As depth increases, so does the pressure and hence the severity of the narcosis. The effects may vary widely from individual to individual, and from day to day for the same diver. Because of the perception-altering effects of narcosis, a diver may not be aware of the symptoms, but studies have shown that impairment occurs nevertheless.

Since the choice of breathing gas also affects the depth at which narcosis occurs, the table below represents typical manifestations when breathing air. An EEG recording netis the least of all gases, and divers may use containing a proportion of helium for dives exceeding about 40 metres (130 ft) deep. In the 1960s it was expected that helium narcosis would begin to become apparent at depths of 300 metres (1,000 ft). However, it was found that different symptoms, such as, occurred at shallower depths around 150 metres (500 ft). This became known as, and its effects are found to result from both the absolute depth and the speed of descent. Although the effects vary from person to person, they are stable and reproducible for each individual; the list below summarises the symptoms observed underwater and in studies using simulated dives in the dry, using and (EEG) monitors.

Signs and symptoms of HPNSSymptomNotesImpairmentBoth intellectual and are. A 20% decrease in the ability to perform calculations and in is observed at 180 metres (600 ft), rising to 40% at depths of 240 metres (800 ft)Dizziness, and may occur in divers at depths of 180 metres (600 ft).

Animal studies under more extreme conditions have produced.Tremorsof the hands, arms and torso are observed from 130 metres (400 ft) onward. The tremors occur with a frequency in the range of 5–8 (Hz), and their severity is related to the speed of compression; the tremors reduce and may disappear when the pressure has stabilised.EEG changesAt depths exceeding 300 metres (1,000 ft), changes in the (EEG) are observed; the appearance of (4–6 Hz) and depression of (8–13 Hz).SomnolenceAt depths beyond the onset of EEG changes, test subjects intermittently, with 1 and 2 observed in the EEG.

Even when decompressed to shallower depths, the effect continues for 10–12 hours.Oxygen toxicity. During Professor Kenneth Donald carried out extensive testing for oxygen toxicity in divers. The chamber is pressurised with air to 3.7 bars (370 kPa; 54 psi).

The subject in the centre is breathing 100% oxygen from a mask.Although is essential to life, in concentrations greater than normal it becomes, overcoming the body's natural defences , and causing in any part of the body. The and are particularly affected by high of oxygen, such as are encountered in diving.

The body can tolerate partial pressures of oxygen around 0.5 (50 kPa; 7.3 psi) indefinitely, and up to 1.4 bars (140 kPa; 20 psi) for many hours, but higher partial pressures rapidly increase the chance of the most dangerous effect of, a resembling an. To oxygen toxicity varies dramatically from person to person, and to a much smaller extent from day to day for the same diver.

Prior to convulsion, several symptoms may be present – most distinctly that of an.During 1942 and 1943, Professor Kenneth W Donald, working at the Admiralty Experimental Diving Unit, carried out over 2,000 experiments on divers to examine the effects of oxygen toxicity. To date, no comparable series of studies has been performed. In one seminal experiment, Donald exposed 36 healthy divers to 3.7 bars (370 kPa; 54 psi) of oxygen in a chamber, equivalent to breathing pure oxygen at a depth of 27 metres (90 ft), and recorded the time of onset of various signs and symptoms.

Five of the subjects convulsed, and the others recovered when returned to normal pressure following the appearance of acute symptoms. The table below summarises the results for the relative frequency of the symptoms, and the earliest and latest time of onset, as observed by Donald. The wide variety of symptoms and large variability of onset between individuals typical of oxygen toxicity are clearly illustrated. Signs and symptoms of oxygen toxicity observed in 36 subjectsSigns and symptomsFrequencyEarliest onset (minutes)Latest onset (minutes)19911515predominanceNote. ^ Brubakk, Alf O; Neuman, Tom S, eds. '9: Pressure Effects'.

Bennett and Elliott's physiology and medicine of diving (5th Revised ed.). United States: Saunders Ltd. Pp. 265–418. Abraini, JH; Gardette-Chauffour, MC; Martinez, E; Rostain, JC; Lemaire, C (1994).

Journal of Applied Physiology. American Physiological Society. 76 (3): 1113–8. Retrieved 1 March 2009. Powell, Mark (2008). Deco for Divers.

Southend-on-Sea: Aquapress. P. 70. Francis, T James R; (2003). '10.6: Manifestations of Decompression Disorders'. In Brubakk, Alf O; Neuman, Tom S (eds.).

Bennett and Elliott's physiology and medicine of diving (5th Revised ed.). United States: Saunders Ltd. Pp. 578–99. Neuman, Tom S (2003). '10.5: Arterial Gas Embolism and Pulmonary Barotrauma'. In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th ed.).

United States: Saunders Ltd. Pp. 557–8. Neuman, Tom S (2003).

'10.5: Arterial Gas Embolism and Pulmonary Barotrauma'. In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th ed.). United States: Saunders Ltd. Pp. 568–71. Bennett, Peter B; Rostain, Jean Claude (2003). '9.2: Inert Gas Narcosis'.

In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th ed.).

United States: Saunders Ltd. P. 301. Lippmann, John; (2005). 'Nitrogen narcosis'.

Deeper into Diving (2nd ed.). Victoria, Australia: J L Publications. P. 105.; Rostain, Jean Claude (2003). '9.3: The High Pressure Nervous Syndrome'. In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th ed.). United States: Saunders Ltd.

Pp. 323–8. Clark, James M; Thom, Stephen R (2003). '9.4: Oxygen under pressure'.

In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th ed.). United States: Saunders Ltd. Pp. 358–360. Clark, James M; Thom, Stephen R (2003).

'9.4: Oxygen under pressure'. In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving (5th ed.). United States: Saunders Ltd. P. 376. Donald, Kenneth W (1947). British Medical Journal.

1 (4506): 667–72.Further reading. (PDF). European Committee for Hyperbaric Medicine. September 2003. Retrieved 22 November 2010. Donald, Kenneth W (1947).

British Medical Journal. 1 (4507): 712–7. Francis T James R, Smith DJ (eds) (1991). 42nd Undersea and Hyperbaric Medical Society Workshop. UHMS Publication Number 79(DECO)5-15-91. Archived from on 2011-07-27. Retrieved 2011-03-11.

CS1 maint: extra text: authors list. Bevan, John (1999). Retrieved 2011-06-13.

Technical diving is a term used to describe all diving methods that exceed the limits imposed on depth and/or immersion time for recreational scuba diving. Technical diving often involves the use of special gas mixtures (rather than compressed air) for breathing. The type of gas mixture used is determined either by the maximum depth planned for the dive or by the length of time that the diver intends to spend underwater. While the recommended maximum depth for conventional scuba diving is 130 feet, technical divers may work in the range of 170 feet to 350 feet, sometimes even deeper.

Technical diving almost always requires one or more mandatory decompression 'stops' upon ascent, during which the diver may change breathing gas mixes at least once. Decompression stops are necessary to allow gases that have accumulated in the diver's tissues (primarily nitrogen) to be released in a slow and controlled manner.

If an individual exceeds the limits of time and/or depth for recreational diving, and/or ascends too quickly, large bubbles can form in the tissues, joints, and bloodstream. The formation of these bubbles leads to an extremely painful condition known as Decompression Sickness (DCS), more commonly known as the 'bends,' which can cause paralysis and even death.

Nitrox

People have used compressed air as their breathing medium since the advent of diving in the 1950s. Its main advantage is that it is readily available and relatively inexpensive to compress into cylinders. Nevertheless, air is not the 'ideal' breathing mixture for diving. With a concentration of approximately 79 percent nitrogen, compressed air poses two potential problems for all divers: susceptibility to nitrogen narcosis (a condition resembling alcoholic intoxication) at deeper depths; and decompression sickness (DCS). Both of these can prove fatal to a diver. In an effort to reduce the ill effects of nitrogen on divers, nitrox was developed.

Nitrox is a generic term that can be used to describe any gaseous mixture of nitrogen and oxygen. In the context of technical diving, nitox is a mixture containing more oxygen than air. The two most commonly used nitrogen-oxygen mixtures contain 32 percent and 36 percent oxygen by volume. This differs significantly from compressed air, which contains approximately 21 percent oxygen by volume. While an increase of 12 to 16 percent oxygen by volume may not seem drastic, it allows divers to significantly extend their bottom time, and decreases their risk of developing DCS.

While diving with nitrox has definite benefits, it also has clearly associated risks. The major hazard is oxygen toxicity. This comes about when oxygen is inhaled in high concentrations for an extended period of time; this occurs primarily when a diver exceeds the recreational limits for depth. Under these circumstances, a diver can experience an epileptic-like seizure, which may lead to drowning. Due to this potentially fatal hazard, divers using nitrox must adhere to special dive tables. These tables list the maximum safe amount of time that a diver can stay underwater at a certain depth.

Mixed-Gas Diving

The term 'mixed-gas diving' refers to any activity in which the diver breathes a mixture other than air or nitrox. The main incentive to dive with 'non-air' gas mixtures is to avoid nitrogen narcosis. Mixed-gas diving can also be beneficial in improving decompression and avoiding oxygen toxicity. Mixed-gas diving operations require detailed planning, sophisticated equipment, and, at times, extensive support personnel and facilities. The fact that such dives are often conducted at great depths and for extended periods of time increases the risks associated with them. It is extremely important for the breathing mixture to be properly identified, because breathing the wrong mix can lead to a fatal accident.

One type of mixed gas diving involves the use of heliox. This (79 percent helium and 21 percent oxygen) mixture is often used for very deep diving. Unlike nitrogen, helium is not known to have an intoxicating effect at any depth; it has a lower density than nitrogen, making it easier to breathe; and in cases of extended submersion, it improves decompression.

Still, heliox has its drawbacks. It is expensive, has a limited availability, and its thermal conductivity is six times greater than that of nitrogen. This means that a diver breathing heliox will lose body heat six times faster than someone breathing compressed air or nitrox, making them susceptible to hypothermia. To prevent this, divers often wear special suits filled with hot water that is pumped down from the surface. Heating the heliox before the diver inhales it is another strategy used to combat hypothermia. Either of these procedures require specialized equipment and highly trained personnel.

Surface-supplied Diving

Surface-supplied diving (SSD) is an alternative to self-contained equipment. This method consists of lowering divers into the water on a support platform, or stage, and supplying them with breathing gas (air or another gas mixture) through a flexible hose attached to a diving helmet. Since the diver does not need to be concerned with a limited supply of breathing gas, SSD gives divers the flexibility they need to perform a variety of underwater tasks. The diver's helmet is connected to an 'umbilical' that supplies breathing gas, two-way communications, a depth measurement tube and, optionally, hot water to warm the dive suit. In many cases, a camera and lights are mounted on the diver's helmet. Video from the diver's cameras, as well as audio communication, allow a dive supervisor to monitor activity throughout the dive, and provide recommendations if any difficulties arise. SSD is particularly effective on deep or extended operations when divers are working in a relatively restricted area.

The heiresses series. Surface-supplied diving provides several advantages over scuba. These include a direct physical link between the diver and the surface; a continual, unlimited supply of breathing gas; a means of controlling the diver's depth and location; and a means of providing video and audio links to the surface.

Surface-supplied diving does, however, have disadvantages. Among them, a diver's mobility and range are limited by the length of the umbilical; in strong currents, the pull on the umbilical can be severe; divers must walk along the bottom on weighted boots, and are unable to swim effectively; and SSD operations require a large support crew and a great deal of equipment.

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