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Diving as a sport has become much more popular in the past 30 years and there are at least 53,000 current divers in the UK - 5,000 commercial divers, around 8,000 members of the armed forces and more than 40,000 members of the public paying for some form of instruction every year. Exploitation of underwater sources of gas and oil have also increased the need for professional divers.
Diving accidents can occur with any form of diving whilst entering the water. They can also be associated with snorkel diving, SCUBA diving, or deep sea diving.
There is a British Diving Safety Group with representatives from many organisations including the Health and Safety Executive, the Royal Navy and the Maritime and Coastguard Agency.
The act of diving can affect normal body structure and function. Ambient pressure increases by 1 atmosphere (1 bar or 100 kPa) for every 10 metres descent in sea water.
- Gas volumes change inversely with pressure - this means that on descent gas in body cavities undergoes compression, whilst on ascent it expands. Tissue damage can result.
- The partial pressure of gases increases proportionally as ambient pressure increases. This means that inert gases such as nitrogen can dissolve in tissues at depth and come out of solution when the diver ascends.
- Because the density of inhaled gas increases as pressure increases, breathing can be restricted. Lung volume is also reduced because of displacement of blood from the periphery to the thorax.
- The British Sub Aqua Club reported 364 diving incidents in 2010, including 98 decompression incidents and 17 fatalities.
- The Health and Safety Executive reported 24 fatal accidents in the 8-year period from 1996/97 to 2003/04, many amongst people receiving recreational diving training by instructors. 
Accidents on entering the water
- Accidents may occur due to hyperextension of the spine, especially the neck.
- If it is necessary to enter the water from a great height (eg abandoning a sinking ship), it is much safer to enter the water feet first than head first. When people take their own life by jumping from a very high bridge, it is often the force of the impact with the water that is fatal.
- However, many accidents related to entering the water also occur when diving into shallow water.
- Nocturnal swimming (particularly when associated with the consumption of alcohol) is a major risk factor.
- Acute spinal cord injury is discussed elsewhere. See separate article Spinal Cord Injury and Compression.
Different forms of diving
- SCUBA diving: SCUBA stands for 'self-contained underwater breathing apparatus'. It first became available for civilian use in the late 1940s. It enables the diver to reach greater depths and is generally regarded as suitable for depths of up to 40 metres for advanced, experienced divers.
- Snorkel diving: a snorkel is a J-shaped tube from the mouth that enables the swimmer to breathe whilst face down in water. In normal breathing there is a dead space that ventilates the airways and does not reach the gas-exchanging tissues of the alveoli. This represents about 1 ml for every 1 cm of the person's height. A snorkel effectively increases this dead space by its own volume. If the swimmer goes deeper so that the snorkel tip is below the surface, they can prevent water from entering by blowing against it. When they reach the surface, they can then blow out the water. An extra piece may be present in the snorkel and is like a small table tennis ball in a cage: if the swimmer dives, the ball floats up and blocks the snorkel so that water will not enter.
- Deep sea diving: this is conducted by professionals. They may be in the Royal Navy or working for the gas and oil industry. There are strict regulations regarding fitness, training and behaviour but it is a high-risk occupation and accidents are not uncommon. For depths down to 60-70 metres, a proper diving suit is required. There are heavy boots to help take the diver down feet first. Rather than a SCUBA apparatus, air is pumped through a line and the diver does not swim freely but is raised and lowered from above. For greater depths, special high-pressure suits are necessary and for very great depths a bathysphere is required.
General safety precautions
Whether diving is a hobby or an occupation, it carries significant risks. This applies to both shallow or deep water diving.
- Diving requires the individual to be in peak physical and mental condition. See 'Fitness to dive', below.
- It is essential that in any diving at any level the diver always should have a 'buddy'. This means not being alone. The pair look after each other and they must be able to communicate by sign language, as verbal communication is impossible. Some well-known signs are thumbs up to show intended ascent, thumbs down for intended descent, and index finger and thumb tips in opposition to indicate that all is well.
- Training is essential before embarking on a proper dive. Some will be classroom learning and some will be basic training in a swimming pool.
- Equipment should be properly checked before attempting a dive. Appropriate, uncontaminated gas mixtures should be used and the dive should be well planned.
- These may be simple matters such as becoming entangled in something. Physical trauma can also occur due to injury from, for example, coral, and wrecks. Divers should carry a knife to help in any necessary disentanglement.
- Even small amounts of alcohol must be avoided. Narcosis is enhanced by alcohol.
- Loss of buoyancy control and drowning can also be reasons for accidents. Divers need to be able to swim in strong currents.
- Under normal conditions, there are many facets to the drive to breathe but the main one is pCO2.
- Breathing gases at pressures of several atmospheres may totally distort the picture of partial pressures.
- Even with snorkel diving, gas pressures can be a problem. One way to delay the irresistible urge to breathe, and hence to prolong time under water for snorkelling, is to hyperventilate before diving. This can drop the pCO2 markedly but it does little to the pO2. Hence, pO2 may fall to dangerous levels before pCO2 rises and unconsciousness from hypoxia can be very swift. The practice is to be discouraged.
- As discussed above, the behaviour of gases changes at higher pressures. Gases that are usually thought of as inert (eg nitrogen, oxygen and hydrogen) can become toxic and produce narcosis.
- Obvious mental deterioration occurs at depths below 50 metres. This is caused by nitrogen dissolving in parts of nerve membranes, thickening them. Effects develop within minutes and can be reversed in the same time by ascent.
- Replacing nitrogen with helium allows divers to go deeper. An oxygen-helium mixture instead of air, can allow a diver to reach 700 metres depth without narcosis.
- At greater depths, neurological disturbance occurs in divers breathing oxygen-helium mixtures. This is caused by partial thinning of the neurone membranes from compression of nerve tissues so that nerve impulses travel more quickly, causing convulsions. Addition of nitrogen to the mix thickens the tissues and can prevent this.
- There are a number of people who get a toxic response from breathing oxygen at a higher partial pressure.
- This appears to be an idiosyncratic response that affects only a minority of people. It is possible to screen for it as it may have serious consequences in anyone undertaking deep sea diving. Nausea is the most common feature, followed by muscle twitching.
- This is less likely to be a problem when SCUBA diving in shallow water in a warm environment such as the Mediterranean or the Caribbean. But even there the temperature of the water can fall rapidly on descent.
- It is a significant problem when working on North Sea rigs and it limits the time underwater.
- As well as being very uncomfortable, hypothermia impairs intellect and judgement and is potentially very dangerous.
- The provision of hot water suits to North Sea saturation divers appears to be adequate to prevent a dangerous fall in core temperature.
- If serious hypothermia does occur, re-warming should be slow, especially if the core temperature falls below 32°C.
Dangers of ascent
Ascent is more dangerous than descent. There is considerable risk of decompression sickness ("the bends") if it is too fast (see 'Decompression sickness', below). If a diver is having trouble, there is a natural urge to get them out of the water as soon as possible. However, a slower ascent, or even a temporary descent to a greater depth, may be what is required. Ascent problems can include:
- Sinus problems:
Expansion of gas in sinuses may be very painful and can eventually burst them. Rarely, the ethmoid sinus ruptures with risk of cerebral infection.
- Ear problems: Gas trapped in the middle ear can cause problems. Attempts to clear the ears can cause a situation where one ear clears before the other, causing disorientation due to uneven stimulation (alternobaric vertigo). If the external auditory canals are unequally blocked before diving, cold water entering one canal can lead to caloric vertigo.
- Pulmonary barotrauma:
- As a diver ascends, compressed air in the lung expands and divers are taught to exhale continuously and ascend at a rate no faster than the bubbles they are exhaling.
- With these measures, the lungs have time to empty adequately and there is a low risk of rupture. Lung rupture usually occurs in divers holding their breath on ascent or ascending too fast.
- A central tear may cause mediastinal emphysema.
- Peripheral tears lead to pneumothorax:
- Symptoms include chest pain, shortness of breath and hoarseness.
- Surgical emphysema of the neck and upper chest may be apparent, as well as signs of pneumothorax.
- Treatment with oxygen can be given but avoid positive-pressure ventilation.
- Tension pneumothorax requires immediate needle thoracocentesis with emergency chest drain.
- Arterial gas embolism: Expansion of the lungs on ascent can also lead to the formation of emboli which can expand as the ascent continues. They can travel in the bloodstream to the brain. Neurological symptoms and signs will be evident, including numbness or tingling of the skin, weakness, paralysis or loss of consciousness. The presence of neurological signs requires immediate recompression.
- Decompression sickness ("the bends"), is well known and justifiably feared by divers. Those that dive deep are at highest risk but it can even occur in breath-holding divers.
- It occurs because nitrogen or helium that is stored in the body tissues or blood comes out of solution on ascent, forming bubbles within the circulation and tissues. The bubbles continue to increase in size and number as ascent continues. They can block blood vessels and distort or rupture cells.
- With sufficiently slow ascent, the gas diffuses into the bloodstream and is removed from the lungs without the formation of a significant number of bubbles.
- It is imperative to control ascent and decompression schedules can be obtained from diving clubs. They take into account both the depth reached and the time at any given depth. Even when these schedules are followed, around 1% of dives result in decompression sickness with a rather higher figure when they are not followed.
- Symptoms are:
- Limb pain, usually in the shoulders or elbows. It may appear a few minutes after the dive or up to 24 hours later. Pain is often dull, poorly localised, of gradual onset and not exacerbated by movement of the joint. Untreated pain will reduce and disappear over 2 or 3 days with rapid improvement on recompression.
- Neurological symptoms may also occur. There is usually sensory disturbance with numbness and paraesthesia but no clear dermatomal or peripheral nerve distribution.
- In its severe form, it starts with girdle pain with loss of sensation and movement in lower limbs.
- Cerebral involvement is often found but it may be subtle. It frequently involves denial with higher functions affected, such as short-term memory, mood and vision.
- There may be loss of consciousness.
- Any patient who shows signs of decompression illness within 24 hours of diving should be treated as if they have the illness.
- The risk is increased by flying soon after diving.
- First-line treatment involves:
- Rescuing the person from the water, drying, and rewarming them as appropriate.
- Cardiopulmonary resuscitation should be performed if necessary.
- 100% oxygen (via tracheal intubation if possible) and rehydration with intravenous fluids can be given.
- Body position does not influence the haemodynamic effects of massive venous air embolism.The Trendelenburg position is no longer recommended; it promotes the development of cerebral oedema. The supine position increases the rate of nitrogen washout and may prevent hypotension.
- The diver should be transported for definitive care to a decompression chamber.
- If transfer by air is required, the diver should be transported at below 1,000 feet or the cabin should be pressurised to sea level pressure. 100% oxygen should also be given during transport. Keep all of the diving equipment with the diver as this may give an idea of what caused the problem.
- Obtain expert opinion from either the British Hyperbaric Association or the Institute of Naval Medicine. Please refer to the links under 'Internet and further reading' below for contact details.
- The foramen ovale is the link between the atria in the fetal circulation. It usually closes soon after birth. This is not invariable and failure to close is usually asymptomatic, as the higher pressure in the left heart tends to push it shut.
- It can sometimes be a problem if right-sided pressure rises, as may happen with pulmonary embolism. The other time of risk is with the considerable pressure and physiological changes of diving, when a right to left shunt may occur. Gas emboli that should be discharged to the lungs can reach the systemic circulation.
- A patent foramen ovale occurs in 30% of the population. It increases the risk of decompression sickness by approximately 5 times.
- The implications for screening all potential divers are uncertain. It may be responsible for as many as two thirds of "unexplained" cases of decompression sickness.
Fitness to dive
The British Thoracic Society (BTS) has published guidelines on respiratory aspects of fitness for diving. Prospective divers should undergo a medical examination. The following are regarded as exclusion criteria:
- Lung bullae or cysts.
- History of previous pneumothorax may be a contra-indication (see BTS website for more details).
- Asthma may be a contra-indication (see BTS website for more details).
- Chronic obstructive pulmonary disease.
- Existing sarcoidosis.
- Active tuberculosis.
- Cystic fibrosis with pulmonary involvement.
- Fibrotic lung disease.
Other contra-indications may include:
- Ear surgery.
- Poorly controlled diabetes.
- Addiction to alcohol or drugs.
- Ruptured eardrums.
- Mental instability.
Assessment of respiratory fitness to dive
- Routine CXR is not required but spirometry should be undertaken.
- Forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), and peak expiratory flow rate (PEF) should be measured. FEV1 and PEF should normally be greater than 80% of predicted and the FEV1/FVC ratio greater than 70%.
- There is much more detail available via the BTS website link below.
Further reading and references
; Decompression Sickness, Medscape, Sep 2009
; Snorkelling deaths in Australia, 1987-1996. Med J Aust. 1999 Dec 6-20171(11-12):591-4.
; Breathing at depth: physiologic and clinical aspects of diving while breathing compressed gas. Clin Chest Med. 2005 Sep26(3):355-80, v.
; Effects of inert gas narcosis on behavior--a critical review. Undersea Biomed Res. 1985 Dec12(4):369-402.
; Screening for oxygen intolerance in U.S. Navy divers. Undersea Biomed Res. 1986 Mar13(1):91-8.
; Thermal status of saturation divers during operational dives in the North Sea. Undersea Hyperb Med. 2001 Fall28(3):149-55.
; Pathophysiology of cold exposure. Minn Med. 2001 Nov84(11):30-6.
; Decompression sickness following breath-hold diving. Res Sports Med. 2006 Jul-Sep14(3):163-78.
; The relative risk of decompression sickness during and after air travel following diving. Aviat Space Environ Med. 2002 Oct73(10):980-4.
; Diving emergencies. Resuscitation. 2003 Nov59(2):171-80.
; Dysbarism, Medscape, Apr 2011
; Risk of decompression illness among 230 divers in relation to the presence and Eur Heart J. 2004 Jun25(12):1014-20.
; Patent foramen ovale and diving. Cardiol Clin. 2005 Feb23(1):97-104.
; Patent foramen ovale: a review of associated conditions and the impact of physiological size. J Am Coll Cardiol. 2001 Sep38(3):613-23.