Oxygen Poisoning
Oxygen is essential for life, but the body is very sensitive to the correct level within our bodies.
We already no that a reduction in the levels of oxygen can result in Hypoxia, and then on to Anoxia, which is the lack of oxygen.
The opposite can also happen, Hyperoxia is an increase in the levels of oxygen in the body.

The table below shows the main effects of oxygen Imbalance

We normally breathe air at 1 bar atmospheric pressure (the pressure which surrounds us on the planets surface). This air contains 21 percent oxygen, which has a partial pressure of 0.21 bar. Our bodies will tolerate slight reductions and quite large increases in the partial pressure of the oxygen before it has any adverse effects. Once the effects cause our bodies systems to fail, death could ultimately follow.

Increases in the partial pressure of oxygen may occur if the concentration of the oxygen in the breathing air is increased, if the ambient pressure is increased or a combination of both.

A large increase in the partial pressure of oxygen can produce significant changes in the metabolism of many tissues, but for a diver, only the acute (sudden onset, short term) effects on the central nervous system or CNS (mainly the brain) and the chronic (slow onset, long term) effects on the lungs are important.

Acute Oxygen Toxicity
If the breathing mix contains oxygen at a partial pressure of 1.6 - 2.0 bar absolute, oxygen poisoning can occur. The effects do not immediately occur, but usually within minutes of exposure to oxygen at this pressure, As with nitrogen narcosis, people will be effected differently and on a different time scale.
It is thought that the high partial pressure of oxygen inhibits enzymes that are required for the biochemical reactions in the brain.

If a diver is breathing 100 percent oxygen this will occur at depths greater than 6 metres, and if the diver is breathing air with 21 percent oxygen, it will occur at depths of 66 metres or more.

In acute Oxygen poisoning the symptoms are similar to those occurring in an epileptic fit and will almost certainly cause death by drowning should it happen underwater
Prior to becoming unconscious the diver commonly notices twitching of the lips or other facial muscles, they may also become dizzy and feel nauseated, less commonly the breathing pattern will alter and breathing may become rapid, slow or simply difficult. There may also be difficulty in concentration, disorientation, drowsiness, numbness, tingling or the inability to focus may occur. This is called the Pre-tonic Build-up.

The diver's buddy may notice that they start acting strangely or become aware of a different breathing pattern. They may notice that the diver's facial pallor may change as the veins and capillaries close down due to Hyperoxia, this closing down of the blood supply is called Peripheral Vasoconstriction. They may also be able to notice the twitching lips or facial muscles, both of which are sure sign of impending convulsions.

However it is more usual that the buddy not to notice anything until the casualty starts to convulse.

The next phase of oxygen poisoning is called the Tonic Phase, here the diver's muscles will become rigid and they will not be able to breathe out, it has been known for some muscles to become detached at this point. This phase may last up to a few minutes. Because all the muscles become rigid, care should be taken that no rescue is attempted as the casualty will suffer a burst lung, also the buddy should be aware that the effected diver may bite through the regulator mouthpiece and loose the ability to breathe once this phase has passed.

Once this phase has passed the casualty then enters the Clonic Phase, here breathing will restart, but the casualty will start to have major convulsive fits. This phase may last a few minutes.

The final stage is called the Relaxed Phase, but is also known as Post Convulsive Depression, here the casualty will become relaxed and limp, breathing will have restarted and casualty can be lifted to the surface, though care should be taken to ensure the regulator remains in place, and the head pushed back so that the airway is extended.

One the surface has been reached the casualty may experience further convulsions, in some cases the convulsions do not start until the casualty has reached the surface. In the event of this happening, the rescuer should simply hold onto the casualty until they have relaxed. It should be remembered that it is better to have the convulsive fits on the surface than underwater where the threat of drowning is present.

During the Recovery Phase, the casualty may have no recollection of what occurred on the dive, they may be drowsy, confused, lethargic or exhausted, it is often said that they appear to be dazed and in a zombie like state. Place in the recovery position, making sure that the airway is extended and the mouth is open.

If the casualty has further fits, make sure that they do not harm themselves or others and that they do not inhale vomit. Do not place your fingers in the casualty's mouth in the misguided thought of keeping the airway open, they could be bitten off by a convulsing diver.

Fatigue, stress, exertion, a high partial pressure of carbon dioxide, cold immersion, poor physical condition or a hangover will all increase the susceptibility of acute oxygen toxicity.

The current trend of divers learning to use NITROX may lead to more cases of oxygen poisoning cases.

As we already know from previous lectures, air that we breathe contains the following

The amount of nitrogen in the breathing air is the cause of Decompression Sickness.

Nitrox is air with a higher partial pressure of oxygen to combat the effects of the nitrogen, however it also places the diver at a greater risk of oxygen poisoning, as there are a number of different mixes of Nitrox, ( Nitrox 36 has 36% oxygen in the gas whereas Nitrox 32 has only 32%) If a diver forgets what mix they are diving with and exceed their depth restriction, then they are in danger of oxygen poisoning.

For this reason only divers who have attended a Nitrox course and carry a valid card can use the gas.

Chronic Oxygen Toxicity
Chronic oxygen toxicity occurs at a partial pressure greater than 0.6 bar. It will occur in someone on the surface who breaths a gas containing 60 percent oxygen for long enough. If 100 percent oxygen is breathed at atmospheric pressure (1 bar) the effects of chronic oxygen toxicity will probably be experienced within 24 hours.
The symptoms include an initial soreness of the chest as the high level of oxygen irritates the lungs. There may also develop a condition similar to pneumonia, with congestion, coughing and considerable discomfort, blood stained sputum may also be produced.

The vital capacity of the lungs may also be reduced with the walls of the alveoli and pulmonary arteries thickening.

All the symptoms will gradually diminish once the casualty starts to breathe normal air.

If a diver suffers from decompression sickness and is taken into a recompression chamber, they will be breathing high levels of oxygen, but to prevent the onset of chronic oxygen toxicity they are required to take air breaks.