Hypoxia is a shortage of oxygen in the blood, as opposed to anoxia which is a total lack of oxygen. Hypoxia is something that is more likely to be experienced by a breath-holding diver rather than a SCUBA diver.

The brain's response to hypoxia is to render the individual unconscious in a vain attempt to make them breathe. This happens because of the way the brain monitors how much oxygen and carbon dioxide there is in the blood. The brain has two sensors at the base of the brain, monitoring the level of gases in the blood entering the brain. The carbon dioxide sensor responds to high levels by stimulating the person to breathe. The body produces carbon dioxide as part of the metabolic process.

If there is a high level of carbon dioxide and a low level of oxygen, the brain comes to the conclusion that there is oxygen available, otherwise there would not be a high level of carbon dioxide, and overrides the conscious effort of breath-holding by rendering the individual unconscious. This is fine on land where most humans spend their time, but in or underwater it can be fatal.

If the blood is deprived of oxygen because of toxic gases such as carbon monoxide, the effect is anoxia. This is why so many people die in their sleep from carbon monoxide poisoning or commit suicide by breathing exhaust fumes. Because of the lack of oxygen, there is no corresponding build up of carbon dioxide, consequently there is no urge to breathe which would wake a sleeping person.

Hypoxia is a particular hazard to a breath-holding diver because of the way the brain measures the amount of absorbed gases. It measures the partial pressure, not necessarily the quantity. See the article on Nitrogen Narcosis for information on partial pressure.

At the surface, the partial pressure of oxygen is 200 millibars. As the oxygen is consumed in a breath-holding diver, the partial pressure will be reduced. At approximately 50 millibars, about 25% of the available oxygen, the oxygen sensor clicks in and renders the individual unconscious. By the time this stage is reached, the urge to breathe would become so great that most people would have given up and started to breathe again. Experienced breath-holding divers can, with practice and a little determination, override the urge to breathe, to such an extent that it can be dangerous.

When a diver descends to 10 metres, the partial pressure will double to 400 millibars. Once 75% of the available oxygen is consumed the partial pressure will have only reduced to 100 millibars. Not enough to cause hypoxia. However, when the diver starts to ascend the partial pressure will fall rapidly, both from the effort of swimming to the surface and the decrease in partial pressure. There will also be a corresponding decrease in the partial pressure of carbon dioxide, thus reducing the urge to breathe. Consequently, the diver will arrive at the surface with the partial pressure of oxygen way below the 50 millibar bar threshold.

Hyperventilation

Hyperventilation is the practice of taking a few deep breaths before finally holding the breath and diving. This does little to increase how much oxygen is in the tissues but does reduce the residual carbon dioxide. Consequently it can delay the onset of the breathing urge, causing the diver to lose consciousness without warning.

There are breath-holding divers who regularly dive to depths greater than 100 metres. The world record on a single lungful of air is 162 metres, holding the breath for more than ten minutes. It should be stressed that this is a highly dangerous activity. Divers who regularly dive to such depths have been training for years - some even use yoga techniques to help them. They also have extensive safety systems in place, including SCUBA divers in the water in case something should go wrong, which it often does.

Carbon Monoxide Poisoning

Carbon monoxide is a highly toxic, colourless and odourless gas. It is a by-product of the combustion of any organic material. This includes wood, coal, domestic gas and petrol. It is particularly dangerous to SCUBA divers if the air in the cylinder is contaminated. What might be a tolerable level at the surface can become lethal at depth because of the increase in partial pressure. At the extreme pressure inside a fully-charged SCUBA cylinder (3000 psi) carbon monoxide can even become explosive.

The way it affects humans is by displacing the oxygen in the hæmoglobin in the blood. The hæmoglobin has a greater affinity for carbon monoxide than it does for oxygen. So smokers get short of breath as the carbon monoxide displaces the oxygen generated by the burning tobacco. Once carbon monoxide has entered the system, it can take up to six hours to disperse, providing no more carbon monoxide is inhaled in the meantime.

Compressed air can become contaminated by careless filling of cylinders from petrol-driven compressors. The air intake to the compressor should always be upwind from the exhaust and not be allowed to draw its own fumes into the compressor. Health regulations stipulate that compressed air for breathing should contain no more than five parts per million of carbon monoxide. There are test kits available that can detect carbon monoxide at such low levels. Unfortunately, no filters can filter out carbon monoxide.

The symptoms in order of severity are headaches, dizziness, mental confusion, slurred speech, red lips and cheeks, coma and death. Once the later symptoms have begun to show, the individual is in serious trouble and they will require immediate treatment if they are going to live. In minor cases, where the victim just complains of a headache, breathing fresh air is usually enough. More serious cases require oxygen. Treatment in a decompression chamber may be necessary where oxygen can be administered under pressure.

It is interesting that the headache that most people complain of as part of a hangover is more likely to be due to the smoky atmosphere of the previous night rather than the alcohol consumed.

Drowning

Drowning is often the ultimate form of the ills described above. Drowning occurs when water enters the lungs. With fresh water it is absorbed into the blood stream, thus diluting the blood and reducing its ability to carry oxygen to the vital organs. Eventually the blood is so badly diluted, the victim dies, usually of heart failure. This can take some, 2-3 minutes. In sea water, the process is different. Because of the high content of salt in sea water, the blood enters the lungs as it attempts to dilute the sea water. This also can lead to heart failure as the body is deprived of a blood supply. This process takes 8-12 minutes.

The drowning process only starts once the victim has become unconscious. This is because normal reflexes will close off the windpipe and direct water into the stomach. Once the victim becomes unconscious from anoxia, the reflexes relax and water can then enter the lungs. When the victim is resuscitated, they will often regurgitate water. This water is coming from the stomach, not the lungs as is often believed. There will be some water in the lungs, but comparatively little compared with what the stomach can hold. Consequently, it is important when resuscitating a drowning victim, not to waste time trying to expel water out of the lungs. Most of the water will be coming from the stomach, and as the victim is still unconscious much of it will enter the lungs making matters worse. The important thing is to get oxygen into the lungs via mouth-to-mouth resuscitation. As the victim regains consciousness, the water in the stomach will be regurgitated and expelled from the mouth. Warning: If you are ever called upon to resuscitate a drowning victim via mouth-to-mouth resuscitation, be ready for it!

It is important that once a drowning victim has been resuscitated they are sent to hospital for observation, although they may appear to have made a full recovery. It has been known for drowning victims to collapse and die 24 hours after being resuscitated due to the contamination of the blood.