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• Breath
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Apnea

Under normal conditions, humans cannot store much oxygen in the body. Apnea of more than approximately one minute's duration therefore leads to severe lack of oxygen in the blood circulation. Permanent brain damage can occur after as little as three minutes and death will inevitably ensue after a few more minutes unless ventilation is restored. However, under special circumstances such as hypothermia, hyperbaric oxygenation, apneic oxygenation (see below), or extracorporeal membrane oxygenation, much longer periods of apnea may be tolerated without severe consequences.

Untrained humans cannot sustain voluntary apnea for more than one or two minutes. The reason for this is that the rate of breathing and the volume of each breath are tightly regulated to maintain constant values of CO₂ tension and pH of the blood. In apnea, CO₂ is not removed through the lungs and accumulates in the blood. The consequent rise in CO₂ tension and drop in pH result in stimulation of the respiratory centre in the brain which eventually cannot be overcome voluntarily. However, tolerance of apnea can be trained. The ancient technique of free-diving requires breath-holding and trained free-divers can indeed hold their breath for up to seven minutes. An apneist, in this context, is someone who can hold his breath for a long time.

Many people have discovered, on their own, that voluntary hyperventilation before beginning voluntary apnea allows them to hold their breath for a longer time. Some of these people incorrectly attribute this effect to increased oxygen in the blood, not realizing that it is actually due to a decrease in CO₂ in the blood and lungs. Blood leaving the lungs is normally fully saturated with oxygen, so hyperventilation of normal air cannot increase the amount of oxygen available. Lowering the CO₂ concentration increases the time before the respiratory center becomes stimulated, as described above.

This error has led some people to use hyperventilation as a means to increase their diving time, not realizing that there is a danger that their body may exhaust its oxygen while underwater, before they feel any urge to breathe, and that they can suddenly lose consciousness as a result. If a person loses consciousness underwater, especially in fresh water, there is a considerable danger that they will die by drowning.

Table of contents

1 Apneic oxygenation 2 References 3 See also 4 External links

Apneic oxygenation

However, no CO₂ is removed during apnea. The partial pressure of CO₂ in the airspace of the lungs will quickly equilibrate with that of the blood. As the blood is loaded with CO₂ from the metabolism, more and more CO₂ will accumulate and eventually displace oxygen and other gases from the airspace. CO₂ will also accumulate in the tissues of the body, resulting in respiratory acidosis.

Under ideal conditions (i.e., if pure oxygen is breathed before onset of apnea to remove all nitrogen from the lungs, and pure oxygen is insufflated), apneic oxygenation could theoretically be sufficient to provide enough oxygen for survival of more than one hour's duration in a healthy adult. However, accumulation of carbon dioxide (described above) would remain the limiting factor.

Apneic oxygenation is more than a physiologic curiosity. It can be employed to provide a sufficient amount of oxygen in thoracic surgery when apnea cannot be avoided, and during manipulations of the airways such as bronchoscopy, intubation, and surgery of the upper airways. However, because of the limitations described above, apneic oxygenation is inferior to extracorporal circulation using a heart-lung machine and is therefore used only in emergencies and for short procedures.

References

• Sleep apnea
• Mechanical ventilation
• Free-diving

• http://freediving.dyndns.org/ - comprehensive apnea diving site
• http://freediving.angelosbar.com/
• http://www.umich.edu/~oseh/history.pdf - notes on the history of diving