Apnea

Apnea, apnoea, or apnœa (Greek απνοια, from α-, privative, πνεειν, to breathe) is a technical term for suspension of external breathing. During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged. Depending on the patency (openness) of the airways there may or may not be a flow of gas between the lungs and the environment; gas exchange within the lungs and cellular respiration is not affected. Apnea can be voluntarily achieved (i.e., "holding one's breath"), drug-induced (e.g., opiate toxicity), mechanically induced (e.g., strangulation), or it can occur as a consequence of neurological disease or trauma.

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Mechanism

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.

When a person is immersed in water, physiological changes due to the mammalian diving reflex enable somewhat longer tolerance of apnea even in untrained persons. Tolerance can in addition be trained. The ancient technique of free-diving requires breath-holding, and world-class free-divers can indeed hold their breath underwater up to depths of 214 metres and for more than nine minutes. An apneist, in this context, is someone who can hold their breath for a long time.

Hyperventilation

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 — a shallow water blackout — as a result. If a person loses consciousness underwater, especially in fresh water, there is a considerable danger that they will drown. An alert diving partner would be in the best position to rescue such a person.

Apneic oxygenation

Because the exchange of gases between the blood and airspace of the lungs is independent of the movement of gas to and from the lungs, enough oxygen can be delivered to the circulation even if a person is apneic. This phenomenon (apneic oxygenation) is explained as follows:

With the onset of apnea, an underpressure develops in the airspace of the lungs, because more oxygen is absorbed than CO₂ is released. With the airways closed or obstructed, this will lead to a gradual collapse of the lungs. However, if the airways are patent (open), any gas supplied to the upper airways will follow the pressure gradient and flow into the lungs to replace the oxygen consumed. If pure oxygen is supplied, this process will serve to replenish the oxygen stores in the lungs. The uptake of oxygen into the blood will then remain at the usual level and the normal functioning of the organs will not be affected.

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

• J.F.Nunn; Applied Respiratory Physiology; Butterworth-Heinemann Ltd; ISBN 0-7506-1336-X (4th edition, 1993)
• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

See also

• Hypopnea
• Sleep apnea
• Mechanical ventilation
• Free-diving
• Shallow water blackout
• Deep water blackout