Reference Terms
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Apnea, apnoea, or apnœa 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.

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.

Note:   The above text is excerpted from the Wikipedia article "Apnea", which has been released under the GNU Free Documentation License.
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