Apnea, apnoea, or apnœa ( , from α-, privative, πνεειν, to breathe) is a 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 (e.g., "holding one's breath"), drug-induced (e.g., opiate toxicity), mechanically induced (e.g., strangulation or choking), or it can occur as a consequence of neurological disease or trauma.

Mechanism

Under normal conditions, humans cannot store much oxygen in the body. Prolonged apnea 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. (Smokers, in most cases, can only endure much shorter periods of voluntary apnea due to damages to the lung's alveoli and thus their lung capacities and gas exchange efficiency are decreased). The reason for the time limit of voluntary apnea 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 hold their breath underwater up to depths of 214 metres and for more than four minutes . Apneists, in this context, are people who can hold their breath for a long time.

Hyperventilation

Voluntary hyperventilation before beginning voluntary apnea allows the person involved to hold their breath for a longer period. Some have incorrectly attributed 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.