Apnea

Apnea is the cessation of breathing. During apnea, there is no movement of the muscles of inhalation, and the volume of the lungs initially remains unchanged. Depending on how blocked the airways are (patency), 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. Voluntarily doing this is called holding one's breath. Apnea is the cessation of breathing. During apnea, there is no movement of the muscles of inhalation, and the volume of the lungs initially remains unchanged. Depending on how blocked the airways are (patency), 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. Voluntarily doing this is called holding one's breath. Apnea can be involuntarily achieved, drug-induced (such as opiate toxicity or tryptamine toxicity), mechanically induced (for example, by strangulation or choking), or it can occur as a consequence of neurological disease or trauma. During sleep in patients who are suffering from sleep apnea, these events can occur up to more than a hundred times per hour, every night. Apnea can also be observed during periods of heightened emotion, such as during crying or accompanied by the Valsalva maneuver when a person laughs. Apnea is a common feature of sobbing while crying, characterised by slow but deep and erratic breathing followed by brief periods of breath holding. Another example of apnea are breath-holding spells; these are sometimes emotional in cause and are observed in children as a result of frustration, emotional stress and other psychological extremes. Voluntary apnea can be achieved by closing the vocal cords, simultaneously keeping the mouth closed and blocking the nasal vestibule, or constantly activating expiratory muscles. 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. 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. Voluntary hyperventilation before beginning voluntary apnea is commonly believed to allow the person involved to safely hold their breath for a longer period. In reality, it will give the impression that one does not need to breathe, while the body is actually experiencing a blood-oxygen level that would normally, and indirectly, invoke a strong dyspnea. Some have incorrectly attributed the effect of hyperventilation 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 pH of the blood, thus increasing the time before the respiratory center becomes stimulated, as described above. While hyperventilation will yield slightly longer breath-holding times, any small time increase is at the expense of possible hypoxia. One using this method can suddenly lose consciousness—a shallow water blackout—as a result. If a person loses consciousness underwater, there is considerable danger that they will drown. An alert diving partner would be in the best position to rescue such a person. Static apnea blackout occurs at the surface when a motionless diver holds a breath long enough for the circulating oxygen to fall below that required for the brain to maintain consciousness. It involves no pressure changes in the body and is usually performed to enhance breath-hold time. It should never be practiced alone, but under strict safety protocols with a safety beside the diver.