Apnea (British spelling - apnoea) is the absence 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 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.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 oxygenationIn intensive care medicine, extracorporeal membrane oxygenation (ECMO) is a technique of providing oxygen to patients whose lungs are so severely diseased that they can no longer serve their function. An ECMO machine is similar to a heart-lung machine., 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 CO2Carbon dioxide is an atmospheric gas composed of one carbon and two oxygen atoms. One of the best known of chemical compounds, it is frequently called by its formula: :CO (pronunciation: "see oh two") Carbon dioxide results from the combustion of organic tension and pHAlternate uses: see Ph pH (potential (of) hydrogen) is a measure of the concentration of hydrogen ions (H+) in a solution and, therefore, its acidity or alkalinity. The concept was introduced by S. Sorensen in 1909. The p stands for the German potenz mean 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-divingFree-diving is an aquatic sport, considered an extreme sport, in which divers attempt to reach great depths unassisted by breathing apparatus. The world record for the "No Limits" category of competitive free diving is held by French diver Loic Leferme. requires breath-holding and world-class free-divers can indeed hold their breath for more than eight 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 hyperventilationIn physiology hyperventilation can mean the following: Hyperventilation is a state of decreased ratio of carbon dioxide production to alveolar respiration. In this sense it means literally overventilation of the lungs. It can be done voluntarily, by takin 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.
