Under normal conditions, humans can not store much oxygen in the body. Apnea of more than approximately one minutes' 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 extracorporal circulation, much longer periods of apnea may be tolerated without severe consequences.
Untrained humans can not 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 can not be overcome voluntarily. However, tolerance of apnea can be trained. The ancient technique of free-diving requires breath-holding and trained free-divers can indeed hold their breath for up to seven minutes. An apneist, in this context, is someone who can hold his breath for a long time.
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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 collaps 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.
Apneic oxygenation is more than a physiologic curiosity. It can be employed to provide a sufficient amount of oxygen in thoracic surgery when apnea can not 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.
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, a sufficient amount of oxygen can be delivered to the circulation even if a person is apneic. This phenomenon (apneic oxygenation) is explained in the following.References
See also
External links