Editing Apnea (section)

==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, enough oxygen can be delivered to the circulation even if a person is apneic, and even if the [[thoracic diaphragm|diaphragm]] does not move. With the onset of apnea, low pressure develops in the airspace of the lungs because more oxygen is absorbed than CO<sub>2</sub> is released. With the airways closed or obstructed, this will lead to a gradual collapse of the lungs and suffocation. However, if the airways are 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 stored in the lungs and resume sufficient ventilation. 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. A consequence of this hyperoxygenation is the occurrence of "nitrogen washout", which can lead to [[atelectasis#Absorption (resorption) atelectasis|atelectasis]].<ref>{{cite web|title=preoygenation, reoxygenation and Delayed Sequence Intubation in the Emergency Department|website=medscape.com|url=http://www.medscape.com/viewarticle/745228_2|access-date=10 August 2015|archive-date=4 March 2016|archive-url=https://web.archive.org/web/20160304235821/http://www.medscape.com/viewarticle/745228_2|url-status=live}}</ref>

However, no CO<sub>2</sub> is removed during apnea. The [[partial pressure]] of CO<sub>2</sub> in the airspace of the lungs will quickly equilibrate with that of the blood. As the blood is loaded with CO<sub>2</sub> from the metabolism without a way to remove it, more and more CO<sub>2</sub> will accumulate and eventually displace oxygen and other gases from the airspace. CO<sub>2</sub> 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 supplemental oxygen is [[Insufflation (medicine)|insufflated]]), apneic oxygenation could theoretically be sufficient to provide enough oxygen for survival of more than one hour's duration in a healthy adult.{{Citation needed|date=August 2011}} However, accumulation of carbon dioxide (described above) would remain the limiting factor.

Apneic oxygenation is more than a physiologic curiosity. It can be employed to provide a sufficient amount of oxygen in [[thoracic surgery]] when apnea cannot 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, short procedures, or where extracorporal circulation cannot be accessed. Use of [[PEEP valve]]s is also an accepted alternative (5&nbsp;cm H<sub>2</sub>O in average weight patients and 10&nbsp;cm H<sub>2</sub>O significantly improved lung and chest wall compliance in morbidly obese patients).<ref>{{cite book|title=Perioperative Medicine: Managing for Outcome|publisher=PerioperBy Mark F. Newman, Lee A. Fleisher, Mitchell P. Fink|pages=517}}</ref>

In 1959, Frumin described the use of apneic oxygenation during anesthesia and surgery. Of the eight test subjects in this landmark study, the highest recorded [[Arterial blood gas|PaCO<sub>2</sub>]] was 250 [[Torr|millimeters of mercury]], and the lowest arterial [[pH]] was 6.72 after 53 minutes of apnea.<ref>{{cite journal|first1=M.J. |last1=Frumin |first2=R.M. |last2=Epstein |first3=G. |last3=Cohen |title=Apneic oxygenation in man|journal=Anesthesiology| volume=20|issue=6| pages=789–798|date=November–December 1959|doi=10.1097/00000542-195911000-00007|pmid=13825447|s2cid=33528267 |doi-access=free}}</ref>