Apneic oxygenation is an adjunct “ventilation” technique that involves insufflation of oxygen at varying flows through a catheter that is inserted through the endotra-cheal tube and is positioned above the carina. Apneic oxygenation improves gas exchange efficiency and preserves the arterial oxygenation at an acceptable level. The understanding of the mechanism that is responsible for the sustained high al-veolar and blood oxygenation levels requires the substantial knowledge of the physiology regarding the transport and exchange of alveolar gases. Application of apneic oxygenation may be necessary in some clinical situations, in which no movement of the chest or the lungs may be desirable, such as during cardiothorac-ic surgery or during some radiological procedures, in order to eliminate respirato-ry motion artifacts. Moreover, it is used during the apnea test for the diagnosis of brain death, to ensure an adequate arterial oxygenation. Under ideal conditions, apneic oxygenation could theoretically be sufficient to provide enough oxygen for survival for a longer time period. However, accumulation of carbon dioxide would remain the limiting factor. Hypercapnia and subsequent acidosis are re-sponsible for most of the respiratory and cardiovascular effects of apneic oxyge-nation.