Oxygen: Origin, Physiology, Pathophysiology, and Use in the Critically Ill

Oxygen is probably the most often used drug in medicine in general, especially in critical care. The earth’s atmosphere with 21% oxygen is sufficient in healthy human beings to attain adequate oxygenation, but critically ill patients frequently need extra oxygen administration. The oxygenation status can continuously and non-invasively be monitored as oxygen saturation by pulse oximetry, while oxygen pressure is measured discontinuously and invasively. However, pressure is the driving force of oxygen diffusion and in the high saturation range (97 to 100%), oxygen saturation is an unreliable predictor of pressure. Hyperoxemia may go undetected if only oxygen saturation is measured. When breathing air, the ambient oxygen pressure is reduced via the oxygen cascade to 0.5–3 kPa in the mitochondria were oxygen is used in the tricarboxylic acid cycle to produce energy. Hypoxemia will hamper aerobic glycolysis, causing energy depletion in the cells. Hyperoxia has direct toxic effects on the airways and lungs. Hyperoxemia in the cells and the mitochondria stimulates production of reactive oxygen species, which can cause cellular and organ injury. The results of studies on the effects of liberal or conservative oxygen administration in critically ill patients are not uniform. However, it seems prudent to avoid far-conservative and far-liberal oxygenation values. It is presently unclear if there are specific subgroups within the critically ill population that could benefit from oxygenation targets outside the normoxemia ranges. Two currently ongoing large randomized controlled trials (Mega-ROX and UK-ROX) might answer the question of benefit of different oxygenation targets in specific subgroups.