Partial pressure of oxygen, hyperoxemia and hyperoxia in the intensive care or anesthesia setting

In clinical studies, the partial pressure of oxygen (PaO2) and oxygen pulse saturation are the main variables used to assess blood oxygenation and define the threshold of hypoxia/hyperoxia and hypoxemia/hyperoxemia. Determination of the optimal oxygenation target has generated a lot of interest in recent years, mainly because of the potential risk of worse outcomes associated with hyperoxia, whereas the risk associated with hypoxia has been already well known. In this short narrative review, we recall some fundamental elements of physiology regarding the meaning of PaO2, the diffusion of oxygen to cells, the definitions of hyperoxemia and hyperoxia and the mechanisms of oxygen toxicity to provide a better understanding of these concepts, to which intensive care clinicians are frequently confronted. PaO2 provides only limited information about oxygen concentration carried by blood and does not allow to determine whether cells are exposed to hyperoxia. This should be considered for the design of future studies that aim to determine optimal oxygenation target and by clinicians for their daily practice.