Hypoxic, anemic and cardiac hypoxemia: When does tissue hypoxia begin?

In case of hypoxemia, the oxygen content is often still in the lower normal range, so that there is no hypoxia in the tissue. If the hypoxia-threshold is reached in the tissue in hypoxic, anemic and also cardiac-related hypoxemia, identical counterregulations occur in the cell metabolism, regardless of the cause of hypoxemia. In clinical practice, this pathophysiologic fact is sometimes ignored, although depending on the cause of hypoxemia, assessment and therapy vary widely. While restrictive and generally accepted rules are specified in the transfusion guidelines for anemic hypoxemia, in the case of hypoxic hypoxia, the indication for invasive ventilation is made very early. The clinical assessment and indication are limited to the parameters oxygen saturation, oxygen partial pressure and oxygenation index. During the corona pandemic, misinterpretations of pathophysiology have become evident and may have led to unnecessary intubations. However, there is no evidence for the treatment of hypoxic hypoxia with ventilation. This review addresses the pathophysiology of the different types of hypoxia focusing on the problems associated with intubation and ventilation in the intensive care unit.