Administration of supplementary O2 is considered to be safe, as exemplified by one editorial comment1: “Oxygen should be used as soon as possible, in as near 100% as possible in all resuscitation situations, and for the early management of injury and illness. Its use will never disadvantage [our emphasis] a patient under these circumstances.” We believe this claim merits examination.
The rationale for administering O2 is that it increases the O2 content of blood and, therefore, O2 delivery to tissues. In a healthy person, hemoglobin is nearly saturated, and switching from air to pure O2 at sea level will increase O2 content by < 10% due almost exclusively to the increase in O2 dissolved in the plasma. In these people, the more influential determinant of O2 delivery is tissue perfusion that is determined by perfusion pressure and local tissue vascular resistance. Vascular resistances in the brain, heart, and placenta are affected by the Pco2 in arterial blood.