oxygen saturation

Imagine a parking garage with a fixed number of spaces, and you ask what fraction of those spaces is currently occupied. Oxygen saturation answers exactly that question for the blood: of all the oxygen-binding sites that hemoglobin offers, what percentage are actually carrying an oxygen molecule right now. It is reported as a percentage.

Each hemoglobin molecule has four sites that can each hold one oxygen, so saturation reflects how loaded the body's oxygen carriers are on average. In a healthy person breathing room air it usually sits around 95 to 100 percent. Two letters distinguish how it was obtained: SaO2 is measured directly from an arterial blood sample, while SpO2 is the estimate from a pulse oximeter clipped to the skin. They normally agree closely.

Saturation is linked to dissolved oxygen pressure by the S-shaped dissociation curve, and that link carries a warning. Because the curve is flat on top, saturation can remain comfortably high while the underlying oxygen pressure is already sliding; but once saturation begins to fall below roughly 90 percent, it can drop steeply and quickly as the patient slips onto the steep part of the curve. Saturation also tells you nothing about how much hemoglobin is present — anaemic blood can be fully saturated yet still carry little oxygen.