Overview
What SpO₂ Tells You—and What It Does Not
A pulse oximeter provides a non invasive estimate of arterial oxygen saturation, reported as SpO₂.
A pulse oximeter provides a non-invasive estimate of arterial oxygen saturation, reported as SpO₂. It uses red and near-infrared light to analyse the changing absorption produced by pulsatile arterial blood. The result is a percentage of haemoglobin carrying oxygen; it is not a direct measurement of oxygen tension. An arterial blood gas (ABG) provides information that the oximeter cannot: PaO₂, PaCO₂, pH, and, depending on the method, a laboratory measurement of arterial saturation. SpO₂ therefore answers one question—how saturated the circulating haemoglobin appears to be—but does not tell you whether the patient is ventilating adequately. A patient may retain carbon dioxide while displaying an acceptable SpO₂, particularly while receiving oxygen. The number also does not describe total oxygen delivery to tissues. A patient with severe anaemia may have a normal saturation but reduced oxygen-carrying capacity because there is less haemoglobin available. Shock and poor cardiac output can further impair tissue oxygen delivery even when the displayed saturation looks acceptable. Accuracy decreases at low saturations. Readings may also be less reliable in patients with dark skin and can be falsely reassuring...