Pulse Oximetry and SpO2

A pulse oximeter estimates arterial oxygen saturation without drawing blood. Its sensor sends red and infrared light through a pulsatile vascular bed, usually a finger, toe, or ear. Oxygenated and deoxygenated hemoglobin absorb those wavelengths differently; the device isolates the pulsatile portion of the signal, which represents arterial blood, and calculates SpO2. SpO2 is an estimate of SaO2, not a direct measurement. Medical-purpose devices are validated against arterial blood co-oximetry, and the labeled accuracy is expressed as a root-mean-square error. An individual reading can therefore differ from the patient's true arterial saturation by several percentage points. Check the specific device labeling and never use a borderline number as though it were exact. The monitor answers one question: how saturated the detected arterial hemoglobin appears to be. It does not measure ventilation, carbon dioxide removal, work of breathing, or the adequacy of circulation. A patient can have an acceptable SpO2 while retaining carbon dioxide, and a patient with poor perfusion may have an unreliable display. Respiratory effort, rate, level of consciousness, color, and perfusion give the number its clinical meaning.