Remember that Pulse Oximetry is Inaccurate at Lower Saturation Levels
Nirav G. Shah MD
Pulse oximetry is a noninvasive method to measure arterial hemoglobin saturation. The probe for the pulse oximeter consists of two light-emitting probes and a photodetector. Pulse oximeters are able to distinguish between oxygenated and deoxygenated hemoglobin on the basis of their differential absorption of two wavelengths of light. The photodiodes turn on and off several hundred times per second, allowing the absorption of light by oxyhemoglobin and deoxyhemoglobin to be continuously recorded. It is the absorption during pulsatile flow that is a measure of arterial hemoglobin saturation, while the absorption during nonpulsatile flow is a function of the surrounding tissue and venous blood.
Watch Out For
Although pulse oximetry is universally used in the intensive care unit (ICU), the experienced practitioner will understand the limitations and inaccuracies inherent in using it. First, microprocessors in pulse oximeters are calibrated using reference tables compiled by exposing healthy volunteers to decreasing FIO2 (fraction of inspired oxygen) to produce SaO2 (oxygen saturation) ranging from 75% to 100% by co-oximetry. Thus, any reading below 75% is a calculated value extrapolated from healthy volunteers. At 75% to 83% the bias on pulse oximetry is 8% from co-oximetry and this value does not reach an acceptable 3% bias until the saturation is >83% on pulse oximetry.
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