The pulse oximeter (“pulse ox“) is perhaps the best non-invasive, continuous monitor ever created. It uses a light emitter and photodetector to analyze light absorbance across an extremity (typically a finger or toe). In doing so, the pulse ox gives us the hemoglobin oxygen saturation, heart rate, a rough idea of the rhythm, and even the fact that there’s distal perfusion present with the aid of a waveform.
Oxyhemoglobin (oxyHb) and deoxyhemoglobin (deoxyHb) have varying absorbances of light across different wavelengths. The extinction coefficient describes the degree to which oxyHb and deoxyHb absorb light at a given wavelength (940 nm and 660 nm, respectively). These two wavelengths are selected to maximize the ratios of the absorbances. In other words, at 660 nm (red), the deoxyHb to oxyHb ratio is maximized, where at 940 nm (infrared), the converse is true.
The key to an accurate pulse ox reading is minimizing noise and amplifying the arterial signal. The pulse ox picks up an arterial component (‘AC’) and a nonpulsatile component (‘DC’). The ratio (‘R’) of AC to DC at the 660 nm and 940 nm wavelengths is determined. This ratio is then correlated to a hemoglobin oxygen saturation based on a calibration curve derived from desaturation studies in healthy patients. SpO2 is unreliable below ~70% because, well, it’s unethical to have volunteers desaturate below that. Mathematical models try to extrapolate desaturation < 70%.
Drop me a comment below with questions and read my post on what pulse oximeters tell us.