How Do Pulse Oximeters Work?
The pulse oximeter (“pulse ox“) is perhaps the greatest noninvasive perioperative/acute care monitor created in the modern era. 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 a hemoglobin saturation, heart rate, 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 were 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 calculated. This ratio is then correlated to a hemoglobin saturation based on the manufacturer’s calibration curve. And this happens continuously. 🙂