Pulse oximetry is a noninvasive method for monitoring a person's blood oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy (within 4% accuracy in 95% of cases) of the more accurate (and invasive) reading of arterial oxygen saturation (SaO2) from arterial blood gas analysis.[1] But the two are correlated well enough that the safe, convenient, noninvasive, inexpensive pulse oximetry method is valuable for measuring oxygen saturation in clinical use.[citation needed]
A standard pulse oximeter passes two wavelengths of light through tissue to a photodetector. Taking advantage of the pulsate flow of arterial blood, it measures the change in absorbance over the course of a cardiac cycle, allowing it to determine the absorbance due to arterial blood alone, excluding unchanging absorbance due to venous blood, skin, bone, muscle, fat, and, in many cases, nail polish.[2] The two wavelengths measure the quantities of bound (oxygenated) and unbound (non-oxygenated) hemoglobin, and from their ratio, the percentage of bound hemoglobin is computed.
Pulse oximeter forehead sensor
The most common approach is transmissive pulse oximetry. In this approach, one side of a thin part of the patient's body, usually a fingertip or earlobe, is illuminated, and the photodetector is on the other side. Fingertips and earlobes have disproportionately high blood flow relative to their size, in order to keep warm, but this will be lacking in hypothermic patients.[1] Other convenient sites include an infant's foot or an unconscious patient's cheek or tongue.
Reflectance pulse oximetry is a less common alternative, placing the photodetector on the same surface as the illumination. This method does not require a thin section of the person's body and therefore may be used almost anywhere on the body, such as the forehead, chest, or feet, but it still has some limitations. Vasodilation and pooling of venous blood in the head due to compromised venous return to the heart can cause a combination of arterial and venous pulsations in the forehead region and lead to spurious SpO2 results. Such conditions occur while undergoing anaesthesia with endotracheal intubation and mechanical ventilation or in patients in the Trendelenburg position.[3]
^ abNitzan, Meir; Romem, Ayal; Koppel, Robert (8 July 2014). "Pulse oximetry: fundamentals and technology update". Medical Devices: Evidence and Research. 7: 231–239. doi:10.2147/MDER.S47319. PMC 4099100. PMID 25031547.
^Brand TM, Brand ME, Jay GD (February 2002). "Enamel nail polish does not interfere with pulse oximetry among normoxic volunteers". Journal of Clinical Monitoring and Computing. 17 (2): 93–96. doi:10.1023/A:1016385222568. PMID 12212998. S2CID 24354030.
^Jørgensen JS, Schmid ER, König V, Faisst K, Huch A, Huch R (July 1995). "Limitations of forehead pulse oximetry". Journal of Clinical Monitoring. 11 (4): 253–256. doi:10.1007/bf01617520. PMID 7561999. S2CID 22883985.
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