How do o2 saturation monitors work

T and R configurations lead to different electromagnetic absorption and therefore different emission of light. The oximeter utilizes an electronic processor and a pair of small light-emitting diodes LEDs facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe.

One LED is red, with wavelength of nm, and the other is infrared with a wavelength of nm. Absorption of light at these wavelengths differs significantly between blood loaded with oxygen and blood lacking oxygen.

Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated hemoglobin allows more infrared light to pass through and absorbs more red light. Figure 3: Oxy and Deoxy Hemoglobin Absorption The LEDs sequence through their cycle of one on, then the other, then both off about thirty times per second. The amount of light that is transmitted in other words, that is not absorbed is measured. These signals fluctuate in time because the amount of arterial blood that is present increases literally pulses with each heartbeat.

By subtracting the minimum transmitted light from the peak transmitted light in each wavelength, the effects of other tissues is corrected for allowing for measurement of only the arterial blood. The ratio of the red light measurement to the infrared light measurement is then calculated by the processor which represents the ratio of oxygenated hemoglobin to deoxygenated hemoglobin.

This ratio is then converted to SpO 2 by the processor via a lookup table based on the Beer—Lambert law. Photoplethysomography: An important tool for any SpO 2 reading is plethysmography tracings or "pleth" which is a measure of volumetric changes associated with pulsatile arterial blood flow. Therefore, plethysomography ensures reliability of the calculated oxygen saturation. Oxygen saturation levels are also generally slightly lower for those living at higher altitudes.

Pulse oximeters have limitations and a risk of inaccuracy under certain circumstances. In many cases, the level of inaccuracy may be small and not clinically meaningful; however, there is a risk that an inaccurate measurement may result in unrecognized low oxygen saturation levels.

Therefore, it is important to understand the limitations of pulse oximetry and how accuracy is calculated and interpreted. FDA-cleared prescription pulse oximeters are required to have a minimum average mean accuracy that is demonstrated by desaturation studies done on healthy patients. However, real-world accuracy may differ from accuracy in the lab setting. While reported accuracy is an average of all patients in the test sample, there are individual variations among patients.

The SpO 2 reading should always be considered an estimate of oxygen saturation. Due to accuracy limitations at the individual level, SpO 2 provides more utility for trends over time instead of absolute thresholds. Many patient factors may also affect the accuracy of the measurement. In the recently published correspondence by Sjoding, et. It is important to note that this retrospective study had some limitations. It relied on previously collected health record data from hospital inpatient stays and could not statistically correct for all potentially important confounding factors.

However, the FDA agrees that these findings highlight a need to further evaluate and understand the association between skin pigmentation and oximeter accuracy.

All premarket submissions for prescription use oximeters are reviewed by the FDA to ensure that clinical study samples are demographically representative of the U. Although these clinical studies are not statistically powered to detect differences in accuracy between demographic groups, the FDA has continued to review the effects of skin pigmentation on the accuracy of these devices, including data from controlled laboratory studies and data from real world settings.

The FDA is committed to the continued evaluation of the safety, effectiveness, and availability of medical devices, especially devices in high demand during the COVID pandemic. The FDA is evaluating published literature pertaining to factors that may affect pulse oximeter accuracy and performance, with a focus on literature that evaluates whether products may be less accurate in individuals with darker skin pigmentation.

The FDA has been working on additional analysis of premarket data, as well as working with outside stakeholders, including manufacturers and testing laboratories, to analyze additional postmarket data to better understand how different factors including skin pigmentation may affect pulse oximeter accuracy.

Pulse oximetry may be useful in both inpatient and outpatient settings. In some cases, your doctor may recommend that you have a pulse oximeter for home use. In pulse oximetry, small beams of light pass through the blood in your finger, measuring the amount of oxygen. According to the British Lung Foundation , pulse oximeters do this by measuring changes in light absorption in oxygenated or deoxygenated blood.

This is a painless process. The pulse oximeter will be able to tell you your oxygen saturation levels along with your heart rate. This is especially true when using high quality equipment found in most medical offices or hospital settings.

With this equipment, medical professionals can carry out the tests accurately. The Food and Drug Administration FDA requires that prescription oximeters must provide results within an accuracy range of 4 to 6 percent. The American Thoracic Society says that typically, more than 89 percent of your blood should be carrying oxygen. This is the oxygen saturation level needed to keep your cells healthy. Having an oxygen saturation temporarily below this level may not cause damage. But repeated or consistent instances of lowered oxygen saturation levels may be damaging.

An oxygen saturation level of 95 percent is considered typical for most healthy people. A level of 92 percent or lower can indicate potential hypoxemia, which is a seriously low level of oxygen in the blood. A report compared the accuracy of pulse oximetry tests and blood gas measurements in detecting hypoxemia in Black and white patients.

Researchers found that among Black patients, there were three times as many cases of pulse oximetry tests failing to detect occult hypoxemia when blood gas measurements did so. Tests like these were developed without considering a diversity of skin tones.

The authors concluded that more research is needed to understand and correct this racial bias. Once the test is over, your doctor will have the readings available immediately.