A pulse oximeter is a medical device that measures the oxygen saturation of a person’s blood and their heart rate. It typically consists of a small clip or probe that attaches to a finger, toe, or earlobe and uses light to measure the amount of oxygen in the blood. Pulse oximeters are commonly used in hospitals and clinics to monitor the oxygen levels of patients with respiratory or cardiac conditions, but they are also available for use at home.
The Misunderstood Mechanism
A pulse oximeter works by shining two different wavelengths of light, typically red and infrared, through the blood vessels in the finger, toe, or earlobe. The amount of light absorbed by the blood varies depending on the oxygen saturation level. The device uses this information to calculate the percentage of oxygenated hemoglobin, also known as the oxygen saturation level, in the blood.
The pulse oximeter also uses the same light source to detect the pulsation of blood flow through the blood vessels in the finger, toe, or earlobe. It uses this information to measure the heart rate.
The device then displays the results in the form of a numerical value, typically as a percentage, which represents the oxygen saturation level. The device also displays the heart rate in beats per minute.
Most of the textbooks written within the domain of medicine and biomedical engineering describes the functional mechanism of Pulse oximeters as a classical example of Beer-Lambert Law.
Beer-Lambert Law
Beer-Lambert law, also known as Beer’s law or the Beer-Lambert-Bouguer law, is an empirical relationship that describes the absorption of light by a substance. The law states that the absorbance of a solution (A) is directly proportional to the concentration of the absorbing species in the solution (c) and the distance the light travels through the solution (b), also known as the pathlength.
The Beer-Lambert law is described by the equation: A = εbc
Where ε (epsilon) is the molar absorptivity of the substance, which is a measure of how efficiently the substance absorbs light.
This law is widely used in spectrophotometry, which is a technique used to determine the concentration of a substance in a solution by measuring the amount of light that is absorbed by the solution. .
It’s important to note that Beer-Lambert law is not applicable to all solutions, and it has some limitations, such as the assumption of homogenous solutions and the neglect of the refraction and scattering of light.
Real Mechanism of Oximeters is Data Driven
If we follow closely, we will see that the sampling areas mentioned above, from where the reading of the pulse oximeter is taken(i.e finger, toe, or earlobe) defy all the assumptions of the Law (i.e homogenous solutions and the neglect of the refraction and scattering of light.) In a finger, for example, the light has to traverse through different types of tissues, like fat, skin, blood vessels and bone an each tissue layer refracts light according to its unique properties. And presence of these layers show that there is non-homogeneity between them.
If Beer-Lambert law is not applicable, how can the pulse oximeter measure oxygen saturation?
The answer is that, the process is completely data driven. Pulse oximeter measures different values of red and infrared passing through a medium and then uses statistical methods to match those inputs with a oxygen saturation value of test subjects. This process is called ‘calibration’. In machine learning, this is analogous to ‘training’.
The problem here is now the representation of the population and its limitation in differently coloured individuals. If the data has been trained on of lighter colour tones, this calibration of the pulse oximeter would not be accurately ported on people of darker tones.
Problem and the Future Scope
Pulse oximeters have been in clinical use for about 40 years. However, the realization that darker-skinned patients, compared with lighter-skinned patients, are more likely to have falsely elevated oxygen saturation when measured with pulse oximetry, masking hypoxemia otherwise detected with arterial blood gas tests, has been recently addressed. This phenomenon, referred to as “occult hypoxemia,” has been associated with various disparities in health care outcomes. On November 1, 2022, the US Food and Drug Administration (FDA) convened a panel of its Medical Device Advisory Committee to discuss this long-known flaw. The November 2022 meeting appears to be the first public action since that time and the first time experts have gathered publicly to discuss the issue.
A recent report published in JAMA on 9th January 2023 summarizes and highlights the policy changes on the way since November 2023 meeting.
Kupke A, Shachar C, Robertson C. Pulse Oximeters and Violation of Federal Antidiscrimination Law. JAMA. Published online January 09, 2023. doi:10.1001/jama.2022.24976
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