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The Physics Behind Oximeters

Updated: Nov 4, 2023

Author: Samiha Sehgal



What is an oximeter?


An oximeter/pulse oximeter is a medical device that aims to measure the oxygen saturation in a person’s blood, changes in the blood volume and also the pulse rate by using red and infrared light. Oxygen saturation is simply the amount/percentage of available hemoglobin in blood which carries oxygen.

A regular oximeter consists of light emitting diodes (LEDs) as the source of light, mounted opposite to light detectors or sensors. The device is in the form of a clip which can easily be attached onto a finger. An illustration of what has been explained till now is shown in Fig. 1.


Figure 1: A typical oximeter. [Source: Dr Medcable.]


In the current scenario of the ongoing global pandemic, oximeters are extremely useful in detecting early infections which can lead to low arterial oxygen saturation, going unnoticed initially.


Advantages


Oximeters are a non-invasive method for the determination of oxygen saturation and pulse rate.

They can be used anywhere – in intensive care units, by pilots in unpressurised aircrafts and even by mountain climbers.

Due to its size and simplicity, these devices are very compact, easy to carry and use.

The LEDs used are cheap and easily accessible.

The LEDs also emit light of accurate wavelengths and do not heat up easily during use.

Working and Functionality


Oxygenated hemoglobin/oxygen saturation is the percentage of available hemoglobin that carries oxygen.

When oxygen is attached to the hemoglobin (Hb) molecule, it is known as oxygenated Hemoglobin, and when the Hb molecule is without oxygen, it is called deoxygenated Hemoglobin.

Now, when a finger is placed between the light source and the detector, the path of light gets blocked by the finger. Part of the light gets absorbed by the finger and only the remaining light reaches the detector.

In the images to follow, arteries are shown bordered in red as they carry the blood, and veins serve as a passage for the blood to exit and are shown in blue.

The amount of light absorbed by the finger depends on the following factors –


1. Concentration of the absorbent

What is the light-absorbing substance in your finger? Hemoglobin. The amount of light absorbed is directly proportional to the Hb concentration in the blood. This has been shown in Fig. 2 and is governed by an important law in physics.

Beer’s law simply states that a more concentrated solution absorbs more light than a dilute solution.


Figure 2: Beer's law using pulse oximetry. [Source: How Equipment Works.]


2. Length of the path of light

Even if the Hb concentration per unit area is the same, the width of the artery plays an important role in determining the amount of light absorbed. In Fig. 3 below, the artery on the right is wider than the one on the left. And so, light has to travel a longer path. Another law summarises this.

Lambert’s law states that the amount of light absorbed is directly proportional to the length of the path the light has to travel in the absorbent.


Figure 3: Lambert's law using pulse oximetry. [Source: How Equipment Works.]


3. Amount of red and infrared light absorbed

An oximeter uses red and infrared (IR) light to measure oxygen saturation. Red light has a wavelength of about 650 nm and IR light (which is not visible to us) has a wavelength of almost 950 nm.

If we consider a source of light with changing wavelengths (i.e., not monochromatic), and allow it to pass through oxygenated Hb, the graph given in Fig. 4 will be observed.

(Graphs are not drawn to scale.)


Figure 4: Oxygenated Hb absorbing lights of different wavelengths.


As can be seen from the graph, oxygenated Hb absorbs more light of 950 nm, or infrared light, as compared to red light.

Deoxygenated Hb also absorbs lights of different wavelengths differently. But it absorbs more red light (650 nm) than infrared light as can be seen from the graph in Fig.5.


Figure 5: Deoxygenated Hb absorbing lights of different wavelengths.


Thus, oxygenated Hb absorbs more IR light than red light, and deoxygenated Hb absorbs more red light than IR light. So, an oximeter compares the amounts of red and IR lights being absorbed by the detector to display the oxygen saturation. More the amount of IR light absorbed, greater will be the oxygenated Hb and higher will be the oxygen saturation. Similarly, if more red light is absorbed, the deoxygenated Hb will be more, resulting in a lower oxygen saturation percentage.


However, there are some important factors we must consider.


1. Calibration adjustment

Beer’s law and Lambert’s law only hold true if light travels straight from the source to the detector, as shown in the left of Fig. 6. This does not happen in reality as when the light passes through the blood, it interacts with red blood cells, white blood cells, plasma and platelets, which cause an obstruction in the path, as shown in the right of Fig. 6.


Figure 6: An image showing light rays travelling in a straight line and light scattering because of various components in its path, respectively. [Source: How Equipment Works.]


To resolve the potential errors of this problem, a test oximeter is used on a person who is asked to breathe in extremely low concentrations of oxygen. The oxygen levels, however, are not allowed to drop below 75-80 %. While doing so, blood samples are taken at intervals and the readings are simultaneously compared to those shown on the oximeter. The errors shown on the oximeter are taken into account and a calibrated graph is made. A copy of this corrected graph is available inside the device, and the computer refers to it before displaying the final reading. As oxygen levels are not made to fall below 75-80%, the oximeter is not very accurate for readings lesser than those.


2. Plethysmography

Usually, along with the oxygen saturation (in percent) on the display, an oximeter also shows the quality of the pulsatile signal (plethysmographic graph) absorbed. This is more important than just the percentage. This is because with slight changes, or movement, the percentage values can be inaccurate and give you false information. This quality of the pulsatile signal mentioned above, is in the form of a graph (Fig. 7), which is always accurate and must be referred to at all times.


Figure 7: An oximeter showing the graph of the pulsatile signal along with the percentage

[Source: Wikipedia.]


3. Ambient light

The oximeter consists of red light and infrared light. In addition to this, there is a third source of (unwanted) light. This is the light present in the surroundings/room.

An important fact to know is that the device does not switch on both the LEDs together.

The red light passes through the finger first. It is then switched off and only then is the IR light switched on. This process continues. Now, the oximeter switches on the red light first, causing both red and the ambient light to reach the detector. The red light is switched off and the same process takes place, now between IR and ambient light. Next, the oximeter switches off both LEDs, allowing only the ambient light to fall on the detector. This gives the correct measurement of the (pure) ambient light. This amount is subtracted from the previous mixed readings, giving results of only red and infrared light. However, the ambient light should not be too strong, causing the lights from the LEDs to fade.


Limitations


1. Small signal strength

An oximeter is able to analyse only about 2% of the total light it receives from the detector. Thus, the device is extremely sensitive and even the slightest of movements in a person can result in very different readings.


2. Optical shunting

The light is supposed to pass through the arteries for correct detection. However, if the probe of the device is of the wrong size or if the finger is placed incorrectly, light, instead of passing through the artery, goes by its side, shunting the artery. Naturally, this will result in false readings.


3. Electromagnetic interference

If an oximeter is used in the vicinity of surgical devices that rely on electromagnetic waves (MRI machine, X ray machine, diathermy, etc), strong electric waves are emitted by them. These waves cause small currents that are “read” by the oximeter, assuming the currents to be from the detector.


4. Poor peripheral perusion

In cases such as during hypotension, the arteries are not very pulsatile, causing the absorption rate to be much lower. The oximeter may find the signal inadequate to correctly display results.


5. Hyperoxia

Hyperoxia is a medical condition when the body receives oxygen in excess. This can be harmful in cases. Usually, oxygen is only attached to Hemoglobin. However, some additional oxygen can also get easily dissolved in the plasma. The oximeter is unable to take into account this extra oxygen and will not display it. So, people with hyperoxia cannot rely on an oximeter for correct readings.


6. Dyes and color

Dyes such as methylene blue, or nail polish on the fingernails can serve as an artificial barrier and can lower the oxygen saturation percentage.


7. Carboxy-hemoglobin

Carbon monoxide (CO) combines with Hb to form carboxy-Hb. The oximeter cannot distinguish between carboxy-Hb and oxygenated Hb. So, it will result in a high level of oxygen saturation, which is actually false. Carboxy-Hb does not consist of oxygen and is harmful.


Conclusion


In conclusion, oximeters are useful devices, especially during the present COVID-19 situation. Many doctors are advising patients to keep an oximeter. Patients undergoing treatment for infection at home are also being provided with oximeters, to help monitor their oxygen levels.

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