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ALL ABOUT: Pulse oximeter – How does it work, and what is its history?(2/2)

Tags: Technology and concepts | Smart

10.8.2021 | 3 MIN

Smartwatches with pulse oximeter are slowly but surely becoming the standard. At first, pulse oximeter was a key feature of more expensive smartwatches, but now we can find it in smart bands with a price tag below 40 euros. But how does this technology work, and what is its history?

How does oxygen get into the blood?

Let's start with the basics. We receive oxygenated air through the oral cavity, where it proceeds to the larynx and trachea, which then branches into two bronchi. These then enter the lungs and branch to smaller bronchioles. At the end of the bronchioles are the alveoli, where oxygen enters the blood. (Source in Czech)

Hemoglobin, a metalloprotein of red blood cells, is then responsible for the transfer of oxygen to the tissues and the removal of carbon dioxide.

One molecule of hemoglobin can bind up to 4 molecules of oxygen, and almost 99% of all oxygen in our body is bound to hemoglobin.

Oxygen-bound hemoglobin is called oxyhemoglobin, and non-oxygenated hemoglobin is called deoxyhemoglobin.

How can a watch measure blood oxygenation?

As opposed to the heart rate sensor, which lights the skin with green light, the pulse oximeter uses red light. Two LEDs to be exact – one emits red light (wavelength of 660 nm), and the other one emits infrared light (wavelength of 940 nm).

These two types of light are there for a reason because every wavelength absorbs different types of hemoglobin. Red light absorbs better deoxyhemoglobin (deoxygenated), and  infrared light absorbs oxyhemoglobin (oxygenated).

Both diodes can shine through the skin, the tissue, and also the blood (arterial, venous, and capillary). When the diodes on the case back begin to illuminate the skin with both types of light of a certain intensity, the detector on the watch is then able to determine how much infrared and red light is reflected and how much light has been absorbed by both types of hemoglobin. And therefore also what is the ratio of oxygenated and deoxygenated blood.

What is needed for a watch to correctly measure the oxygenation of blood?

To stay calm, for sure. While the watch does not have a problem with moving when measuring the heart rate, to measure the oxygenation of the blood, it is necessary to stay calm. Therefore, the pulse oximeter measurement will not start if you are on the move, and the measured values ​​may be several hours old. It is ideal to be sitting down, not moving and not doing anything else during each measurement. To get more accurate results, it also helps to place your hand at heart level.

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For correct measurements, it is necessary to have the strap tightened enough (but it must not strangle the hand), and therefore the best option is a silicone strap, which adapts to the creases of the hand.

You should also pay attention to the good condition of both your wrist and the optical sensor itself. Wash and dry your hand well before putting on the watch. Do not use sunscreens or repellents in these areas before measuring. Be careful not to scratch the optical sensor and rinse the watch underwater after each workout.

How did pulse oximetry originate?

The first mention of pulse oximetry came from the 1930s as they started to study the passage of light through the skin and what could all of this tell us. The first company to develop an ear pulse oximeter was Hewlett Packard. The device was so big that it was primarily used in labs.

The very first non-invasive way of using light passing through the ear was the work of Japanese bioengineer Takuo Aoyagi (early 70's). Since then, the size of this technology has been steadily declining and has entered devices such as smartwatches.

Fingertips, earlobes or wrists are used to measure blood oxygenation because these areas are well translucent.

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