How Pulse Oximetry Works in a Fitness Watch?
Hey friends, Happy Wednesday!
Answer to the question I posed last week
Last week, I asked about the negative impacts of using a Pacemaker. There are possibilities of swelling, bruising, bleeding, or infections near the site where the pacemaker is implanted.
This week, we’ll look at how a pulse oximeter works in a fitness watch. These are features that tell you about the oxygenation level in your blood. If transporting oxygen into the blood is an issue, the amount of oxygen in your blood declines which can cause potential danger to our organs. A pulse oximeter can detect this drop through non-invasive means and will alert the user.
I aim to write my newsletter issues in a way one can follow them while traveling on a bus, having a coffee, waiting for food, etc. Let’s jump in!
Hardware Setup
Red LED and Infrared LED lights are mounted in the fitness watch to generate periodic light pulses directed toward our skin, alternatively. Photodiodes are light-based sensors that produce a current when illuminated by light. They are placed between the LEDs and are used to absorb the photons (light) reflected from our arteries and generate a current based on the intensity of light.
Light’s Interaction with Blood
Blood carries the oxygen necessary for our life through hemoglobin molecules. But not all of them carry oxygen.
The hemoglobin molecules that carry oxygen are called oxyhemoglobins, which absorb more infrared light and a lesser amount of red light. This is because oxygenated blood appears bright red by reflecting the red light in the first place.
While deoxyhemoglobin absorbs more red light and a lesser amount of infrared light.
This difference in absorption of infrared light and red light in our different hemoglobin molecules is leveraged in Pulse Oximetry in the following way.
How does Pulse Oximetry work?
Alternatively, Red LED and Infrared LED light is emitted periodically onto our skin. The light passes through the skin to arteries and veins in our wrists. A portion of the light is reflected and absorbed by the photodiode placed in between the LEDs.
The amount of red light and infrared light absorbed can thus be derived.
But we want to find and are concerned only about the oxygenation in the arteries and not in the veins. This is because veins are vessels that carry blood that is low in oxygen to the heart for re-oxygenation. But the problem is that light emitted by LEDs interacts with veins and arteries.
So, we use the following insight. Every time our heart beats, the blood flow in our arteries increases (which means more light is reflected onto the photodiodes), but they remain constant in our veins. The photodiodes generate current accordingly over time with peaks during the heartbeats. These different components of the current generated due to veins and arteries are then further analyzed to compute the oxygenation level.
Read my blog post through the link attached to read in-depth about how the photodiode current is manipulated to find the oxygenation level.
Blog posts
My blog post S1.E10 on How Pulse Oximetry works in a fitness watch is out! Read it here, happy learning!
Learn how Pulse Oximeter works in a Fitness Watch
My next episode S1.E11 is definitely coming out next week!!
It is the last 3 blog posts of Season 1 from here. We’re moving to Season 2 after completing S1: Wellness Gadgets. Thanks a lot for traveling with me on this journey that has a lot more to come!
Also, let me know if you’d like to read about a specific gadget/device.
Question of the week
What are some cases where there can be inaccuracies in the measurement of the oxygenation level using a fitness watch? And do you have any ideas on how they can be rectified? Share your thoughts by replying to this email, and we can have a discussion. I’ll answer this in next week’s issue.
Gadget of the week
The visually impaired people cannot enjoy live soccer as others do with the millions of fans. So Irish haptic-tech startup Field of Vision created a device that is around a size of an iPad that allows them to follow the ball’s movement across the field with their fingertips. Cameras track the ball and map its exact position to the device, which vibrates when something like slide and tackle happens, etc. The company was co-founded by David Deneher who currently is living in Ireland. Super cool gadget right?
Thank you for reading. Have a nice rest of the week, and take care!
Until next Wednesday,
Chendur
