S1 E10: How is the Oxygen level in your blood measured by a Fitness watch?
Introduction
Wearables like fitness watches have numerous apps and metrics that they can measure. This includes finding out the level of oxygenation in our blood (arterial blood), also called pulse oximetry. If the amount of oxygen in your blood declines it can cause potential danger to your organs. So a pulse oximeter can detect this drop through non-invasive means and will alert the user. The exact hardware specifics can change from model to model but the basic principle has been the same.
And here is how it works under the hood!
Hardware Setup
The setup used for pulse oximetry is the same as what is used for detecting heartbeats in a fitness watch.
LEDs
Red LED and Infrared LED lights are mounted in the fitness watch to generate periodic light pulses directed toward our skin, alternatively.
Photodiodes
Photodiodes are sensors that produce a current when illuminated by light. They are placed between the LEDs and are used to absorb the light reflected from our arteries and generate a current based on the intensity of light.
Figure 1: Side of a fitness watch with relevant components used for pulse oximetry
Light’s Interaction with blood
Blood carries the oxygen necessary for our life through hemoglobin molecules.
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 hemoglobin molecules that do not carry oxygen are called deoxyhemoglobin, which absorbs more red light and a lesser amount of infrared light.
This difference in absorption of infrared light and red light in our different hemoglobins is leveraged in Pulse Oximetry in the following way.
How does Pulse Oximetry work?
Red LED and Infrared LED light are emitted periodically onto our skin, alternatively. The light passes through the skin to arteries and veins in our wrist. 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 based on the photodiode current.
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 reoxygenation. But the problem is that light emitted by LEDs interacts with both veins and arteries. So how do we separate the current in photodiodes due to arteries from veins?
So here is how we classify the current generated!
Here is the graph of the photodiode current generated due to the red LED light that gets reflected. Infrared LED light will have a similar graph of its own.
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.
Calculating oxygen rate
The key is to separate the alternating current and the direct (constant) current. Because we are interested only in the current generated due to light reflected from arteries (marked as red in the graph) and not veins (marked as blue in the graph). The amplitude of the AC component and DC component of both red light and infrared light are used to compute the following ratios.
R1 = AC current (Red light) / DC current (red light)
R2 = AC current (infrared light) / DC current (infrared light)
R = R1/R2
The ratio R is then mapped to provide the Blood oxygen level as a percentage, on your fitness watch.
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