How does a Pulse Oximeter work?

I was one of the many people who bought a pulse oximeter during these corona pandemic times, had a basic understanding of what it used to measure and which values to look out for. Today I came across this comprehensive answer on Quora posted by Sunny Dhondkar.

I am sharing the same for public information. The below mentioned words and illustrations have been taken from his quora answer.

The tough times going on all around the world have brought special attention to the device shown in the picture above. You could have seen and even used this device to check the health status of your body.

In case you don’t know about it, we call it Oximeter. The primary job of this device is to measure the Oxygen Saturation of your blood.

Months ago, my dad was posted to a site for analyzing the situation of pandemic and checking people who are positive with the virus. At such sites, for the purpose of suspecting whether a person is positive or negative, the government has provided Oximeters.

So my dad had one of these Oximeters with him. When I inserted my fingertip inside it, the device calculated my oxygen saturation level and pulse rate. I was stunned.

Fine with the pulse rate measurement, but how could a device measure oxygen saturation level without even getting a blood sample? How could a device measure the level of oxygen in the blood without even entering the body?

You might have gone through the same thoughts after seeing the device work and measure your oxygen saturation level. Today, I’ll discuss the interesting mechanism of the Oximeter and how it measures the level of oxygen. That too by staying outside the body.

Before we get into the mechanism directly, you must have a basic knowledge of what the term oxygen saturation level means. Don’t worry, you don’t need to set off towards google… I’ll explain it.

Oxygen is present in the blood in two forms:
1. Dissolved in the blood
2. Bound to Hemoglobin

In terms of binding to the oxygen and its transportation, Hemoglobin is classified into two types:
1. Functional Hemoglobin
2. Non-functional Hemoglobin

Functional Hemoglobin is further classified into two categories:

Oxyhemoglobin → Contains oxygen bound to it

Deoxyhemoglobin → Does not have oxygen bound to it

Non-functional Hemoglobin is further classified into two categories:

Carboxyhemoglobin → Hemoglobin that’s bound to CO (Carbon Monoxide)

Methemoglobin → Hemoglobin that’s bound to… not Meth, obviously! (Breaking Bad vibes at their peak).
This hemoglobin is bound to ferric ion Fe+3Fe+3.

To measure the level of oxygen in your blood, we’re more interested in Oxyhemoglobin. This hemoglobin is bound to oxygen and transports it throughout the body. The amount of this oxygen in the blood is what we call Oxygen Saturation Level. Now that you’ve understood what this term actually means. Let’s move on to its measurement part.

To calculate the saturation level of oxygen, we need to know the amount of oxyhemoglobin present in your blood. That is where the Pulse Oximeter comes into play.

The Pulse Oximeter measures the percentage saturation of oxygen bound to Hemoglobin in arterial blood. This term is represented as SpO2SpO2.

I’ve marked the term here in this picture:

The number you see as the final output on the device (here it’s showing 96) is actually the %SpO2%SpO2.

Now, how is this number calculated?

This is where the engineering part comes in!

To make you understand the engineering part, I’ll be using my illustrations.

Here’s a cute little Oximeter:

On the top of this device, there’s a display.
This display provides output values like the Oxygen Saturation Level (in terms of SpO2SpO2) and the Pulse Rate.

Inside the Oximeter, we have two Light Emitting Diodes (LEDs) and a Photodiode.

One of these diodes emits Red Light waves (having a wavelength of 660 nm) and the other diode emits Infrared waves (having a wavelength of 940 nm).

The job of the Photodiode is to detect these electromagnetic waves emitted by the two LEDs. On detecting the amount of Red and Infrared light, respectively, the photodiode sends electrical signals for further calculations.

Now, how do these light waves help detecting the oxygen level in your body?

Let me explain…

Here’s a H.I.C.C.U.P (Hint to Initiate Curiosity and Consequently Understand the Point): Oxyhemoglobin and Deoxyhemoglobin absorb light waves of different wavelength.

You insert your finger in the Oximeter:

Your finger contains arteries inside it, which transport blood from the heart to the other parts of the body. The blood inside these arteries contains lots and lots of Hemoglobin.

To measure the oxygen saturation level, we need to find the amount of Oxyhemoglobin.

Since we can’t calculate the total Oxyhemoglobins present in your blood, we’ll have to somehow find the percentage of Oxyhemoglobin present in the mixture of Oxyhemoglobin and Deoxyhemoglobin in a specific amount of blood flowing in the arteries of your finger.

Take an oversimplified example here:

If the blood flowing through your arteries has 20 Hemoglobin molecules. Out of these, 18 molecules are of Oxyhemoglobin and the remaining 2 are Deoxyhemoglobin.

Then we can calculate oxygen saturation level as:

%SpO2=No. of OxyhemoglobinTotal Hemoglobin×100%SpO2=No. of OxyhemoglobinTotal Hemoglobin×100

⇒%SpO2=1820×100⇒%SpO2=1820×100

⇒%SpO2=90%⇒%SpO2=90%

Hence, the number 90 will be displayed on your Oximeter.

To find the amount of Oxyhemoglobin & Deoxyhemoglobin, the Light Emitting Diodes fire Red Light Waves and Infrared Light Waves, respectively, at a certain frequency.

Now that light waves are fired, we will determine the concentration of Oxyhemoglobin and Deoxyhemoglobin, since both of these molecules have different light absorption characteristics.

To be more specific:
Oxyhemoglobin → Absorbs more infrared light (940 nm) and lets more red light (660 nm) travel through the tissues of your fingertip
Deoxyhemoglobin → Absorbs more red light (660 nm) and lets more infrared light (940 nm) travel through the tissues of your fingertip

Depending on the amount of Red and Infrared light hitting on the surface of the Photodiode, further electrical signals are passed to the Integrator (where the calculations happen).

To find the concentration of the respective Hemoglobin molecules from the absorbed light waves, the Integrator of the Oximeter uses Beer’s Law.

Don’t panic… It’s not that hard to understand. The Beer’s Law is easier to Digest than a Beer.

According to Beer’s Law:

Absorbance ∝∝ Concentration

In a layman’s language, the amount of light (emitted by one of the LEDs) absorbed by your fingertip is directly proportional to the concentration of the corresponding Hemoglobin molecules.

Using this law, the Oximeter calculates the concentration of Oxyhemoglobin (by measuring the amount of Infrafed Light absorbed) and that of Deoxyhemoglobin (by measuring the amount of Red Light absorbed).

Now that we have the concentration of both, we can calculate the percentage of Oxyhemoglobin present in your blood.

%SpO2=conc. of Oxyhemoglobinconc. of Oxyhemoglobin + Deoxyhemoglobin×100%SpO2=conc. of Oxyhemoglobinconc. of Oxyhemoglobin + Deoxyhemoglobin×100

i.e., %SpO2=HbO2HbO2+Hb×100%SpO2=HbO2HbO2+Hb×100

→ Oxygen Saturation Level

And that’s how a cute and little Pulse Oximeter calculates the saturation level of oxygen in your blood, by staying outside of your body and without even getting a blood sample.

A reunion of Physics, Chemistry, and Biology!

Sunny Dhondkar

Post Credits:

Going through my analysis on the mechanism of oximeter, there are two interesting questions to ask:

Why the fingertip? Why not any other part of the body is used to calculate oxygen saturation level?

Why so specific wavelengths of light are used (660 nm & 940 nm)?

Answering the 1st one:
Fingertips and Earlobes have higher blood flow rates compared to the other tissues of your body.
The Pulse Oximeter I talked about is for household use, so fingertip is preferred to comfortably calculate oxygen saturation level.
In case you visit any hospital, you’ll see that the patient’s earlobes are also used to check for pulse rates and oxygen saturation level.

Answering the 2nd one:
Have a look at this graph:

The graphs shows the behaviour of HbO2HbO2 (Oxyhemoglobin) and HbHb (Deoxyhemoglobin) towards different wavelengths of light.

Having a look at the curves, we can directly interpret that both molecules show opposite behaviours with regards to absorption of light for wavelengths 660 nm and 940 nm. (Like the red curve has local minima around 660 nm and local maxima around 940 nm).