How does your smartwatch measure oxygen saturation (SpO2)?

The human body is a work of art and health is the primary colour of this art. We human beings are capable of understanding the condition of our health, maintaining it, improving it, or ignoring it. The Health of a human being is determined by the quantification of the Vitals. The 4 main vital signs that act as health indicators are:

• Body Temperature
• Heart Rate
• Respiration Rate
• Blood Pressure

These vital signs help determine the health condition of the human being, but not limited to this. Recent times have introduced us to a disastrous pandemic that has set health as our top priority, The paranoia attached to it has forced us to take notice of the slightest change in our body conditions. SpO2, the measurement of the blood oxygen saturation has entered into the realm of vital signs. All these measurements can now be done at home using simple, affordable, non-invasive devices. Most of us are using Smartwatches or oximeters to measure our blood oxygen levels, but how does a smartwatch or an oximeter measure our blood oxygen level? To understand that let’s understand the blood oxygen level first.

Blood Oxygen Level

Blood circulation is really important for the functioning of the human body, Oxygenated blood is pumped from the heart to all the parts of the body and the deoxygenated blood from the body is carried to the heart then to the lungs. This circulatory function is called Systematic Circulation and is aided by Systematic arteries and Systematic Veins.

Systematic Arteries — Carry oxygenated blood from the left ventricle to the Capillaries of tissues.

Systematic Veins — Picks Deoxygenated blood from the tissue and returns it to the Right Atrium.

On the microscopic level, the oxygen carrier in the blood is a protein called Hemoglobin (Hb or Hgb). Hemoglobin also returns the carbon dioxide in the body to the heart. It is this protein molecule that gives Red Blood Cells(RBCs) their shape and the bright red colour. To deliver oxygen throughout the body each Hemoglobin can carry four oxygen molecules via the bloodstream. And on its journey back it carries carbon dioxide from the body through the heart into the lungs where it dissociates from carbon dioxide and the carbon dioxide is expelled from the body. The deoxygenated blood is a different hue of red as compared to the bright red colour of oxygenated blood, it is this difference in the colours of oxygenated blood (Bright Red) and Deoxygenated Blood (Dark Red) that makes it easy to measure the Oxygen Concentration level in the human body. To understand how this change in colour and composition of blood helps to measure Blood Oxygen Saturation we need to understand Spectroscopy.

Spectroscopy

Remember the prism experiment in school, White light when passed through a prism produces a spectrum of colours that resembles a rainbow. The phenomenon responsible for the dispersion of white light is refraction, these dispersed elements are the electromagnetic components of white light that vary in wavelength. Each electromagnetic component refracts according to its wavelength and the refractive index of the surface thus producing the spectrum. This study of the interaction between electromagnetic radiation and matter is spectroscopy. The function of wavelength or frequency can be applied not just to refraction but also absorption of electromagnetic waves by the matter, this is called absorption spectroscopy.

Absorption spectroscopy is an analytical chemistry tool that helps determine the constituents of a matter. Particles have different absorption coefficients based on the energy gap of the molecules. All molecules absorb and emit light differently, so an absorption spectrum obtained when electromagnetic radiation is passed through a sample helps quantify the presence of certain particles. This spectroscopy method is what helps determine the Blood oxygen saturation.

Non-Invasive SpO2 measurement

Before the advent of Pulse Oximeters, Smartwatches, blood oxygen saturation was measured by a test called the Arterial Blood Gas Test (ABG). ABG involved drawing blood from the patient’s radial artery and measuring a series of values through a long and tedious procedure. The end of the test would give the value of Arterial Oxygen Saturation (SaO2).

Riding on the fast-paced journey of technology we now have non-invasive, fast, and affordable devices that can measure blood oxygen saturation in the comfort of our homes. The measured value of oxygen saturation is called SpO2 — Oxygen Saturation as measure by Pulse oximeter.

Pulse Oximeter and Smartwatch

Both the devices measure SpO2 by the absorption spectroscopy method. The devices carry out calculations (Calibrating for ambiance light), and Analysis (Analysing the absorption spectrum) in the background to give us the oxygen saturation value. But they use a slightly different variation of absorption spectroscopy.

Pulse Oximeters use Transmittance Oximetry, A pulse oximeter is a clip-like structure that houses your finger in between two ends to get the reading, The finger is placed between two sensors on either side, The side touching your fingernail is a light-emitting sensor that emits red, and infrared lights that pass through the finger and reach the sensor on the other end, this sensor absorbs the light and runs calculations in accordance to spectroscopy to give you the oxygen saturation result.

Smartwatches use Reflectance Oximetry, watch sitting on your wrist has sensors on the case cover. It has both the light-emitting and light-absorbing sensors on the same side, the light is emitted from the light-emitting sensor that enters the skin and is reflected by the blood under the skin, the reflected light is absorbed by the other sensor and the calculations are done in accordance to spectroscopy to give you your SpO2 measure.

These variations result in different performing abilities, while transmittance absorption is the closest to the medical-grade Oximeters they are still not considered error-free. Reflectance Oximetry is a method that many experts are skeptical about. We should understand these smart devices, non-invasive methods should be used smartly and not as a substitute to primary healthcare. The future of wearable technology is promising with a projected Compound Annual Growth Rate (CAGR) of 18% from 2021 to 2026. The wristwear product is the frontrunner in the wearable technology market, with a market share of 45% of the total sales in the year 2019. The pace of innovation in the health and technology amalgamation is exciting.

Till next time, Wear your heart on your sleeve, stay home and stay safe.