Oxygen saturation

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Oxygen saturation describes how much of the hemoglobin – the oxygen-transport protein in red blood cells (erythrocytes) – is currently saturated with oxygen. It is typically given as a percentage relative to the total hemoglobin. Expected normal blood oxygen saturation values are between 95% - 100 % and values <90% are considered to be hypoxemia .

Definition

The general definition of blood oxygen saturation is saturated hemoglobin / (saturated hemoglobin + unsaturated hemoglobin).

Measuring blood oxygen saturation

There are different ways to measure the blood oxygen saturation in practice, with arterial blood gas tests and pulse oximetry being widely used.

Arterial blood gas test

An arterial blood gas (ABG) test can measure the amounts of arterial gases, including oxygen and carbon dioxide. It is an invasive lab test for which a bit of blood needs to be drawn from the radial artery. The collected blood then needs to be analyzed in a specialized blood gas analyzer. Amongst the measured parameters is the arterial oxygen saturation (SaO₂).

Given the need for specialized equipment and blood draw, these tests are typically done within medical centers and are not readily available for longitudinal testing by individuals.

Pulse Oximetry

Pulse oximetry is a non-invasive alternative to measure blood oxygen saturation, which generates peripheral oxygen saturation (SpO₂) readings that can be within 2% accuracy of SaO₂ readings . Pulse oximeters work by shining light of different wave lengths through the skin and tissue (or reflect from it) to measure how much of the light is absorbed. As saturated and non-saturated hemoglobin absorb light at these wave lengths to different amounts, it becomes possible to calculate the blood oxygen saturation from these differences.

There are different ways to perform a pulse oximetry, ranging from using a smartphone, over wearable devices to using dedicated pulse oximeters.

Smartphone apps

There is an increasing number of smartphone apps that can be used to measure SpO₂ values . The work by using the flash(light) and camera of the phone to shine light through the user's finger and recording the live video image at the same time to calculate the blood oxygen levels. A study found that these recordings might not be the most accurate ones

SpO₂ sensors in wearables

Wearables such as newer Apple Watch models, the Oura Ring, Fitbit or Withings wearables include the capability to record SpO₂ values. There has been discussion on whether these sensors are accurate enough for medical use.

Dedicated pulse oximeters

There are dedicated pulse oximeters which can be clipped to a person's finger to measure blood oxygen values in this way. Additionally there are some in the form factor of a ring, designed to facilitate longitudinal use cases. Some of these devices come as connected devices that can store recordings in companion apps. Consumer variants can sell for as little as $14 US, while approved medical device variants can be $100 US+.

Limitations

One of the main limitations of pulse oximeters is that it does not measure oxygen saturation in itself, but rather uses hemoglobin saturation as a proxy measurement, assuming that hemoglobin is only saturated by oxygen. Gases that have a higher binding-affinity to hemoglobin than oxygen will saturate hemoglobin while crowding out oxygen. In such cases, as e.g. happens during carbon monoxide poisoning, oximeters will falsely record a high oxygen saturation levels due to this .

Additionally, studies have found that pulse oximeters are not working as reliable on people with dark skin, with one study finding that pulse oximeters are 3x as likely to miss hypoxemia in black patients compared to white patients .

Uses

Oxygen saturation is of great importance when affected by sleep apnea as it lowers the saturation. During the COVID-19 pandemic, this metric has gained larger interest as a lowering of the oxygen saturation can be both an indicator and a result of infection.

Projects involving oxygen saturation

• The Quantified Scientist did a project in which he measured how his blood oxygen saturation changes during commercial flights