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What is it?

Oxygen saturation refers to the percentage of hemoglobin that is bound to oxygen when in the artery. Hemoglobin is the protein in red blood cells that binds oxygen, carbon dioxide, and carbon monoxide. Since arterial blood is on the way to the capillaries from the left ventricle of the heart, a high amount of oxygen is expected on hemoglobin, typically greater than 95% saturation.

This oxygen is what is required for metabolic processes, namely ATP production, which provides the energy necessary for vital function. Reduction in oxygen carrying capacity often results in altered or diminished cellular and bodily functions, which can lead to acute or chronic disorders.

How it’s measured

Oxygen saturation is measured non-invasively by photoplethysmography (PPG). PPG utilizes red and infrared light exposure through the skin, which absorbs much of the light. Each form of hemoglobin (unbound or bound to oxygen, carbon dioxide, carbon monoxide) absorbs wavelengths of light differently.

Oxygenated hemoglobin absorbs more infrared light, whereas deoxygenated hemoglobin absorbs more red light. By understanding the light absorption curves of each kind of hemoglobin at red and IR wavelengths, the amount of oxygen-carrying hemoglobin relative to total hemoglobin can be determined and expressed as a percentage.

Correlation with health conditions

Because normal function depends on aerobic processes, impairment of oxygen delivery can lead to worsening symptoms, diminished function, and decreased ability to recover.

Many clinical settings use oxygen saturation to monitor the severity and progression of illnesses. SpO2 is a predictor of all-cause mortality and mortality caused by pulmonary diseases.

What is a “normal” range?

Oxygen saturation greater than 95% is considered normal. Values between 90-95% represent a slightly blunted capacity to carry oxygen and may or may not indicate a significant deviation from normal. However, oxygen saturation below 90% (hypoxemia) is considered low and usually suggests an abnormality in oxygen handling.

95% = Normal
90-95% = Low
<90% = Hypoxemia

For individuals with chronic lung conditions or breathing problems, these “normal” ranges typically do not apply. In these cases, individuals should consult with their healthcare professional for information on acceptable oxygen levels.

Interpreting trends

Deep sleep is a complex biometric that is difficult to quantify. EEG devices provide a strong understanding of sleep stages and progressions but are less realistic for an individual on a regular basis. However, using accelerometers and PPG wearables, light and deep sleep can be approximated on a nightly basis and easily tracked over time.

As with total sleep duration, tracking deep sleep can provide insight into its contribution to changes in health-related outcomes. As a more challenging variable to quantify, monitoring deep sleep over time can also provide insight into lifestyle changes and how they affect deep sleep. For example, tracking how a medication affects deep sleep may provide insight into its efficacy or side effects.

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In a 2018 study, the standard deviation of absolute error in SpO2 values from clinical reference devices was 1.41. Researchers concluded that this preliminary data suggests that this device may be suitable for prospective clinical trials such as evaluating the utility of wearable physiological monitoring in digitally-enabled preventative service models for respiratory disorders.

In another 2018 study published in Circulation investigating the utility of the Biostrap device as a screening tool for Obstructive Sleep Apnea, researchers concluded that the correlation between the Biostrap and clinical reference PSG support further evaluation of wrist-worn health wearables for OSA screening in high risk CVD patients.

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