SpO2 (peripheral capillary oxygen saturation) is calculated by determining the ratio of oxygenated hemoglobin to deoxygenated hemoglobin in your blood using a pulse oximeter. Here's a breakdown of the process:
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Light Emission and Absorption: A pulse oximeter emits two specific wavelengths of light: red light (around 660 nm) and infrared light (around 940 nm). Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through.
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Light Detection: A sensor in the pulse oximeter detects the amount of red and infrared light that passes through your tissue (usually a finger or earlobe).
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Ratio Calculation: The device's processor calculates the ratio of the red light measurement to the infrared light measurement. This ratio reflects the proportion of oxygenated hemoglobin to deoxygenated hemoglobin.
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SpO2 Conversion (Lookup Table): The processor then uses this ratio and compares it to a lookup table based on the Beer-Lambert Law. This law relates the absorption of light to the concentration of the absorbing substance (in this case, oxygenated and deoxygenated hemoglobin). The lookup table translates the calculated ratio into an SpO2 percentage. The higher the ratio of oxygenated hemoglobin, the higher the SpO2 reading.
In summary, pulse oximetry does not directly measure the concentration of oxygen or hemoglobin. Instead, it relies on the different light absorption characteristics of oxygenated and deoxygenated hemoglobin to estimate the percentage of hemoglobin in your blood that is carrying oxygen.
