Pulse oximetry (SpO2) measures oxygen saturation using Beer–Lambert law. The two wavelengths of light used are:

• A 400 nm (ultraviolet) and 700 nm (red)
• B 500 nm (green) and 800 nm (infrared)
• C 630 nm (orange) and 1000 nm (near infrared)
• D 660 nm (red) and 940 nm (infrared) ✓

Explanation

Standard pulse oximetry uses two wavelengths: 660 nm (red light), at which deoxyhaemoglobin absorbs more than oxyhaemoglobin, and 940 nm (near-infrared), at which oxyhaemoglobin absorbs more. The device computes the ratio of pulsatile to non-pulsatile absorption at each wavelength (R/IR ratio) and converts this to SpO2 via a stored look-up algorithm derived from volunteer calibration studies. Carboxyhaemoglobin and methaemoglobin absorb at similar wavelengths and cause falsely elevated SpO2 readings, which is a major limitation of standard two-wavelength pulse oximetry.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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