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How a Pulse Oximeter Works

A pulse oximeter estimates the oxygen saturation of arterial blood (SpO2) noninvasively, using a small clip-on probe (usually on a fingertip, earlobe, or toe).
1. Basic components The probe contains a light emitter and a photodetector positioned on opposite sides of the tissue (transmission type) or side by side (reflectance type). The emitter sends light through the tissue at two specific wavelengths, typically 660 nm (red) and 940 nm (infrared) - Miller's Anesthesia, 10e.
2. Why two wavelengths Oxygenated hemoglobin (O2Hb) and deoxygenated hemoglobin (deO2Hb) absorb these wavelengths differently:
  • At 660 nm, deoxyhemoglobin absorbs more light than oxyhemoglobin.
  • At 940 nm, oxyhemoglobin absorbs more light than deoxyhemoglobin.
By comparing how much of each wavelength is absorbed, the device can calculate the relative proportion of oxygenated versus deoxygenated hemoglobin - Costanzo Physiology, 7th ed.
3. Isolating the arterial signal (the "pulse" in pulse oximetry) Light passing through tissue is absorbed by skin, venous blood, capillary blood, and arterial blood. Only arterial blood pulses with each heartbeat, so light absorption has:
  • A pulsatile ("AC") component from arterial blood
  • A steady ("DC") component from venous blood, capillary blood, and other tissue
The device calculates the ratio (R) of the AC/DC signal at 660 nm versus the AC/DC signal at 940 nm. Because it isolates the pulsatile component, background absorbance from venous and capillary blood is effectively subtracted out, leaving a signal that reflects arterial saturation specifically - Miller's Anesthesia, 10e; Costanzo Physiology, 7th ed.
4. Converting the ratio to a saturation number This ratio R is then matched against a calibration curve, built by each manufacturer from studies where volunteers breathed hypoxic gas mixtures to generate known saturation values between 70-100%, measured against a reference (in vitro co-oximetry of arterial blood samples). The FDA recommends the device's accuracy stay within about 3% root-mean-square difference from true SaO2; typical device accuracy is around ±2-3% SpO2 in that range - Miller's Anesthesia, 10e.
5. What it does and doesn't tell you Pulse oximetry gives % hemoglobin saturation, not PaO2 (the partial pressure of dissolved oxygen) directly. You can estimate PaO2 from SpO2 using the oxygen-hemoglobin dissociation curve, but the relationship is not linear - at high saturations the curve is flat, so SpO2 can look reassuring even as PaO2 falls significantly - Costanzo Physiology, 7th ed.
Limitations worth knowing:
  • Accuracy depends on adequate pulsatile blood flow; poor perfusion, hypotension, cold extremities, or motion artifact can degrade the reading.
  • Carbon monoxide poisoning falsely elevates SpO2 because carboxyhemoglobin absorbs light similarly to oxyhemoglobin at these wavelengths.
  • Nail polish, ambient light, and severe anemia can also affect accuracy.
Recent evidence flag: Several recent systematic reviews and meta-analyses (2024-2025) have found that skin pigmentation can meaningfully affect pulse oximetry accuracy, generally causing overestimation of true oxygen saturation in patients with darker skin tones, which has clinical implications for hypoxemia detection (PMID: 38368234, PMID: 39388258). This is an active area of device recalibration and FDA guidance updates, so it's worth factoring into interpretation, particularly in acutely ill patients.
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