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Now I have enough to give a complete, well-grounded explanation.
How SpO₂ (Pulse Oximetry) Works
SpO₂ stands for peripheral capillary oxygen saturation - the percentage of hemoglobin in arterial blood that is carrying oxygen (bound as oxyhemoglobin), estimated non-invasively via a finger, earlobe, or forehead probe. It's often called the "fifth vital sign."
1. The core physics: two colors of light
Oxygenated hemoglobin (O₂Hb) and deoxygenated hemoglobin (deO₂Hb) absorb light differently depending on wavelength - this is why oxygenated blood looks bright red and deoxygenated blood looks dark red/blue. A pulse oximeter probe has two LEDs and one photodetector on opposite sides of the tissue:
- Red light (~660 nm): absorbed much more by deoxygenated hemoglobin
- Infrared light (~940 nm): absorbed much more by oxygenated hemoglobin
The device rapidly alternates the two LEDs (often flickering at up to several hundred Hz, with brief "both off" intervals to subtract ambient light) and measures how much of each wavelength gets through the tissue (Costanzo Physiology, p. 226; Miller's Anesthesia, 10e).
2. The Beer-Lambert Law
The underlying math is the Beer-Lambert law, which relates transmitted light intensity to the concentration of an absorbing substance in solution:
I_trans = I_in × e^(-D·C·ε)
where D = path length (tissue thickness), C = concentration of the solute (hemoglobin species), and ε = the extinction coefficient of that solute at a given wavelength. Since blood contains multiple hemoglobin species (oxy-, deoxy-, carboxy-, met-, sulfhemoglobin), fully solving this requires absorption measurements at multiple wavelengths - that's what a lab co-oximeter does (5 wavelengths, considered the gold standard). A pulse oximeter simplifies this to just two wavelengths and therefore only estimates functional saturation: O₂Hb / (O₂Hb + deO₂Hb) × 100, ignoring the minor dyshemoglobin species (Miller's Anesthesia, 10e).
3. Isolating arterial blood: photoplethysmography
A key trick: skin, bone, venous blood, and capillary blood also absorb light, but they don't pulse. Only arterial blood volume changes with each heartbeat. The oximeter measures light absorption continuously and separates it into:
- A DC (constant) component - absorption from tissue, bone, venous/capillary blood
- A pulsatile AC component - the small extra absorption that appears only with each arterial pulse (this is the photoplethysmographic waveform you see as the oximeter's waveform trace)
By analyzing only the AC pulsatile fraction at each wavelength, the device isolates the signal attributable to arterial blood, effectively subtracting out the non-pulsating background (Costanzo Physiology, p. 226; Miller's Anesthesia, 10e).
4. The "ratio of ratios" and calibration
The device computes a ratio, often called R:
R = (AC_red/DC_red) / (AC_infrared/DC_infrared)
Because real tissue causes light scattering (not just simple absorption as Beer-Lambert assumes), manufacturers don't calculate SpO₂ from pure theory. Instead, each oximeter is calibrated empirically: R values are correlated against actual arterial oxygen saturation (SaO₂) measured by co-oximetry in healthy volunteers across a range of saturations, producing a built-in calibration curve/algorithm that converts R directly to a displayed SpO₂ percentage.
5. From SpO₂ back to PaO₂ - the oxyhemoglobin dissociation curve
SpO₂ doesn't directly measure the partial pressure of oxygen in blood (PaO₂); it measures percent saturation. The two are linked by the sigmoid-shaped oxyhemoglobin dissociation curve, so clinicians can roughly estimate PaO₂ from SpO₂ (e.g., SpO₂ 90% ≈ PaO₂ 60 mmHg - the steep part of the curve, where saturation drops quickly). Because the curve is flat at the top, SpO₂ is poor at detecting hyperoxia (can't distinguish PaO₂ of 100 from 300 mmHg), but very sensitive to falling oxygenation at the steep, lower portion of the curve (Costanzo Physiology, p. 226-227).
6. Why SpO₂ can be wrong: key limitations
- Carboxyhemoglobin (CO poisoning): absorbs light similarly to oxyhemoglobin at these wavelengths, so standard pulse oximeters read a falsely high SpO₂ even when the patient is critically hypoxic - a major clinical pitfall.
- Methemoglobinemia: drives SpO₂ toward ~85% regardless of true saturation.
- Poor perfusion / hypotension / cold extremities / vasoconstriction: weak pulsatile signal reduces accuracy or gives no reading.
- Motion artifact, ambient light, nail polish (especially dark colors), and skin pigmentation: can degrade or bias the signal.
- Lag time: SpO₂ can take minutes to fall after ventilation stops, especially if the patient was preoxygenated - so it is not a substitute for direct ventilation monitoring like capnography (Roberts & Hedges' Clinical Procedures in Emergency Medicine, p. 711-712).
- Anemia and dyshemoglobinemias affect oxygen content and delivery even when SpO₂ looks normal, since SpO₂ only reflects the fraction of available hemoglobin that's saturated, not total oxygen-carrying capacity.
Note on an active area of research: Several recent systematic reviews and meta-analyses (2024-2025, e.g., British Journal of Anaesthesia 2024, PMID 38368234; JMIR 2024, PMID 39388258; Clinical Nursing Research 2025, PMID 41045137) have found that pulse oximeters can overestimate true oxygen saturation in patients with darker skin pigmentation, particularly during hypoxemia, likely because melanin also absorbs light at these wavelengths and calibration datasets historically underrepresented darker skin tones. This is a recognized limitation that clinicians should factor in, especially in critically ill patients, and regulatory bodies (FDA) have been revising testing/labeling standards in response.
Quick summary
Two LEDs (red + infrared) shine through a pulsating capillary bed → a photodetector measures how much of each wavelength gets through → the device isolates the pulsatile (arterial) component of the signal → it computes a ratio of red-to-infrared absorption → that ratio is matched against an empirical calibration curve to output an SpO₂ percentage, which approximates the fraction of hemoglobin carrying oxygen.
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