Pulse oximetry: principles and limitations
Stem
A 63-year-old Black man, 92 kg, with COPD is having a robotic radical cystectomy under general anesthesia. Two hours in, his hands are cool, the MAP is 62 mmHg on a phenylephrine infusion, and the pleth trace is small. Moments after the urologist asks for intravenous methylene blue to check the ureters, the SpO2 falls from 96% to 87%. End-tidal CO2 and airway pressures are unchanged.
Question
How do you interpret this saturation, and what do you do?
Ideal answer
HeadlineMost likely artifact from methylene blue and a weak pleth, but the patient comes first: FiO2 100% while I prove it.
- FiO2 to 100% now. Real hypoxemia in a robotic case would usually move airway pressure, capnogram or tidal volume; none has. I listen to both lungs and confirm inspired oxygen on the analyzer.
- Name the two artifacts. Methylene blue gives a transient, falsely low reading;1 cold hands and vasopressor make a small pleth the device may misread.1
- Check the pleth, warm the hand, probe to the ear, which vasoconstricts less than the finger.2
- Arterial gas with co-oximetry, which measures saturation directly and settles it.
- Higher SpO2 target once the dye clears: oximeters can overestimate by about 2% in darker skin,1 so a gas beats a borderline reading.
Watch for: Variable-pitch tone and low alarm audible throughout, per the ASA monitoring standard.3
Breaking down the answer
- Changes if the airway moved — had airway pressure or the capnogram changed, I would run the full hypoxemia differential instead.
- Know how the device works — two LEDs, red at 660 nm and infrared at 940 nm, read the pulsatile absorbance of deoxygenated and oxygenated hemoglobin through the Beer-Lambert law.1 Anything that changes absorbance or the pulsatile signal changes the number.
- Name the artifacts by direction — dyes (methylene blue, indocyanine green) read falsely low;1 carboxyhemoglobin reads falsely high because it absorbs like oxyhemoglobin; methemoglobin drives the reading toward 85% whatever the true saturation.2
- Low perfusion and motion — hypotension, vasoconstriction and cold extremities shrink the pulse amplitude and degrade accuracy.1 The ear or forehead keeps a signal when the finger fails.2
- Calibration limits — devices are calibrated for 70–100% and are accurate to about 2–4%; below 70% they track the blood gas poorly.1
- Skin pigmentation is a patient-safety issue — FDA lists pigmentation alongside poor circulation, skin thickness, temperature, tobacco and nail polish as factors affecting accuracy.4 Its January 2025 draft guidance asks for about 150 participants across skin tones graded with the Monk Skin Tone scale.5
Follow-up question
In the PACU an hour later he is drowsy on 2 L/min nasal cannula, SpO2 reads 93%, and the arterial gas shows an SaO2 of 87% with a PaO2 of 54 mmHg. The nurse asks whether to trust the monitor. What do you do?
Follow-up answer
HeadlineOccult hypoxemia: the oximeter reads about six points above the measured saturation, fitting overestimation in darker skin.1 The gas is the truth.
I tell the nurse the monitor is not reliable for him, and his target needs to be higher. I treat the cause: I sit him up, increase oxygen, stimulate him, and look for residual neuromuscular blockade, opioid effect and atelectasis, listening for wheeze given his COPD. I reassess with a repeat gas after the change, not with the oximeter alone, and set the alarm threshold higher. I document the discrepancy so the ward team carries the same rule forward.
Pitfalls that cost points
- Saying carbon monoxide makes the SpO2 low; it makes it falsely high.
- Treating a sudden drop after a dye injection as hypoxemia and escalating airway interventions.
Rapid-fire
Q: What does methemoglobinemia do to the SpO2? A: It fixes the reading near 85% regardless of the true saturation.2
Q: Which probe site keeps a signal during vasoconstriction? A: The ear, because it vasoconstricts less than the finger.2
Q: Does nail polish matter? A: Black or blue polish and artificial nails are often implicated, although studies are inconclusive.1
Q: What does the ASA standard require of the oximeter? A: A quantitative measure of oxygenation such as pulse oximetry for every anesthetic, with the variable-pitch tone and low alarm audible.3
Q: What measures the true saturation? A: A co-oximeter, which uses multiple wavelengths and separates the dyshemoglobins.2
Numbers to know
| Item | Value |
|---|---|
| Wavelengths | 660 nm (red), 940 nm (infrared)1 |
| Calibrated range | 70–100%, accuracy about 2–4%1 |
| Methemoglobin | Reading fixed near 85%2 |
| Darker skin bias | Overestimates by about 2%, device-dependent1 |
| FDA 2025 draft study size | About 150 participants across skin tones5 |
Links
- Methemoglobinemia and carbon monoxide, Capnography: waveforms and sudden loss of ETCO2, Hypoxic gas mixture and oxygen supply failure, burns
- Atlas: Monitor atlas: perioperative respiratory events
- ASA Standards for Basic Anesthetic Monitoring (2025); FDA draft guidance, Pulse Oximeters for Medical Purposes (2025)
Sources
- 1.Pulse Oximetry (StatPearls) · StatPearls Publishing (NCBI Bookshelf) · ncbi.nlm.nih.gov
- 2.Pulse oximetry (keyword) · OpenAnesthesia · openanesthesia.org
- 3.Standards for Basic Anesthetic Monitoring · ASA, 2025 · asahq.orgLast amended October 15, 2025.
- 4.Pulse Oximeters · FDA, 2025 · fda.govAccuracy factors; January 2025 draft guidance 'Pulse Oximeters for Medical Purposes'.
- 5.FDA Issues Draft Guidance for Improving Skin Tone Disparities of Pulse Oximeters · TCTMD, 2025 · tctmd.comNews summary of the FDA January 2025 draft guidance (150 participants, Monk Skin Tone scale).
Facts on this page were checked against these sources on Oct 15, 2026. Guidelines change; confirm doses against current institutional protocols.