Pulse oximetry: principles and limitations

8 min read Last updated April 8, 2026

Clinical approach (how to interpret a pulse oximeter reading)

  • Confirm the signal is trustworthy before acting on SpO2
    • Check plethysmograph waveform quality, pulse rate agreement with ECG/manual pulse, and probe position/site perfusion
    • If pleth is poor: treat as 'no reliable SpO2' and troubleshoot (site, perfusion, motion, ambient light, probe)
  • If SpO2 is low: treat patient first, then device
    • Increase FiO2, ensure airway/ventilation, check circuit and end-tidal CO2, auscultate, consider bronchospasm/atelectasis/pneumothorax
    • Correlate with ABG if unexpected, severe, or discrepant with clinical picture
  • Remember what SpO2 does NOT measure
    • Ventilation (PaCO2), oxygen content (CaO2), or adequacy of oxygen delivery (DO2)

What pulse oximetry measures (definitions)

  • SpO2 is an estimate of arterial oxygen saturation (SaO2) derived from light absorption changes in pulsatile arterial blood
  • Oxygen saturation (functional) = HbO2 / (HbO2 + HHb)
  • Fractional saturation = HbO2 / (HbO2 + HHb + COHb + MetHb + other dysHb)
  • Most standard 2-wavelength pulse oximeters approximate functional saturation and can be misleading in dyshemoglobinaemias

Principles (physics and signal processing)

  • Beer–Lambert law: light attenuation through a medium relates to concentration and path length (I = I0 e^(-εcl))
    • In vivo limitations: scattering, variable path length, and multiple absorbers mean empirical calibration is required
  • Two wavelengths: red (~660 nm) and infrared (~940 nm)
    • DeoxyHb absorbs more red light; oxyHb absorbs more infrared light (relative differences exploited)
  • Photoplethysmography separates pulsatile (AC) from non-pulsatile (DC) components
    • DC: constant absorption from venous blood, tissue, bone, skin pigmentation
    • AC: pulsatile arterial component (small, typically 1–5% of total signal)
  • Ratio-of-ratios: R = (ACred/DCred) / (ACinfra/DCinfra); R mapped to SpO2 via calibration curve
    • Calibration is based on healthy volunteers with controlled hypoxia (typically down to ~70% SaO2); accuracy degrades below this
  • Probe designs: transmission vs reflectance
    • Transmission: LED and photodiode opposite sides (finger, toe, ear lobe)
    • Reflectance: LED and detector on same side (forehead, nasal alar); useful in low perfusion but more motion/venous artefact prone
  • Displayed pleth waveform is a processed signal (not a direct arterial pressure trace)

Performance characteristics

  • Typical accuracy: ±2–3% (1 SD) between ~70–100% saturation under ideal conditions; worse with low perfusion/motion/dysHb
  • Lag time: depends on averaging/algorithm and circulation time; may delay detection of rapid desaturation
    • Longer averaging reduces noise but increases delay (important in apnoea/RSI)
  • Relationship to PaO2: sigmoid oxyhaemoglobin dissociation curve
    • At high SpO2 (≥97–100%), PaO2 can vary widely; oximetry cannot detect hyperoxia
    • At lower saturations, small falls in PaO2 produce large falls in SpO2 (steep portion)

Limitations and sources of error (systematic and random)

  • Low perfusion states reduce AC component → unreliable or absent reading
    • Shock, hypothermia, vasoconstriction (including vasopressors), low cardiac output, PVD
    • Troubleshoot: warm limb, change site (ear/forehead), reduce vasoconstriction if possible, ensure correct probe size/fit
  • Motion artefact: creates spurious AC signals and venous pulsation → false readings
    • Common in shivering, tremor, transport, paediatrics; consider forehead reflectance or secure probe
  • Ambient light and electrical interference
    • Operating lights, phototherapy, sunlight; shield probe or reposition
  • Venous pulsation (e.g., tricuspid regurgitation, tight probe, dependent limb, tourniquet effects) → underestimation of SpO2
  • Dyshemoglobinaemias (major exam theme)
    • Carboxyhaemoglobin (COHb): absorbs similarly to oxyHb at 660 nm → pulse oximeter overestimates saturation (SpO2 may appear normal/high)
    • Methaemoglobin (MetHb): tends to drive SpO2 toward ~85% regardless of true saturation (both wavelengths absorbed)
    • Sulfhaemoglobin: can cause low SpO2; standard oximeters cannot quantify
    • Definitive assessment: co-oximetry on ABG (multi-wavelength)
  • Anaemia: SpO2 may remain normal despite low oxygen content; severe anaemia can reduce signal quality
    • Oxygen delivery depends on CaO2 and cardiac output; SpO2 alone can be falsely reassuring
  • Nail varnish/false nails/skin dyes: can cause under-reading or signal failure (especially dark colours); remove or use alternate site
  • Skin pigmentation: may bias readings (often overestimation at lower saturations reported in some devices); interpret cautiously and confirm with ABG if concerned
  • Intravascular dyes: methylene blue, indocyanine green, indigo carmine can cause transient low SpO2 readings
  • Probe/site issues: poor alignment, excessive pressure, oedema, arterial line cuff inflation on same limb, tourniquet
  • Arrhythmias: irregular pulsation can destabilise pulse detection and averaging
  • Abnormal Hb-O2 affinity (shift of dissociation curve): SpO2 reflects saturation but not PaO2; clinical implications differ
    • Left shift (hypothermia, alkalosis, low 2,3-DPG): higher saturation at a given PaO2; may mask tissue hypoxia risk
    • Right shift (acidosis, hyperthermia): lower saturation at a given PaO2

Safety, standards, and good practice

  • Pulse oximetry is a minimum standard of monitoring during anaesthesia and recovery; alarms must be enabled and audible
  • Set appropriate alarm limits and respond to trends (not just single numbers)
  • Avoid pressure injury and thermal injury (rare): rotate site in long cases; ensure correct probe size and avoid excessive tightness
Explain the principle of pulse oximetry.

Core points expected: Beer–Lambert concept, two wavelengths, pulsatile (AC) vs non-pulsatile (DC), ratio-of-ratios, calibration.

  • Uses differential absorption of red (~660 nm) and infrared (~940 nm) light by oxyHb and deoxyHb
  • Separates pulsatile arterial signal (AC) from constant absorption (DC) due to tissues/venous blood
  • Computes R = (ACred/DCred)/(ACinfra/DCinfra) and converts R to SpO2 using an empirically derived calibration curve
  • Therefore SpO2 is an estimate of arterial oxygen saturation, not PaO2 and not oxygen content
Define what SpO2 represents and distinguish functional from fractional saturation.

A common FRCA viva theme: what exactly is being measured and what is ignored by standard oximeters.

  • SpO2 is a non-invasive estimate of arterial oxygen saturation derived from pulsatile light absorption
  • Functional saturation = HbO2/(HbO2 + HHb)
  • Fractional saturation = HbO2/(HbO2 + HHb + COHb + MetHb + other dysHb)
  • Standard 2-wavelength oximeters approximate functional saturation and can be misleading when dysHb are present
A patient has suspected carbon monoxide poisoning. What will the pulse oximeter show and why? How do you confirm the diagnosis?

Classic exam question: COHb causes falsely reassuring SpO2.

  • SpO2 may appear normal or high despite significant hypoxaemia because COHb absorbs light similarly to oxyHb at 660 nm
  • Therefore pulse oximetry overestimates true oxygenation in CO poisoning
  • Confirm with arterial blood gas including co-oximetry (multi-wavelength) to measure COHb and fractional saturation
  • Management implication: give high-flow oxygen; consider hyperbaric oxygen depending on severity and local guidance
A patient receives methylene blue intra-operatively and the SpO2 suddenly falls. Explain.

Frequently tested: intravascular dyes cause artifactual desaturation.

  • Methylene blue strongly absorbs red light and alters the measured ratio-of-ratios, producing a transient low SpO2 reading
  • This may not represent true hypoxaemia; correlate with clinical signs, pleth quality, and consider ABG if uncertain
How does methaemoglobinaemia affect pulse oximetry readings?

Key number to remember: ~85%.

  • MetHb absorbs both red and infrared light such that the calculated saturation tends to drift toward ~85% regardless of true SaO2
  • Thus SpO2 becomes unreliable: it may read ~85% in both mild hypoxaemia and in normal PaO2 states
  • Confirm with co-oximetry (ABG) which can quantify MetHb
Describe the main causes of a poor plethysmograph waveform and how you would troubleshoot.

Assess signal quality systematically: patient, probe, environment, algorithm/averaging.

  • Low perfusion: shock, hypothermia, vasoconstriction/vasopressors, PVD
  • Motion artefact: shivering, tremor, patient movement, transport
  • Probe problems: malposition, wrong size, excessive pressure, nail varnish/false nails, oedema
  • Ambient light interference: theatre lights/phototherapy; shield the probe
  • Troubleshoot: reposition/replace probe, change site (ear/forehead), warm the limb, reduce motion, check pulse agreement with ECG
Why can a patient be severely hypoxic with a normal SpO2? Give examples.

The examiner is looking for oxygen content vs saturation, dysHb, and hyperventilation/PaO2 issues.

  • Severe anaemia: SpO2 can be 100% but CaO2 is low (low Hb concentration) → reduced oxygen delivery
  • Carbon monoxide poisoning: SpO2 may be falsely normal/high due to COHb
  • Poor peripheral perfusion: displayed SpO2 may be unreliable or intermittently plausible despite true hypoxaemia
  • Hyperoxia cannot be detected: SpO2 100% does not distinguish PaO2 13 kPa from 40 kPa
Explain the relationship between SpO2 and PaO2 and why this matters clinically.

Expect discussion of the oxyhaemoglobin dissociation curve and its clinical consequences.

  • SpO2 reflects saturation; PaO2 reflects dissolved oxygen tension—linked by the oxyhaemoglobin dissociation curve
  • On the flat portion (high saturations), large changes in PaO2 cause little change in SpO2 → cannot detect hyperoxia and early deterioration may be missed
  • On the steep portion (lower saturations), small PaO2 falls cause large SpO2 drops → rapid desaturation once reserve is lost
  • Curve shifts (pH, temperature, CO2, 2,3-DPG) alter PaO2 for a given SpO2, affecting interpretation
What are the main limitations of pulse oximetry in the peri-operative period?

A broad 'limitations' question: structure by patient factors, device factors, and interpretive limits.

  • Does not measure ventilation (PaCO2) or confirm airway patency—capnography is required
  • Does not measure oxygen content or delivery; can be falsely reassuring in anaemia/low cardiac output
  • Unreliable with dyshemoglobins (COHb, MetHb), dyes, motion, low perfusion, venous pulsation, ambient light
  • Accuracy reduced at low saturations (below calibration range) and may be biased by skin pigmentation depending on device
Describe transmission vs reflectance pulse oximetry and give clinical examples of each.

Often asked as an equipment viva: probe types and when to use them.

  • Transmission: LED and detector opposite; common sites finger/toe/ear lobe; generally robust when perfusion is adequate
  • Reflectance: LED and detector on same side; common sites forehead; useful in low perfusion but can be more susceptible to motion/venous artefact

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