Pulse Oximeter


  • pulse oximeter


  • measurement of arterial oxygen-haemoglobin saturation (SaO2) — denoted SpO2 when measured by pulse oximetry


(1) Diodes

  • within the probes produce light of the required wavelengths, usually in the red or infra-red range as absorbance by body tissue is small compared to blood
  • emitted light may alternate between wavelengths at several hundred Hz

(2) Photodetector

  • on opposite side of probe
  • detects transmitted light

(3) Signal converter

  • converted to a dc component = tissue background, venous blood & the constant part of arterial blood flow
  • converted to a ac component = pulsatile arterial blood flow
  • the dc component is disregarded/ subtracted
  • the ac component is amplified and averaged over a few seconds

(4) Display unit

  • signal is displayed ideally as a continuous trace
  • shows quality of signal and numbered value of SpO2


Pulse oximetry is based on the principle that pulsatile blood absorbance of IR or red light changes with regard to degree of oxygenation

  • 2 wavelengths of light are used: red (660nm) & infrared (930nm)
  • signal is divided into two components
    1. ac = pulsatile arterial blood
    2. dc = tissue + capillary blood + venous blood + non-pulsatile arterial blood

-> all pulse oximeters assume that only the pulsatile absorbance is arterial blood

For each wavelength, the oximeter determines the ac/dc ratio

  • the ratio (R) of these is calculated:

R = (ac absorbance/dc absorbance) red / (ac absorbance/dc absorbance )IR

  • R corresponds to SaO2
    — SaO2 100% = R 0.4
    — SaO2 85% = R 1
    — SaO2 0% = R 3.4



  • SpO2 and SaO2 are not measures of blood or tissue oxygenation (if [Hb] and cardiac output known, then oxygen dleivery (DO2) can be calculated from SaO2)
  • insensitive to directional changes in PaO2 above 80mmHg
  • relatively insensitive to perfusion due to gain
  • reading failure
  • lag time

Sources of Error

  • motion artefact
  • signal to noise ratio (shocked, hypothermia, vasoconstrictors)
  • light artefact
  • dyshemoglobinaemias (COHb indistinguishable from HbO2, Met Hb absorbance high -> R = 1.0)
  • anaemia
  • intravenous dyes
  • pigmented skin
  • nail polish
  • abnormal pulses  (venous waves and ventilation)
  • probe position
  • low saturations (progressive inaccuracy below 80%)
  • non-pulsatile flow (bypass)
  • ambient light
  • radiofrequency interference (MRI)



  • measurement error leading to inappropriate interventions
  • intervention may be required before desaturation is detected due to oximetry lag time
  • pressure injuries from the probe

Causes of HIGH Co-oximetery and LOW Pulse Oximetry Readings

  • Poor peripheral perfusion
  • Ambient light
  • Poor probe contact
  • Dyes – methylene blue
  • TR

Causes of LOW Co-oximetry and HIGH Pulse Oximetry Readings

  • COHb
  • MetHb
  • Radiofrequency interference


Journal articles

  • Fouzas S, Priftis KN, Anthracopoulos MB. Pulse oximetry in pediatric practice. Pediatrics. 2011 Oct;128(4):740-52. doi: 10.1542/peds.2011-0271. Epub 2011 Sep 19. Review. PubMed PMID: 21930554. [Free Full Text]
  • Jubran A. Pulse oximetry. Crit Care. 1999;3(2):R11-R17. PubMed PMID: 11094477; PubMed Central PMCID: PMC137227.
  • Jubran A. Pulse oximetry. Critical care (London, England). 19:272. 2015. [pubmed] [free full text]
  • Jubran A. Pulse oximetry. In: Tobin MJ, editor. Principles and Practice of Intensive Care Monitoring. New York: McGraw-Hill, Inc; 1998. pp. 261–87.
  • Mannheimer PD. The light-tissue interaction of pulse oximetry. Anesth Analg. 2007 Dec;105(6 Suppl):S10-7. Review. PubMed PMID: 18048891. [Free Full Text]
  • Mendelson Y. Pulse oximetry: theory and applications for noninvasive monitoring. Clinical chemistry. 38(9):1601-7. 1992. [pubmed] [free full text]
  • Monnet X, Lamia B, Teboul JL. Pulse oximeter as a sensor of fluid responsiveness: do we have our finger on the best solution? Crit Care. 2005 Oct 5;9(5):429-30. Epub 2005 Sep 28. PubMed PMID: 16277729; PubMed Central PMCID: PMC1297637.
  • Petterson MT, Begnoche VL, Graybeal JM. The effect of motion on pulse oximetry and its clinical significance. Anesth Analg. 2007 Dec;105(6 Suppl):S78-84. Review. PubMed PMID: 18048903. [Free Full Text]
  • Pretto JJ, Roebuck T, Beckert L, Hamilton G. Clinical use of pulse oximetry: official guidelines from the Thoracic Society of Australia and New Zealand. Respirology (Carlton, Vic.). 19(1):38-46. 2014. [pubmed] [free full text]
  • Ralston AC, Webb RK, Runciman WB. Potential errors in pulse oximetry. I. Pulse oximeter evaluation. Anaesthesia. 46(3):202-6. 1991. [pubmed] [Free full text]
  • Ralston AC, Webb RK, Runciman WB. Potential errors in pulse oximetry. III: Effects of interferences, dyes, dyshaemoglobins and other pigments. Anaesthesia. 46(4):291-5. 1991. [pubmed] [Free full text]
  • Webb RK, Ralston AC, Runciman WB. Potential errors in pulse oximetry. II. Effects of changes in saturation and signal quality. Anaesthesia. 46(3):207-12. 1991. [pubmed] [Free full text]

FOAM and Web resources

CCC 700 6

Critical Care


Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

One comment

  1. Any ideas on how Raynaud’s phenomenon might affect pulse oximetry readings? Best I could find is a 1991 article (https://pubmed.ncbi.nlm.nih.gov/2248827/) about how cold exposure leads to a decrease in the plethysmograph amplitudes, but it didn’t mention the actual oxygen saturation readings.

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