Pulse Oximeter
OVERVIEW
- pulse oximeter
USES
- measurement of arterial oxygen-haemoglobin saturation (SaO2) — denoted SpO2 when measured by pulse oximetry
DESCRIPTION
(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
METHOD OF USE
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
- ac = pulsatile arterial blood
- 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
OTHER INFORMATION
Limitations
- 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)
COMPLICATIONS
Complications
- 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
References and Links
Introduction to ICU Series
Introduction to ICU Series Landing Page
DAY TO DAY ICU: FASTHUG, ICU Ward Round, Clinical Examination, Communication in a Crisis, Documenting the ward round in ICU, Human Factors
AIRWAY: Bag Valve Mask Ventilation, Oropharyngeal Airway, Nasopharyngeal Airway, Endotracheal Tube (ETT), Tracheostomy Tubes
BREATHING: Positive End Expiratory Pressure (PEEP), High Flow Nasal Prongs (HFNP), Intubation and Mechanical Ventilation, Mechanical Ventilation Overview, Non-invasive Ventilation (NIV)
CIRCULATION: Arrhythmias, Atrial Fibrillation, ICU after Cardiac Surgery, Pacing Modes, ECMO, Shock
CNS: Brain Death, Delirium in the ICU, Examination of the Unconscious Patient, External-ventricular Drain (EVD), Sedation in the ICU
GASTROINTESTINAL: Enteral Nutrition vs Parenteral Nutrition, Intolerance to EN, Prokinetics, Stress Ulcer Prophylaxis (SUP), Ileus
GENITOURINARY: Acute Kidney Injury (AKI), CRRT Indications
HAEMATOLOGICAL: Anaemia, Blood Products, Massive Transfusion Protocol (MTP)
INFECTIOUS DISEASE: Antimicrobial Stewardship, Antimicrobial Quick Reference, Central Line Associated Bacterial Infection (CLABSI), Handwashing in ICU, Neutropenic Sepsis, Nosocomial Infections, Sepsis Overview
SPECIAL GROUPS IN ICU: Early Management of the Critically Ill Child, Paediatric Formulas, Paediatric Vital Signs, Pregnancy and ICU, Obesity, Elderly
FLUIDS AND ELECTROLYTES: Albumin vs 0.9% Saline, Assessing Fluid Status, Electrolyte Abnormalities, Hypertonic Saline
PHARMACOLOGY: Drug Infusion Doses, Summary of Vasopressors, Prokinetics, Steroid Conversion, GI Drug Absorption in Critical Illness
PROCEDURES: Arterial line, CVC, Intercostal Catheter (ICC), Intraosseous Needle, Underwater seal drain, Naso- and Orogastric Tubes (NGT/OGT), Rapid Infusion Catheter (RIC)
INVESTIGATIONS: ABG Interpretation, Echo in ICU, CXR in ICU, Routine daily CXR, FBC, TEG/ROTEM, US in Critical Care
ICU MONITORING: NIBP vs Arterial line, Arterial Line Pressure Transduction, Cardiac Output, Central Venous Pressure (CVP), CO2 / Capnography, Pulmonary Artery Catheter (PAC / Swan-Ganz), Pulse Oximeter
LITFL
- CCC — Co-oximeter
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
- Oximetry.org
- INTENSIVEblog.com – How does pulse oximetry work? (by Dr Ken Hoffman)
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the Clinician Educator Incubator programme, and a CICM First Part Examiner.
He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.
His one great achievement is being the father of three amazing children.
On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.
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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.