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Pulse Oximetry

  • Author: Bruce M Lo, MD, CPE, RDMS, FACEP, FAAEM, FACHE; Chief Editor: Zab Mosenifar, MD, FACP, FCCP  more...
Updated: Sep 03, 2015


Pulse oximetry is a noninvasive method of measuring the oxygenation level in the blood.

Modern pulse oximeters measure the amount of red and infrared light in an area of pulsatile blood flow. Because red light is primarily absorbed by deoxygenated blood and infrared light is primarily absorbed by oxygenated blood, the ratio of absorption can be measured. Because the amount of light absorbed varies with each pulse wave, the difference of measurement between two points in the pulse wave occurs in the arterial blood flow, with more than several hundred measurements per second. This is compared against baseline values, giving both the pulse oximetry oxygen saturation (SpO2) and the pulse rate.



Indications for pulse oximetry include the following:


Technical Considerations

Pulse oximetry probes consist of either transmission probes or reflectance probes. With transmission probes, the light emitter and sensor are placed opposite each other on pulsatile tissue such as a digit or ear. With reflectance probes, the light emitter and sensor are placed side by side on a flat body surface.

Anything that interferes with the transmission or absorbance of light can cause errors in SpO2 readings. This can be seen with a poor-quality plethysmographic tracing, suggesting possible errors in SpO2 readings.

Erroneous readings

Several situations can cause an erroneous SpO2 reading, especially with the use of transmission probes. Darker skin pigments, certain nail polishes, dyshemoglobinemias (eg, carboxyhemoglobin, methemoglobin), intravenous dyes (eg, methylene blue), hypoperfusion, and hypoxia (especially with SpO2 readings< 80%) can cause errors. Motion and exposure to ambient or excessive light has also been shown to cause erroneous SpO2 readings.[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

Delay in change

SpO2 readings in distal extremities may be delayed. Compared with measurements from the earlobe, finger measurements were delayed by around 30 seconds, whereas toe measurements were delayed by up to 90 seconds.[14, 15, 16] Thus, caution must be used when interpreting SpO2 during rapid changes in oxygenation levels.

Forehead probes

Reflectance probes must be used on the forehead for reliable readings. To prevent venous pulsation from causing erroneous readings, a headband with slight pressure should be placed. Venous pooling can be also caused by placing patients in the Trendelenburg position, resulting in inaccurate SpO2 readings. The probe should be placed over a pulsatile bed of tissue, and not over a major vessel (artery or vein) that can confound the sensor and give an inaccurate SpO2 reading.[17]

SpO2 readings from forehead probes are more accurate and can detect hypoxia sooner than SpO2 obtained from digits, including in patients with hypothermia or hypotension.[14, 18, 19, 20]

Contributor Information and Disclosures

Bruce M Lo, MD, CPE, RDMS, FACEP, FAAEM, FACHE Medical Director, Department of Emergency Medicine, Sentara Norfolk General Hospital; Associate Professor, Assistant Program Director, Core Academic Faculty, Department of Emergency Medicine, Eastern Virginia Medical School

Bruce M Lo, MD, CPE, RDMS, FACEP, FAAEM, FACHE is a member of the following medical societies: American Academy of Emergency Medicine, American Association for Physician Leadership, American College of Emergency Physicians, American College of Healthcare Executives, American Institute of Ultrasound in Medicine, Emergency Nurses Association, Medical Society of Virginia, Norfolk Academy of Medicine, Society for Academic Emergency Medicine

Disclosure: Nothing to disclose.

Chief Editor

Zab Mosenifar, MD, FACP, FCCP Geri and Richard Brawerman Chair in Pulmonary and Critical Care Medicine, Professor and Executive Vice Chairman, Department of Medicine, Medical Director, Women's Guild Lung Institute, Cedars Sinai Medical Center, University of California, Los Angeles, David Geffen School of Medicine

Zab Mosenifar, MD, FACP, FCCP is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, American Federation for Medical Research, American Thoracic Society

Disclosure: Nothing to disclose.

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Portable pulse oximeter with finger probe.
Pulse oximeter probe on ear.
Pulse oximeter probe on forehead.
Pulse oximetry with normal plethysmograph waveform.
Pulse oximetry with artifact in the plethysmograph waveform.
Pulse oximetry with no plethysmograph waveform (no signal).
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