Pulse Oximetry

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

Background

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.

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Indications

Indications for pulse oximetry include the following:

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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]

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