Background
Pulse oximetry is a rapid, noninvasive method of estimating oxygenation and is continuous, allowing detection of sudden changes in a patient’s clinical status. 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 2 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. [1, 2, 3, 4, 5, 6, 7, 8, 9]
Pulse oximeters extract and display SpO2 and heart rate from the photoplethysmographic (PPG) waveform every 3 to 6 seconds, and some display the PPG waveform. One side of the pulse oximeter probe contains 2 light-emitting diodes that transmit 2 wavelengths of light, and the other side contains a photodetector. Red light at 660 nm and near-infrared (NIR) light at 940 nm is transmitted through tissue (skin, arteries, capillaries, veins, bone, and fat), and the light that is not absorbed is detected by the photodetector on the opposite side. [7]
Pulse oximetry is a standard monitoring parameter by the American Society of Anesthesiologists for all anesthesia cases. It is also used in hospitalized patients who are receiving medications that may impair respiration, particularly opioids. Pulse oximetry is used to rapidly diagnose hypoxia, as well as to titrate treatment for hypoxia. During the COVID-19 pandemic, pulse oximetry has been a useful device in helping establish the need for supplemental oxygen during triage. [4, 6, 9, 10]
Ongoing research is being undertaken to allow pulse oximeters to also measure methemoglobin and carboxyhemoglobin levels, total hemoglobin, and oxygen levels above 100% saturation. [4, 5, 8]
The equipment used for pulse oximetry includes the following:
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Monitoring unit
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Probe sensor
Currently, the 2 basic types of pulse oximeter probes are transmission probes and reflectance probes.
Interfering factors include nail polish and artificial fingernails, as well as intravenous dyes such as methylene blue and indocyanine green, which can color the serum in the blood and interfere with the light absorption spectrum and affect readings. Dyshemoglobinemias change the color and absorption spectrum of blood and therefore cause false readings. In that case, a co-oximeter should be used for confirmation. Light pollution such as ambient light and light emitted from other probes can cause interference and inaccurate readings. The site or the probe should therefore be convered. [4, 11, 12, 6]
Transmission probes
With transmission probes, the light emitter and sensor are placed opposite each other on pulsatile tissue such as a digit or ear. The lights used to measure tissue oxygenation are typically placed across from a detector surrounding approximately 5-10 mm of tissue that contains pulsatile blood flow, such as a fingertip or ear lobe.
Reflectance probes
With reflectance probes, the light emitter and sensor are placed side by side on a flat body surface. The detector lies adjacent to the light source on a flat surface such as the forehead. This information can be used noninvasively to help evaluate the hemodynamic status of a patient and to detect hypoxemia in various clinical settings.
Indications
Indications for pulse oximetry include the following:
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Endotracheal intubation
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Cardiac arrest
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Respiratory complaints
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Acute respiratory distress syndrome (ARDS) [13]
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Sleep disorders/sleep apnea [14]
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.
Excessive debris should be removed from the area where the probe will be attached. The patient should be out of excessive light.
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. [17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]
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. [30, 31, 32] 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, not over a major vessel (artery or vein) that can confound the sensor and give an inaccurate SpO2 reading. [33]
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. [30, 34, 35, 36]
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Portable pulse oximeter with finger probe.
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Pulse oximeter probe on ear.
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Pulse oximetry with normal plethysmograph waveform.
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Pulse oximetry with artifact in the plethysmograph waveform.
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Pulse oximetry with no plethysmograph waveform (no signal).
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Two components for forehead blood oxygen saturation (SpO2) sensor (Nellcor™ SpO2 Forehead Sensor).
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The sensor is placed on the forehead of the patient.
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The headband is placed to secure the sensor probe.
