Pulse oximetry

Pulse oximetry is a noninvasive method for monitoring blood oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy (within 4% accuracy in 95% of cases) of the more accurate (and invasive) reading of arterial oxygen saturation (SaO2) from arterial blood gas analysis.

A standard pulse oximeter passes two wavelengths of light through tissue to a photodetector. Taking advantage of the pulsate flow of arterial blood, it measures the change in absorbance over the course of a cardiac cycle, allowing it to determine the absorbance due to arterial blood alone, excluding unchanging absorbance due to venous blood, skin, bone, muscle, fat, and, in many cases, nail polish. The two wavelengths measure the quantities of bound (oxygenated) and unbound (non-oxygenated) hemoglobin, and from their ratio, the percentage of bound hemoglobin is computed.

The most common approach is transmissive pulse oximetry. In this approach, one side of a thin part of the patient's body, usually a fingertip or earlobe, is illuminated, and the photodetector is on the other side. Fingertips and earlobes have disproportionately high blood flow relative to their size, in order to keep warm, but this will be lacking in hypothermic patients.

Medical uses

thumb|A pulse oximeter probe applied to a person's finger

A pulse oximeter is a medical device that indirectly monitors the oxygen saturation of a patient's blood (as opposed to measuring oxygen saturation directly through a blood sample) and changes in blood volume in the skin, producing a photoplethysmogram that may be further processed into other measurements.

Because of their simplicity of use and the ability to provide continuous and immediate oxygen saturation values, pulse oximeters are of critical importance in emergency medicine and are also very useful for patients with respiratory or cardiac problems, especially COPD, or for diagnosis of some sleep disorders such as apnea and hypopnea. For patients with obstructive sleep apnea, pulse oximetry readings will be in the 70–90% range for much of the time spent attempting to sleep.

Portable battery-operated pulse oximeters are useful for pilots operating in non-pressurized aircraft above or in the U.S. where supplemental oxygen is required. Portable pulse oximeters are also useful for mountain climbers and athletes whose oxygen levels may decrease at high altitudes or with exercise. Some portable pulse oximeters employ software that charts a patient's blood oxygen and pulse, serving as a reminder to check blood oxygen levels.

Connectivity advancements have made it possible for patients to have their blood oxygen saturation continuously monitored without a cabled connection to a hospital monitor, without sacrificing the flow of patient data back to bedside monitors and centralized patient surveillance systems.

For patients with COVID-19, pulse oximetry helps with early detection of silent hypoxia, in which the patients still look and feel comfortable, but their SpO2 is dangerously low. This happens to patients either in the hospital or at home. Low SpO2 may indicate severe COVID-19-related pneumonia, requiring a ventilator.

Safety

Continuous monitoring with pulse oximetry is generally considered safe for most patients for up to 8 hours. However, prolonged use in certain types of patients can cause burns due to the heat emitted by the infrared LED, which reaches up to 43°C. Additionally, pulse oximeters occasionally develop electrical faults which causes them to heat up above this temperature. Patients at greater risk include those with delicate or fragile skin, such as infants, particularly premature infants, and the elderly. Additional risks for injury include lack of pain response where the probe is placed, such as having an insensate limb, or being unconscious or under anesthesia, or having communication difficulties. Patients who are at high risk for injury should have the site of their probe moved frequently, i.e. every hour, whereas patients who are at lower risk should have theirs moved every 2-4 hours.

Limitations

Fundamental limitations

Pulse oximetry solely measures hemoglobin saturation, not ventilation and is not a complete measure of respiratory sufficiency. It is not a substitute for blood gases checked in a laboratory, because it gives no indication of base deficit, carbon dioxide levels, blood pH, or bicarbonate (HCO3−) concentration. The metabolism of oxygen can readily be measured by monitoring expired CO2, but saturation figures give no information about blood oxygen content. Most of the oxygen in the blood is carried by hemoglobin; in severe anemia, the blood contains less hemoglobin, which despite being saturated cannot carry as much oxygen.

Pulse oximetry also is not a complete measure of circulatory oxygen sufficiency. If there is insufficient bloodflow or insufficient hemoglobin in the blood (anemia), tissues can suffer hypoxia despite high arterial oxygen saturation.

Since pulse oximetry measures only the percentage of bound hemoglobin, a falsely high or falsely low reading will occur when hemoglobin binds to something other than oxygen:

Hemoglobin has a higher affinity to carbon monoxide than it does to oxygen. Therefore, in cases of carbon monoxide poisoning, most hemoglobin might be bound not to oxygen but to carbon monoxide. A pulse oximeter would correctly report most hemoglobin to be bound, but nevertheless the patient would be in a state of hypoxemia and subsequently hypoxia (low cellular oxygen level).
Cyanide poisoning gives a high reading because it reduces oxygen extraction from arterial blood. In this case, the reading is not false, as arterial blood oxygen is indeed high early in cyanide poisoning: the patient is not hypoxemic, but is hypoxic.
Methemoglobinemia characteristically causes pulse oximetry readings in the mid-80s.
COPD [especially chronic bronchitis] may cause false readings.

A noninvasive method that allows continuous measurement of the dyshemoglobins is the pulse CO-oximeter, which was built in 2005 by Masimo. By using additional wavelengths, it provides clinicians a way to measure the dyshemoglobins, carboxyhemoglobin, and methemoglobin along with total hemoglobin.

Conditions affecting accuracy

thumb|Consumer pulse oxymeter

Because pulse oximeter devices are calibrated for healthy subjects, their accuracy is poor for critically ill patients and preterm newborns. Obesity, hypotension (low blood pressure), and some hemoglobin variants can reduce the accuracy of the results. One of the earliest studies on this topic occurred in 1976, which reported reading errors in dark-skinned patients that reflected lower blood oxygen saturation values. Further studies indicate that while accuracy with dark skin is good at higher, healthy saturation levels, some devices overestimate the saturation at lower levels, which may lead to hypoxia not being detected. A study that reviewed thousands of cases of occult hypoxemia, where patients were found to have oxygen saturation below 88% per arterial blood gas measurement despite pulse oximeter readings indicating 92% to 96% oxygen saturation, found that black patients were three times as likely as white patients to have their low oxygen saturation missed by pulse oximeters. Another research study investigated patients in the hospital with COVID-19 and found that occult hypoxemia occurred in 28.5% of black patients compared to only 17.2% of white patients. There has been research to indicate that black COVID-19 patients were 29% less likely to receive supplemental oxygen in a timely manner and three times more likely to have hypoxemia. A further study, which used a MIMIC-IV critical care dataset of both pulse oximeter readings and oxygen saturation levels detected in blood samples, demonstrated that black, Hispanic, and Asian patients had higher SpO2 readings than white patients for a given blood oxygen saturation level measured in blood samples. As a result, black, Hispanic, and Asian patients also received lower rates of supplemental oxygen than white patients. This inaccuracy can lead to potentially missing people who need treatment, as pulse oximetry is used for the screening of sleep apnea and other types of sleep-disordered breathing, This bias is a significant concern, as a 2% decrease is important for respiratory rehabilitation, studies of sleep apnea, and athletes performing physical efforts; it can lead to severe complications for the patient, requiring an external oxygen supply or even hospitalization.

Another concern regarding pulse oximetry bias is that insurance companies and hospital systems increasingly use these numbers to inform their decisions. Pulse oximetry measurements are used to identify candidates for reimbursement. Similarly, pulse oximetry data is being incorporated into algorithms for clinicians. Early Warning Scores, which provide a record for analyzing a patient's clinical status and alerting clinicians if needed, incorporate algorithms with pulse oximetry information and can result in misinformed patient records. Some smart watches with activity tracking incorporate an oximeter function. An article on such devices, in the context of diagnosing COVID-19 infection, quoted João Paulo Cunha of the University of Porto, Portugal: "these sensors are not precise, that's the main limitation ... the ones that you wear are only for the consumer level, not for the clinical level". Pulse oximeters used for diagnosis of conditions such as COVID-19 should be Class IIB medical grade oximeters. Class IIB oximeters can be used on patients of all skin colors, low pigmentation and in the presence of motion. When a pulse oximeter is shared between two patients, to prevent cross-infection it should be cleaned with alcohol wipes after each use or a disposable probe or finger cover should be used.

According to a report by iData Research, the US pulse oximetry monitoring market for equipment and sensors was over $700 million in 2011.

Mobile apps

Mobile app pulse oximeters use the flashlight and the camera of the phone, instead of infrared light used in conventional pulse oximeters. However, apps do not generate as accurate readings because the camera cannot measure the light reflection at two wavelengths, so the oxygen saturation readings that are obtained through an app on a smartphone are inconsistent for clinical use. At least one study has suggested these are not reliable relative to clinical pulse oximeters.

Mechanism

thumb|250px|Absorption spectra of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) for red and infrared wavelengths

thumb|250px|Simplified principle of operation of a transmissive LED pulse oximeter

thumb|upright|The inner side of a pulse oximeter

A blood-oxygen monitor displays the percentage of blood that is loaded with oxygen. More specifically, it uses light spectrometry to measure what percentage of hemoglobin, the protein in blood that carries oxygen, is loaded. Acceptable normal SpO2 ranges for patients without pulmonary pathology are from 95 to 99 percent. For a person breathing room air at or near sea level, an estimate of arterial SpO2 can be made from the blood-oxygen monitor "saturation of peripheral oxygen" (SpO2) reading.

Mode of operation

A typical pulse oximeter uses an electronic processor and a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. One LED is red, with wavelength of 660 nm, and the other is infrared with a wavelength of 940 nm. Absorption of light at these wavelengths differs significantly between blood loaded with oxygen and blood lacking oxygen. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated hemoglobin allows more infrared light to pass through and absorbs more red light. The LEDs sequence through their cycle of one on, then the other, then both off about thirty times per second which allows the photodiode to respond to the red and infrared light separately and also adjust for the ambient light baseline.

The amount of light that is transmitted (in other words, that is not absorbed) is measured, and separate normalized signals are produced for each wavelength. These signals fluctuate in time because the amount of arterial blood that is present increases (literally pulses) with each heartbeat. By subtracting the minimum transmitted light from the transmitted light in each wavelength, the effects of other tissues are corrected for, generating a continuous signal for pulsatile arterial blood. The ratio of the red light measurement to the infrared light measurement is then calculated by the processor (which represents the ratio of oxygenated hemoglobin to deoxygenated hemoglobin), and this ratio is then converted to SpO2 by the processor via a lookup table and a numeric ratio between the pulsatile and baseline absorbance ("perfusion index") can be used to evaluate perfusion.

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