Pulse oximetry
Pulse oximetry | |
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Purpose | Monitoring a person's oxygen saturation |
Pulse oximetry is a
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.[2] 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
Reflectance pulse oximetry is a less common alternative, placing the photodetector on the same surface as the illumination. This method does not require a thin section of the person's body and therefore may be used almost anywhere on the body, such as the forehead, chest, or feet, but it still has some limitations. Vasodilation and pooling of venous blood in the head due to compromised venous return to the heart can cause a combination of arterial and venous pulsations in the forehead region and lead to spurious SpO2 results. Such conditions occur while undergoing
Medical uses
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.[4] The pulse oximeter may be incorporated into a multiparameter patient monitor. Most monitors also display the pulse rate. Portable, battery-operated pulse oximeters are also available for transport or home blood-oxygen monitoring.[5]
Advantages
Pulse oximetry is particularly convenient for
Because of their simplicity of use and the ability to provide continuous and immediate oxygen saturation values, pulse oximeters are of critical importance in
Portable battery-operated pulse oximeters are useful for pilots operating in non-pressurized aircraft above 10,000 feet (3,000 m) or 12,500 feet (3,800 m) in the U.S.
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.[11]
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.[5] This happens to patients either in the hospital or at home. Low SpO2 may indicate severe COVID-19-related pneumonia, requiring a ventilator.[12]
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 be 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.[13]
Limitations
Fundamental limitations
Pulse oximetry solely measures hemoglobin saturation, not
Pulse oximetry also is not a complete measure of circulatory oxygen sufficiency. If there is insufficient
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 hypoxia(low cellular oxygen level).
- hypoxic.
- Methemoglobinemia characteristically causes pulse oximetry readings in the mid-80s.
- ]
A noninvasive method that allows continuous measurement of the dyshemoglobins is the pulse CO-oximeter, which was built in 2005 by Masimo.[15] By using additional wavelengths,[16] it provides clinicians a way to measure the dyshemoglobins, carboxyhemoglobin, and methemoglobin along with total hemoglobin.[17]
Conditions affecting accuracy
Because pulse oximeter devices are calibrated for healthy subjects, their accuracy is poor for critically ill patients and preterm newborns.
Equipment
Consumer pulse oximeters
In addition to pulse oximeters for professional use, many inexpensive "consumer" models are available. Opinions vary about the reliability of consumer oximeters; a typical comment is "The research data on home monitors has been mixed, but they tend to be accurate within a few percentage points".
According to a report by iData Research, the US pulse oximetry monitoring market for equipment and sensors was over $700 million in 2011.[37]
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.[38]
Mechanism
A blood-oxygen monitor displays the percentage of blood that is loaded with oxygen. More specifically, it measures what percentage of
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.[39]
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.[40] 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[40] based on the Beer–Lambert law.[39] The signal separation also serves other purposes: a plethysmograph waveform ("pleth wave") representing the pulsatile signal is usually displayed for a visual indication of the pulses as well as signal quality,[4] and a numeric ratio between the pulsatile and baseline absorbance ("perfusion index") can be used to evaluate perfusion.[41]
where HbO2 is oxygenated hemoglobin (oxyhemoglobin) and Hb is deoxygenated hemoglobin.
Derived measurements
Due to changes in blood volumes in the skin, a plethysmographic variation can be seen in the light signal received (transmittance) by the sensor on an oximeter. The variation can be described as a periodic function, which in turn can be split into a DC component (the peak value)[a] and an AC component (peak minus trough).[42] The ratio of the AC component to the DC component, expressed as a percentage, is known as the (peripheral) perfusion index (Pi) for a pulse, and typically has a range of 0.02% to 20%.[43] An earlier measurement called the pulse oximetry plethysmographic (POP) only measures the "AC" component, and is derived manually from monitor pixels.[41][44]
Pleth variability index (PVI) is a measure of the variability of the perfusion index, which occurs during breathing cycles. Mathematically it is calculated as (Pimax − Pimin)/Pimax × 100%, where the maximum and minimum Pi values are from one or many breathing cycles.[42] It has been shown to be a useful, noninvasive indicator of continuous fluid responsiveness for patients undergoing fluid management.[41] Pulse oximetry plethysmographic waveform amplitude (ΔPOP) is an analogous earlier technique for use on the manually-derived POP, calculated as (POPmax − POPmin)/(POPmax + POPmin)×2.[44]
History
In 1935, German physician Karl Matthes (1905–1962) developed the first two-wavelength ear O2 saturation meter with red and green filters (later red and infrared filters). It was the first device to measure O2 saturation.[45]
The original oximeter was made by
The first pulse oximetry was developed in 1972 by Japanese bioengineers Takuo Aoyagi and Michio Kishi at Japanese medical electronic equipment manufacturer Nihon Kohden, using the ratio of red to infrared light absorption of pulsating components at the measuring site. Nihon Kohden manufactured the first pulse oximeter, Ear Oximeter OLV-5100. Surgeon Susumu Nakajima and his associates first tested the device in patients, reporting it in 1975.[49] However, Nihon Kohden suspended the development of pulse oximetry and did not apply for a basic patent of pulse oximetry except in Japan, which facilitated further development and utilization of pulse oximetry later in U.S. In 1977, Minolta commercialized the first finger pulse oximeter OXIMET MET-1471. In the U.S., the first pulse oximetry was commercialized by Biox in 1980.[49][50][51]
By 1987, the standard of care for the administration of a general anesthetic in the U.S. included pulse oximetry. From the operating room, the use of pulse oximetry rapidly spread throughout the hospital, first to
In 1995, Masimo introduced Signal Extraction Technology (SET) that could measure accurately during patient motion and low perfusion by separating the arterial signal from the venous and other signals. Since then, pulse oximetry manufacturers have developed new algorithms to reduce some false alarms during motion,[54] such as extending averaging times or freezing values on the screen, but they do not claim to measure changing conditions during motion and low perfusion. So there are still important differences in performance of pulse oximeters during challenging conditions.[19] Also in 1995, Masimo introduced perfusion index, quantifying the amplitude of the peripheral plethysmograph waveform. Perfusion index has been shown to help clinicians predict illness severity and early adverse respiratory outcomes in neonates,[55][56][57] predict low superior vena cava flow in very low birth weight infants,[58] provide an early indicator of sympathectomy after epidural anesthesia,[59] and improve detection of critical congenital heart disease in newborns.[60]
Published papers have compared signal extraction technology to other pulse oximetry technologies and have demonstrated consistently favorable results for signal extraction technology.[18][19][61] Signal extraction technology pulse oximetry performance has also been shown to translate into helping clinicians improve patient outcomes. In one study, retinopathy of prematurity (eye damage) was reduced by 58% in very low birth weight neonates at a center using signal extraction technology, while there was no decrease in retinopathy of prematurity at another center with the same clinicians using the same protocol but with non-signal extraction technology.[62] Other studies have shown that signal extraction technology pulse oximetry results in fewer arterial blood gas measurements, faster oxygen weaning time, lower sensor utilization, and lower length of stay.[63] The measure-through motion and low perfusion capabilities it has also allow it to be used in previously unmonitored areas such as the general floor, where false alarms have plagued conventional pulse oximetry. As evidence of this, a landmark study was published in 2010 showing that clinicians at Dartmouth-Hitchcock Medical Center using signal extraction technology pulse oximetry on the general floor were able to decrease rapid response team activations, ICU transfers, and ICU days.[64] In 2020, a follow-up retrospective study at the same institution showed that over ten years of using pulse oximetry with signal extraction technology, coupled with a patient surveillance system, there were zero patient deaths and no patients were harmed by opioid-induced respiratory depression while continuous monitoring was in use.[65]
In 2007, Masimo introduced the first measurement of the
In 2011, an expert workgroup recommended newborn screening with pulse oximetry to increase the detection of
High-resolution pulse oximetry (HRPO) has been developed for in-home sleep apnea screening and testing in patients for whom it is impractical to perform
See also
- Arterial blood gas– A test of blood taken from an artery that measures the amounts of certain dissolved gases
- Capnography – Monitoring of the concentration of carbon dioxide in respiratory gases
- Integrated pulmonary index – single value that describes the patient’s respiratory status
- Respiratory monitoring– Method to mechanically assist or replace spontaneous breathing
- Medical equipment– Device to be used for medical purposes
- Mechanical ventilation – Method to mechanically assist or replace spontaneous breathing
- Oxygen sensor – Device for measuring oxygen concentration
- Oxygen saturation – Relative measure of the amount of oxygen that is dissolved or carried in a given medium
- Photoplethysmogram – Chart of tissue blood volume changes. Also, the measuring of carbon dioxide (CO2) in the respiratory gases
- Sleep apnea – Disorder involving pauses in breathing during sleep
- CO-oximeter
Notes
- ^ This definition used by Masimo varies from the mean value used in signal processing; it is meant to measure the pulsatile arterial blood absorbance over the baseline absorbance.
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