How do pulse oximeters work?

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author: Zack Ding, William Yin

2023-04-03

Pulse oximetry is a noninvasive method for measuring a person's Blood Oxygen Saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2%

accuracy between 70% and 100%, the more high accurate (and invasive) reading of Arterial oxygen saturation (SaO2) by arterial blood gas analysis. But the two

method are correlated well enough that the safe, convenient, noninvasive, inexpensive pulse oximetry method is valuable for measuring oxygen saturation in

clinical use.

The most common technology approach is transmissive pulse oximetry. In this approach, a sensor device is placed on a thin part of the patient's body, usually

a fingertip or earlobe, or an infant's foot. Fingertips and earlobes have higher blood flow rates than other tissues, which facilitates heat transfer. The device

passes two wavelengths ( Red Light and infrared Light )of light through the body part to a photodetector. for example, UTECH company ( UTECH CO., LTD ,

//www.chinautech.com/5264800193915309.html Pulse Oximeter, Thermometer from China Manufacturers - UTECH CO., LTD. ( UTMI ) (chinautech.com)) use

660nm and 904nm wavelength. It measures the changing absorbance at each of the wavelengths, allowing it to determine the absorbances due to the pulsing

arterial blood alone, excluding venous blood, skin, bone, muscle, fat, and, in most cases, nail polish.

Reflectance pulse oximetry is a less common alternative to transmissive pulse oximetry. This method does not require a thin section of the person's body and

is therefore well suited to a universal application such as the feet, forehead, and chest, but it also 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 anaesthesia with endotracheal intubation and mechanical ventilation or in patients in the

Trendelenburg position.

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.

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.

Here's how it works:

Pulse oximetry is particularly convenient for noninvasive continuous measurement of blood oxygen saturation. In contrast, blood gas levels must otherwise be

determined in a laboratory on a drawn blood sample. Pulse oximetry is useful in any setting where a patient's is unstable, including intensive care, operating,

recovery, emergency and hospital ward settings, pilots in unpressurized aircraft, for assessment of any patient's oxygenation, and determining the effectiveness

of or need for supplemental oxygen. Although a pulse oximeter is used to monitor oxygenation, it cannot determine the metabolism of oxygen, or the amount

of oxygen being used by a patient.

For this purpose, it is necessary to also measure carbon dioxide (CO2) levels. It is possible that it can also be used to detect abnormalities in ventilation. However,

the use of a pulse oximeter to detect hypoventilation is impaired with the use of supplemental oxygen, as it is only when patients breathe room air that

abnormalities in respiratory function can be detected reliably with its use. Therefore, the routine administration of supplemental oxygen may be unwarranted

if the patient is able to maintain adequate oxygenation in room air, since it can result in hypoventilation going undetected.

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. we use UTECH company's

UT100, UPM60, UPM50 Oximeter ( Pulse Oximeter, Thermometer from China Manufacturers - UTECH CO., LTD. ( UTMI ) (chinautech.com) )to test, it can use in patient's oximetry

monitoring, its accuracy is better than 2%.

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. 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.

Pulse oximetry is based on the principle that O2Hb absorbs more near-IR light than HHb, and HHb absorbs more red light than O2Hb. Under optimal conditions,

pulse oximeters do not calculate SpO2 of venous blood (and other stationary tissues) but rather only arterial SpO2 by determining changes in absorbance of the

light transmitted over time; i.e., arterial blood volume changes with the cardiac cycle whereas the volumes of light absorbers in non-arterial tissues are relatively

constant. Since pulse oximeters are calibrated for SpO2 between 70 and 100%, displayed values below 70% should only be considered qualitatively accurate and

not quantitatively. Inaccurate SpO2 readings can occur with conditions that decrease arterial blood perfusion.

The SpO2 can overestimate FO2Hb content in blood that contains increased amount of CO. In sickle cell anemia-associated vasoocclusive crises, SpO2 can

overestimate the FO2Hb (due likely to the presence of increased endogenous production of CO) but could underestimate the SaO2 (due to vasoocclusion-induced

under perfusion). Falsely low SpO2 can occur with venous pulsations, excessive movement, pigmented dyes, and certain dyshemoglobins. Although severe anemia

per se does not cause a falsely low SpO2, its presence can cause the SpO2 to underestimate the SaO2 in someone with true hypoxemia. SpO2 can either overestimate

or underestimate the SaO2 with methemoglobinemia, sulfhemoglobinemia, and poor probe positioning. Sulfhemoglobinemia can mimic methemoglobinemia clinically

and by SpO2 and SaO2 measurements. Finally, it is important to emphasize that even in the most optimal patient-pulse oximeter interface and settings, a completely

normal SpO2 does not rule out gas exchange problems in the lungs or the adequacy of ventilation since the alveolar-arterial oxygen difference and PaCO2 are not

measured by pulse oximetry.

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