Pulse Oximetry Basic Principles and Interpretation return to: Pulse Oximetry common misconceptions regarding use Introduction Pulse oximetry is considered by some as the '5th' vital sign. The pulse oximeter gives a rapid estimation of the peripheral oxygen saturation, providing valuable clinical data in a very efficient, non-invasive and convenient manner. When one O 2 molecule binds to one of hemoglobin's four binding sites, the affinity to oxygen of the three remaining available binding sites increases; i.
This property results in a sigmoidal oxygen dissociation curve allowing for more rapid loading of oxygen molecules in oxygen rich environments i. Relaxed R : oxygenated form with high affinity for O 2 , therefore oxygen loading is favored. T and R configurations lead to different electromagnetic absorption and therefore different emission of light.
The oximeter utilizes 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 nm, and the other is infrared with a wavelength of 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.
All premarket submissions for prescription use oximeters are reviewed by the FDA to ensure that clinical study samples are demographically representative of the U. Although these clinical studies are not statistically powered to detect differences in accuracy between demographic groups, the FDA has continued to review the effects of skin pigmentation on the accuracy of these devices, including data from controlled laboratory studies and data from real world settings.
The FDA is committed to the continued evaluation of the safety, effectiveness, and availability of medical devices, especially devices in high demand during the COVID pandemic. The FDA is evaluating published literature pertaining to factors that may affect pulse oximeter accuracy and performance, with a focus on literature that evaluates whether products may be less accurate in individuals with darker skin pigmentation.
The FDA has been working on additional analysis of premarket data, as well as working with outside stakeholders, including manufacturers and testing laboratories, to analyze additional postmarket data to better understand how different factors including skin pigmentation may affect pulse oximeter accuracy.
Based on these findings, the FDA may reassess the content of the pulse oximetry guidance document. Health care personnel employed by facilities that are subject to the FDA's user facility reporting requirements should follow the reporting procedures established by their facilities. GOV or call or Be aware that multiple factors can affect the accuracy of a pulse oximeter reading, such as poor circulation, skin pigmentation, skin thickness, skin temperature, current tobacco use, and use of fingernail polish.
When placing the oximeter on your finger, make sure your hand is warm, relaxed, and held below the level of the heart. Remove any fingernail polish on that finger. Sit still and do not move the part of your body where the pulse oximeter is located. Wait a few seconds until the reading stops changing and displays one steady number.
Write down your oxygen levels with the date and time of the reading so you can easily track changes and report these to your health care provider. This poses a problem , because the pulse oximeter should only analyse arterial blood while ignoring the absorbance of light by surrounding tissues. For an example of how tissues can interfere, take the two situations shown below. One is a thin finger and the other is a fat finger. The tissues in the thin finger absorbs only a little extra light, while the fatter finger shown on the right absorbs much more light.
Fortunately, there is a clever solution to the problem. The pulse oximeter wants to only analyse arterial blood, ignoring the other tissues around the blood. Luckily, arterial blood is the only thing pulsating in the finger.
Everything else is non pulsating. As shown below, the computer subtracts the non changing part of the absorbance signal from the total signal. In this way, the pulse oximeter is able to calculate the oxygen saturation in arterial blood while ignoring the effects of the surrounding tissues. The diagrams used so far have exaggerated the size of the pulsatile part to make it easy for you to see and understand.
However, in reality, the pulsatile signal is very small. The red shows the changing absorbance due to pulsatile arterial blood. See how small this pulsatile signal is. Off all the light that passes through the finger, it is only the small pulsatile part that the pulse oximeter analyses.
Because it is such a small amount of the total light, the pulse oximeter is very susceptible to errors if for an example, the probe is not placed properly or if the patient moves the probe.
Pulse oximeters often show the pulsatile change in absorbance in a graphical form. The pleth is an extremely important graph to see. It tells you how good the pulsatile signal is. If the quality of the pulsatile signal is poor, then the calculation of the oxygen saturation may be wrong.
The pulse oximeter uses very complicated calculations to work out oxygen saturation. A poor pleth tracing can easily fool the computer into wrongly calculating the oxygen saturation. So always look at pleth first, before looking at oxygen saturation.
Just to remind you okay , I promise, this is the last time! The pleth is affected by factors that affect the peripheral blood flow. For an example, low blood pressure or peripheral cold temperature can reduce it. Sophisticated uses of the pleth are being developed. For example, it may be used to guide fluid therapy. These discussions are beyond the scope of this web site.
Light Emitting Diodes come in a variety of types that emit light in specific wavelengths. Fortunately, there are light emitting diodes LED that emit light in the red light and infrared light wavelengths and these are thus conveniently used in pulse oximeters. The exact wavelengths of the LEDs used depends on the manufacturer. For convenience, in our discussions, we made the red LED to have a wavelength of nm and the infrared LED to have a wavelength of nm easy to remember.
However, most text books will quote nm and nm. On the other side, is a light detector. However, you will note that, though there are only two LEDs, the light detector is exposed to three sources of light. In addition to the red and infra red LED light sources, there is also light in the room ambient light that the pulse oximeter is working in.
Some of this room light can also reach the detector. The pulse oximeter has to work with these three sources of light. It wants the red and infra red light to calculate oxygen saturation. The way it does this will be explained. The above diagram shows both LEDs lit to make the explanation easier. In reality, both LEDs are never lit together. First, the pulse oximeter activates the red LED light.
The red light goes through the finger not shown, to make image less crowded and reaches the detector. Stray room light also reaches the detector. The detector therefore records red light and room light that falls on it. The infrared light goes through the finger not shown and reaches the detector.
The detector therefore records infrared light and room light that falls on it. If you think your pulse oximeter might not be accurate, Rizzo suggests checking the pulse reading or heart rate on the pulse oximeter against your own reading of your pulse. If you have no underlying medical conditions, your levels should generally be at 95 or higher. But for people with chronic conditions, it's important to talk with your doctor about what the appropriate reading should be for you, and when you should seek medical attention.
For reference, lower levels of blood oxygen saturation may indicate:. In addition, if your pulse oximeter reading drops during exercise, Rizzo says it can be a sign of an underlying lung or heart condition and you should speak with your doctor.
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