Throughout our description, we will show the infrared light in light blue. In reality, infrared light is invisible to the human eye. Now look at the oxy Hb absorbance graph again, but this time paying attention to the wavelengths of light used in pulse oximeters.
You will see that oxy Hb absorbs more infrared light than red light. Below is the graph that shows the absorbance of deoxy Hb. It is seen from the graph that deoxy Hb absorbs more Red light than Infrared light. To make the comparison of absorbance of oxy Hb and deoxy Hb easier, here is a composite graph showing the absorbance of both.
You will see that :. You might find the memory aide below useful to remember the wavelengths absorbed by oxy Hb and deoxy Hb.
The pulse oximeter works out the oxygen saturation by comparing how much red light and infra red light is absorbed by the blood. Depending on the amounts of oxy Hb and deoxy Hb present, the ratio of the amount of red light absorbed compared to the amount of infrared light absorbed changes. The absorbance ratio i. The blood has both , oxy Hb and deoxy Hb. The absorbance pattern is now somewhere in between the oxy Hb curve and deoxy Hb curve both shown in grey.
The animation below shows what you have seen before. As the amount of oxy Hb and deoxy Hb changes, the light ratio comparing red and infrared light also changes.
The pulse oximeter uses the ratio to work out the oxygen saturation. Unfortunately, there is a problem. In physics, the Beer and Lambert law have very strict criteria to be accurate.
For an example, the light that goes through the sample should go straight through like the lights rays in the image below. However, in real life , this does not happen. Blood is not a neat red liquid. Instead, it is full of various irregular objects such as red cells etc. This makes the light scatter, instead of going in a straight line. Therefore Beer and Lamberts Law cannot be applied strictly. Because Beer and Lamberts law cannot be applied strictly, there would be errors if they were used to directly calculate oxygen saturation.
A test pulse oximeter is first calibrated using human volunteers. The test pulse oximeter is attached to the volunteer and then the volunteer is asked to breath lower and lower oxygen concentrations. At intervals, arterial blood samples are taken. As the volunteers blood desaturates, direct measurements made on the arterial blood are compared simultaneously with the readings shown by the test pulse oximeter. In this way, the errors due to the inability of applying Beers and Lamberts law strictly are noted and a correction calibration graph is made.
A copy of this correction calibration graph is available inside the pulse oximeters in clinical use. When doing its calculations, the computer refers to the calibration graph and corrects the final reading displayed. For saturations below this, the calibration curve is mathematically estimated.
In a body part such as a finger, arterial blood is not the only thing that absorbs light. Skin and other tissues also absorb some light. 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. Deoxygenated hemoglobin allows more infrared light to pass through and absorbs more red light. Figure 3: Oxy and Deoxy Hemoglobin Absorption The LEDs sequence through their cycle of one on, then the other, then both off about thirty times per second. The amount of light that is transmitted in other words, that is not absorbed is measured. 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 peak transmitted light in each wavelength, the effects of other tissues is corrected for allowing for measurement of only the 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.
This ratio is then converted to SpO 2 by the processor via a lookup table based on the Beer—Lambert law. Photoplethysomography: An important tool for any SpO 2 reading is plethysmography tracings or "pleth" which is a measure of volumetric changes associated with pulsatile arterial blood flow. Therefore, plethysomography ensures reliability of the calculated oxygen saturation. Spl4 [Public domain or Public domain], via Wikimedia Commons Interpretation Tips Always evaluate plethysomograph in conjunction with SpO 2 readings to ensure reliability.
Hemoglobin can normally bind approximately 1. When the concentration of Hb is decreased, there is a decrease in total O2 content of the blood, but no change in the O2 saturation, hence oximetry is not an effective test to evaluate for anemia. Similarly, if a patient has abnormal hemoglobin molecules, such as in the case of sickle cell anemia where the oxygen dissociation curve is right-shifted, pulse oximetry is a poor measure of hypoxemia and may lead to over diagnosis and over treatment.
Therefore, arterial blood gas determination of PaO 2 and SaO 2 is much more accurate in patients with abnormal hemoglobin dissociation curves.
This will help them determine if other testing or treatment is necessary. Your doctor will be able to tell you what the next steps are. Pulse oximetry is a quick, noninvasive, and completely painless test. It comes with no risks, aside from potential skin irritation from the adhesive used in some types of probes. Read this article in Spanish. If you have chronic health conditions, your blood oxygen level may fall outside of the normal range.
This includes people with asthma, heart disease…. Looking for pulse oximeter recommendations? We take a closer look at ChoiceMMed and two of their pulse oximeters that are the easiest to find. See what finger pulse oximeters our team has chosen to help you keep an eye on your health. Find information on why a blood gas test done, what to expect during the procedure, and how to interpret the test results.
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Health Conditions Discover Plan Connect. Medically reviewed by Adithya Cattamanchi, M. Takeaway Pulse oximetry is a noninvasive test that measures the oxygen saturation level of your blood.
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