Pulse oximeters how does it work

For an example, the wave on the left has a wavelength of nm and the wave on the right has a longer wavelength of nm. The pulse oximeter uses the property that oxyhemoglobin and deoxyhemoglobin absorb light of different wavelengths in a specific way. This property can be demonstrated in a laboratory as will be now described. We can first demonstrate how oxyhemoglobin absorbs light of different wavelengths in a specific way. We use a special light source of which we can adjust the wavelength of the light it emits.

This light source sequentially passes light of different wavelengths through a sample of oxy Hb. The detector notes how much light, at each wavelength, has been absorbed. A graph for the absorbance of oxy hemoglobin at different wavelengths will look like this. Again notice , how like oxy Hb, Deoxy Hb absorbs different amount of light at different wavelengths. Now let us see the absorbance graph of oxy Hb and the absorbance graph of deoxy Hb together so you can compare them.

Note how each of them absorbs light of different wavelengths very differently. One is a red light, which has a wavelength of approximately nm. The other is an infrared light, which has a wavelength of nm. 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 Food and Drug Administration FDA requires that prescription oximeters must provide results within an accuracy range of 4 to 6 percent. The American Thoracic Society says that typically, more than 89 percent of your blood should be carrying oxygen.

This is the oxygen saturation level needed to keep your cells healthy. Having an oxygen saturation temporarily below this level may not cause damage.

But repeated or consistent instances of lowered oxygen saturation levels may be damaging. An oxygen saturation level of 95 percent is considered typical for most healthy people. A level of 92 percent or lower can indicate potential hypoxemia, which is a seriously low level of oxygen in the blood. A report compared the accuracy of pulse oximetry tests and blood gas measurements in detecting hypoxemia in Black and white patients.

Researchers found that among Black patients, there were three times as many cases of pulse oximetry tests failing to detect occult hypoxemia when blood gas measurements did so. Tests like these were developed without considering a diversity of skin tones. The authors concluded that more research is needed to understand and correct this racial bias. Once the test is over, your doctor will have the readings available immediately.

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? 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. 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.