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.
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.
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? 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.
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Medically reviewed by Adithya Cattamanchi, M. The physical properties that a pulse oximeter employs will be explained using the probe shown below. A finger is shown inserted into the probe. Above the finger are the light sources that emit light.
In the finger is an artery which carries the blood the pulse oximeter is interested in and a vein through which the blood leaves the finger. Below the finger is the light detector. Hemoglobin Hb absorbs light. The amount of light absorbed is proportional to the concentration of Hb in the blood vessel. In the diagram below, the blood vessels in both fingers have the same diameter.
However, one blood vessel has a low Hb concentration i. Each single Hb absorbs some of the light, so more the Hb per unit area, more is the light is absorbed. By measuring how much light reaches the light detector, the pulse oximeter knows how much light has been absorbed. More the Hb in the finger , more is the light absorbed. Look at the two fingers shown below. Both arteries have the same concentration same Hb per unit area, blue square However, the artery on right is wider than the one on the left.
The light emitted from the source has to travel through the artery. The light travels in a shorter path in the narrow artery and travels through a longer path in the wider artery paths are shown as green lines below. Though the concentration of Hb is the same in both arteries, the light meets more Hb in the wider artery, since it travels in a longer path. Therefore, longer the path the light has to travel, more is the light absorbed. We have seen how concentration and light path affect the absorbance of light.
In addition to these, the pulse oximeter makes use of another important property to calculate oxygen saturation. That is, oxy hemoglobin and deoxy hemoglobin absorb light of different wavelengths in a specific way. Before we go further, we need to remember what wavelength is. All light is composed of waves. 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.
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