Pulse Oximeters: How and When They Work
By Lily - Registered Respiratory Therapist
What Is a Pulse Oximeter?
Pulse oximeters provide continuous monitoring/spot checks of blood oxygen saturation with the use of a probe/clip. The clip/probe is usually placed on a finger but can also be applied to the ear or forehead (in cases of poor peripheral perfusion).
Finger Pulse Oximeters
The smallest pulse oximeters are finger pulse oximeters that clip onto the fingertips and have a screen built in. The screen shows oxygen saturation and heart rate.
Some finger pulse oximeters may show plethysmography waveform - more on this later - which provides information on whether or not your pulse oximeter is giving you a good signal.
These pulse oximeters are battery powered and, depending on the manufacturer, may come with the ability to be paired with smartphone apps that can record and store readings.
Next in size are handheld oximeters which are about the size of a smartphone and can connect to different clips/probes. For instance, you can switch out the connection cable from a finger clip oximeter sensor to a forehead adhesive sensor.
Handheld oximeters contain internal memory to record and store oximetry data. These are usually the oximeters you would use when doing an overnight oximetry test because multiple nights of data can be stored in the device for download by your healthcare provider later.
Tabletop Pulse Oximeters
Tabletop pulse oximeters are larger, can connect to different sensors, and have internal memory to store data. You may see these at your doctor’s or in the hospital.
These oximeters may also have the ability to measure end tidal carbon dioxide (EtCO2) which is used in critical care settings to determine adequacy of gas exchange in the lungs.
Technology in Pulse Oximeters
Most people know that a pulse oximeter measures the oxygen saturation level of blood. However, some may not know that pulse oximeters have limitations, and in certain situations they are unable to provide an accurate reading of your blood oxygen level.
A good understanding of how these devices work will prevent false alarms and unnecessary medical visits. First, let us look into the technology in pulse oximeters and why your smartphone pulse oximeter app may not be as handy as you think it is.
A pulse oximeter generates two different wavelengths of light: red at 660 nm and infrared (IR) at 940 nm. It uses these two wavelengths of light and the following two principles to calculate SpO2 (oxygen saturation).
First, oxygenated hemoglobin (O2Hb) and deoxygenated hemoglobin (HHb) have different red and IR light absorption properties. Oxygenated hemoglobin absorbs more IR light and less red light while deoxygenated hemoglobin absorbs more red and less IR light.
Second, the amount of light absorbed fluctuates with the amount of arterial blood that pulses with each heartbeat. Arterial blood volume increases during systole (heart contraction) and decreases with diastole (heart relaxation) while venous and capillary blood volume does not change much.
A pulse oximeter generates red and IR light from a light emitting diode (LED) on one side of the probe/clip. This light travels through the finger/earlobe etc. to a photodiode on the other side of the probe.
The photodiode transmits light energy to electrical energy, and the pulse oximeter can then calculate the ratio of O2Hb to HHb (relative amount of red light measured vs IR light measured).
The device then uses the Beer-Lambert Law to convert the ratio to the SpO2 reading we see on the screen. Since pulse oximetry relies on pulsatile blood flow to calculate the O2Hb to HHb ratio, adequate perfusion is needed for reliable readings.
What about those Mobile Apps that Claim to Measure SpO2 Accurately?
Now that we’ve covered the technology behind pulse oximeters, it should not come as a surprise to learn that mobile apps that claim to measure SpO2 don’t always work.
Whether or not a pulse oximetry app actually works depends on the smartphone hardware. Some smartphones contain built-in photoplethysmography biosensors which are used in medical devices for measuring heart rate, blood pressure, and oxygen saturation.
Others use red and infrared light technology, just like traditional pulse oximeters. Photoplethysmography is a technology that has been around for a while. It uses light to measure changes in blood volume in micro vascular tissue beds.
This technology is also used in traditional pulse oximeters - it is the strange fluctuating wave on your pulse oximeter display. When measuring your SpO2, you should always compare your oxygen reading to the plethysmography waveform because your oxygen reading is reliable only if you have a good signal (more on this later in limitations).
Many smartphone oximetry apps have disclaimers stating that their app is not for clinical use. This is consistent with studies on smartphone oximetry which show variability between different manufacturers. Until there are more studies investigating the clinical utility of smartphone oximeters, it would be best to avoid using them for clinical purposes.
How Come Sometimes My Oximeter Doesn’t Pick Up Signal?
As we mentioned earlier, pulse oximetry relies on pulsatile blood flow to measure and calculate SpO2. This leads to device limitations when perfusion is poor (decreased blood flow).
If you’ve ever tried using a pulse oximeter when your hands are cold, you’ll know what I mean. Your pulse oximeter won’t be able to pick up a signal and give you a reading.
Usually this is easily remedied by rubbing your hands together or soaking them in a basin of warm water to bring some circulation back into your fingers.
For those who suffer from chronic poor peripheral perfusion (eg. due to hypotension, Raynaud’s Disease etc.), readings can be taken from other sites, such as the ear and forehead.
Ear readings can be taken using a reusable clip-on pulse oximeter probe that is attached to a hand-held oximeter or to a table top oximeter. Forehead readings can be taken via an adhesive probe or a headband wrapped around the forehead.
Nail Polish and Acrylic Nails Can Mislead Your Oximeter
Other common factors that cause falsely low oxygen saturation readings on a pulse oximeter are motion artifact (movement) and nail polish/acrylic nails.
It is important to keep your hands stable while measuring your SpO2 and give the pulse oximeter a few seconds to pick up a reading and stabilize. Some oximeters have a light that will flash green when the device picks up a steady signal.
A good way to determine if your oximeter is picking up a good signal is to look at your heart rate reading. If it is very low or not reliably showing a reading, then you know the device is having trouble picking up a good signal.
In the event that the oximeter is having difficulty picking up a signal, keep your hand steady until you see the light flash/heart rate pulse in a steady rhythm on the oximeter. You should have a reliable SpO2 reading then. If not, adjust the finger/ear clip probe or switch to a different finger/ear.
The index and middle fingers are the best ones for taking a reading. If you have nail polish or acrylic nails, you may have to be a bit creative to obtain a reading.
If you clip the oximeter clip to your finger tip over your nails, you may not get a reading because nail polish and acrylic nails block the transmission of light.
A trick is to rotate the finger probe so it clips on to your finger sideways. Clipping the probe sideways will allow the light to bypass the nails and generate a reading on the oximeter.
Let's Talk about the Numbers Showing on the Oximeter
Healthy individuals will have an SpO2 reading between 95% and 100%. SpO2 decreases as you age, so older healthy adults have a lower normal SpO2 than their younger counterparts.
It is more likely for adults over 70 years old to have an SpO2 closer to 95% than it is for a healthy adult in their 30s to have an SpO2 around 95%.
Individuals with respiratory diseases, such as COPD, pulmonary fibrosis, emphysema, lung cancer, etc., will have a lower baseline SpO2.
The same applies to individuals with other chronic illnesses, such as untreated sleep apnea, cardiovascular diseases, or congenital heart problems and obesity.
Having a good idea of your baseline/resting oxygen saturation will prevent unnecessary panic and help you use your pulse oximeter more effectively.
Regardless of your pulse oximetry readings, however, if you experience any shortness of breath, wheezing, lightheadedness, diaphoresis (sweating), headaches, coldness in the extremities, rapid heart rate, or notice a blue tinge in your fingertips/toes/lips, please seek medical attention immediately.
These are all symptoms of hypoxemia (low oxygen in the blood) which can lead to hypoxia (low oxygen in the tissues) that can lead to tissue damage.
In Conclusion - Be Comfortable Taking Your Own Oxygen Saturation Readings
Pulse oximeters are a useful tool for monitoring oxygen levels. However, they have their limitations, and it is important to understand how to use them correctly.
Hopefully, this article has given you an understanding of the principles behind oximeters and how to use an oximeter effectively.
At the very least, I hope it has familiarized readers with pulse oximetry and made you more comfortable with taking your own oxygen saturation readings.