Unveiling Pulse Oximeters: How They Work and Why You Might Need One

By Lily – Registered Respiratory Therapist

Pulse oximeters allow for either continuous monitoring or spot-checking of blood oxygen saturation levels (SpO2) using a probe or clip. Typically, the clip or probe is placed on a finger, although it can also be attached to the ear or forehead. This is usually done when an individual has poor perfusion to their extremities (e.g., cold hands and/or feet) to ensure accurate SpO2 readings.

In this article, we provide a clear explanation of how pulse oximeters work and explore the different types of pulse oximeters. We explain the significance of the readings on your pulse oximeter and discuss the limitations of these useful tools. We also take a critical look at pulse oximeter smartphone apps. Our goal is to provide you with a guide that helps you use your pulse oximeter more confidently and effectively.

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 come equipped with 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 to be downloaded later by your healthcare provider.

Tabletop pulse oximeters, being larger, can connect to various sensors and also feature internal memory for data storage. You’re likely to encounter these in doctor’s offices or hospitals. In addition, some of these oximeters may be capable of measuring end-tidal carbon dioxide (EtCO2). This feature is often used in critical care settings to assess the efficiency of gas exchange in the lungs.

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. 

Developing a good understanding of how pulse oximeters operate can prevent false alarms and unnecessary medical visits. Let’s start by examining the underlying technology in pulse oximeters and why your smartphone’s pulse oximeter app might not be as reliable as you’d expect.

A pulse oximeter emits two different wavelengths of light: one red at 660 nm and the other infrared (IR) at 940 nm. It uses these two light wavelengths and two underlying principles to calculate SpO2 (oxygen saturation).

Firstly, 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.

Secondly, 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, in contrast, remains relatively stable.

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 converts light energy into electrical energy, enabling the pulse oximeter to calculate the ratio of O2Hb to HHb (the relative amount of red light absorbed versus IR light absorbed).

Finally, the device employs the Beer-Lambert Law to convert this ratio into the SpO2 reading displayed on the screen. It’s important to note that pulse oximetry depends on pulsatile blood flow to calculate the O2Hb to HHb ratio, so sufficient perfusion is required for reliable readings.

Now that we’ve covered the technology behind pulse oximeters, it should not come as a surprise to learn that mobile apps that measure SpO2 aren’t always reliable.

The effectiveness of a pulse oximetry app largely depends on the smartphone’s hardware. Some smartphones come equipped with built-in photoplethysmography biosensors, which are utilized in medical devices to measure heart rate, blood pressure, and oxygen saturation. Others employ red and infrared light technology, mirroring the approach of traditional pulse oximeters.

Photoplethysmography is a well-established technology that uses light to detect variations in blood volume within microvascular tissue beds. This technology also forms the basis for traditional pulse oximeters – it’s responsible for the fluctuating waveform you see on your pulse oximeter display. When evaluating your SpO2, you should always compare your oxygen reading with the plethysmography waveform, as the reliability of your oxygen reading hinges on a good signal (we’ll explore this further in the section on limitations).

It’s worth noting that many smartphone oximetry apps include disclaimers stating they are not intended for clinical use. This aligns with studies on smartphone oximetry, which reveal inconsistencies among different manufacturers. Until more research is conducted into the clinical applicability of smartphone oximeters, it’s probably best to refrain from using them for clinical purposes.

Poor peripheral perfusion

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 (for instance, due to hypotension, Raynaud’s Disease, etc.), pulse oximetry readings can be taken from alternative sites like the ear or 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 tabletop oximeter. Forehead readings can be taken via an adhesive probe or a headband wrapped around the forehead.

Motion artifact

Other common factors that cause falsely low oxygen saturation readings on a pulse oximeter are motion artifact (movement) and nail polish/acrylic nails. 

Keeping your hand stable is crucial to achieving an accurate SpO2 reading. The pulse oximeter needs a few seconds to detect and stabilize the reading. Some oximeters come with a light that flashes green when the device acquires a consistent signal.

A good way to determine if your oximeter is picking up a good signal is to look at your heart rate reading. If your heart rate is very low, then you know the pulse oximeter is having trouble picking up a good signal. If this happens, keep your hand steady until your heart rate stabilizes and the indicator light or plethysmography waveform pulse in a steady fashion.

If, after a couple minutes of holding your hand steady, the pulse oximeter still cannot detect a signal, switch to a different finger.

Nail polish/acrylic nails

The index and middle fingers are the best fingers for taking your SpO2 reading. However, if you have nail polish or acrylic nails and don’t want to remove them, you may have to get a bit creative to obtain a reading.

Nail polish and acrylic nails block the transmission of light and can prevent the oximeter from detecting a signal. A trick is to rotate the finger probe so it clips onto your finger sideways. Clipping the probe sideways will allow the light to bypass the nails and generate a reading on the oximeter.

Healthy individuals typically have an SpO2 reading between 95% and 100%. It’s important to note that SpO2 decreases with age, which means older adults generally have a lower normal SpO2 compared to younger people. For instance, it’s more common for adults over 70 years old to have an SpO2 closer to 95%, compared to healthy adults in their 30s.

People with respiratory diseases, such as COPD, pulmonary fibrosis, emphysema, lung cancer, etc., tend to have a lower baseline SpO2. This also holds true for individuals with other chronic illnesses like untreated sleep apnea, cardiovascular diseases, congenital heart conditions, and obesity.

Understanding your baseline or resting oxygen saturation can help avoid unnecessary anxiety and enable more effective use of your pulse oximeter.

However, no matter what your pulse oximetry readings indicate, if you experience symptoms such as shortness of breath, wheezing, lightheadedness, excessive sweating, headaches, a feeling of coldness in the extremities, a rapid heart rate, or a blueish hue in your fingertips, toes, or lips, please seek immediate medical attention.

These symptoms can indicate hypoxemia (low oxygen in the blood), which may lead to hypoxia (low oxygen in the tissues), resulting in potential tissue damage.

Pulse oximeters are a useful tool for monitoring blood 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, we hope it and has made you more comfortable with taking your own oxygen readings.

Sources:

https://www.sciencedirect.com/science/article/pii/S095461111300053X#fig1

Accuracy of Smartphone Pulse Oximeters in Patients Visiting an Outpatient Pulmonary Function Lab for a 6-Minute Walk Test | Respiratory Care