If you’ve ever had a medical team investigating cardiac issues, you’ve probably had a bunch of electrodes stuck all over your chest and been hooked up to an electrocardiogram. This is the gold standard when it comes to understanding electrical activity in the heart and can diagnose a great many conditions. However, sometimes doctors just need the basic information—your pulse rate, and whether or not there’s actually any oxygen in your blood.

Thankfully, there’s a cheap and simple device that can offer that exact information. It’s the pulse oximeter, and it’s a key piece of equipment that’s just about vital for monitoring vitals. Let’s learn how it works!

Pump It

If you’re unfamiliar with pulse oximeters, they’re that little plastic thing that clips on your finger at the doctor’s office. The device places two LEDs on one side of your finger, and a photodiode on the other. With just these simple components, it’s possible to determine the percentage of your blood’s hemoglobin that is currently carrying oxygen. It’s also possible to discern pulse rate, which also comes in handy when you’re trying to determine a patient’s current status at a glance.

Pulse oximetery was the brainchild of Takuo Aoyagi, an electrical engineer at Nihon Kohden in Tokyo. In 1972 he was working on a non-invasive way to measure cardiac output using the dye dilution method, which involves injecting a tracer dye and watching how its concentration in the blood decays over time. He was reading that decay optically through an ear oximeter. These devices used red and infrared light passed through the ear tissue to determine blood oxygen levels, but required frustrating calibration to work properly and often required fussy steps like first squeezing blood out of the tissues prior to measurement. The problem was that early oximeters worked based on the total absorption of light, and were affected by things like the skin, tissue, and venous blood, when really the goal was to measure the oxygen levels in the arterial blood itself.

As Aoyagi worked with the device, he noted that the patient’s pulse kept showing up as an annoying ripple in the output. He spent some effort trying to cancel that ripple by balancing red and infrared signals against each other. Then he noticed that when a patient’s oxygen saturation dropped, the cancellation fell apart. This led to the realization that the ratio of how much red and infrared light was absorbed could be used to determine the oxygen saturation of the arterial blood.

It all comes down to the nature of blood itself. Hemoglobin comes in two flavours relevant here: oxyhemoglobin, which is carrying an O₂ molecule, and deoxyhemoglobin, which isn’t. They are different colours, which is why arterial blood is bright red and venous blood is darker. They absorb light differently, to the point that it’s actually clinically useful. At a wavelength of 660 nm (red)—deoxyhemoglobin absorbs noticeably more light than its oxygenated cousin. At around 940 nm (near-infrared), oxyhemoglobin absorbs more. Almost every pulse oximeter uses these two wavelengths; both penetrate tissue quite easily, and it’s easy to find LEDs that spit out these wavelengths.

Reading the blood oxygen level is relatively straightforward. The device will typically alternate the two LEDs on and off, many times a second, also including a third phase with both off so the photodiode can subtract out ambient room light as well. The photodiode sees light that has passed through an entire finger, including the skin, bone, fat, as well as the venous and arterial blood. Most of that doesn’t change from second to second, but the arterial blood does, with every pump of the heart. Thus, when sampling light from the infrared and red LED pulses, the photodiode puts out a signal that’s mostly a continuous level from light passing through the finger, with a little wiggly bit on top that throbs at a human pulse rate. That’s due to the pulsing of the arterial blood, and the frequency can be used to measure pulse rate. Meanwhile, the continuous component is removed by subtracting the trough of both the infrared and red signals from the peak, which solely leaves the component of light absorption due to the fresh arterial blood itself.

The level of oxygenation in the arterial blood itself can then be measured by comparing the ratio of red to infrared light picked up in this part of the signal. The light ratio is converted into an human-parseable number via a lookup table, based on the Beer-Lambert law of concentration of substances in a solution. The displayed number is flagged as “SpO₂.” The “p” stands for “peripheral,” to indicate it’s an optical measurement rather than determined directly with blood-gas measurement techniques. This distinction is important, as there are a range of conditions under which pulse oximetry readings can be inaccurate. At a very base level, pulse oximeters can get confused if a patient is moving while wearing the device, which makes the pulsatile signal itself less clear.  The device also cannot tell carboxyhemoglobin from oxyhemoglobin, because they absorb light very similarly at 660 nm.  Carboxyhemoglobin is the result of carbon monoxide entering the blood, so a smoke inhalation victim can display a high apparent SpO₂ figure while their blood is carrying very little oxygen. Nail polish and skin tone can impact the amount of light transmitted through the finger, impacting readings, while limited bloodflow to the fingers can also frustrate things.

It may not be perfect, but pulse oximetry is nevertheless very useful a lot of the time. It enables medical teams to get a near-instant look at a patient’s most vital signs in a completely non-invasive manner. The use of this technology has revolutionized both emergency care and surgery, where it has played a huge role in patient monitoring under anaesthesia. Plus, the simplicity of the device has made this critical medical insight accessible to anyone that can afford a $20 device with a few LEDs and a photodiode in it. It’s now even possible to track your oxygen saturation during sleep with an off-the-shelf smartwatch due to developments from this technology, helping aid in the diagnosis of complex conditions like sleep apnea. All because blood tends to pass light a little differently depending on how oxygenated it is. Sometimes you have to thank nature for those little conveniences.