The Pulse Oximeter

What is a pulse ox?
The “pulse ox,” or pulse oximeter, is a cheap, portable device that allows you to measure your pulse (heart rate) and estimate the level of oxygen in your blood. Specifically, it estimates oxygen saturation of your arterial blood (the blood that delivers oxygen from your heart to your tissues). The value that is produced by the pulse oximeter is called “SpO2” (standing for peripheral saturation of oxygen). The device uses a calculation based off the wavelength of light passed through the blood in a peripheral body part (usually a finger or toe) to estimate the saturation of oxygen in that tissue. This is referred to as “non-invasive,” because no blood needs to be drawn.

Photo: A pulse oximeter's light passes through a finger or toe, and measures the saturation of oxygen in that tissue. (Source:

When should I use it?
If you're feeling sick, you could consider measuring your heart rate and oxygen level with the pulse ox. Heart rate and oxygen saturation, as measured by the pulse ox, provide two vital signs that can help guide diagnosis and treatment of illness. The pulse ox component is more likely to be relevant in respiratory infections, like pneumonia, severe bronchitis, asthma and COPD exacerbations, or at altitude.

I used the pulse ox, and it says my heart rate is 100 and my oxygen level is 98%. Is that normal?
A resting heart rate of 100 beats per minute and below is generally considered normal. This will vary by degrees of physical fitness, as Lance Armstrong is famously known to have a heart rate of 39 at rest!

An oxygen saturation of 96-100% is generally considered normal.

My heart rate is above 100 or below 50, what should I do?
Make sure you're measuring your heart rate at rest, which could mean you may have to wait up to 5 minutes after doing exercise (even walking counts as “exercise”). Make sure the skin on your finger/fingernail isn’t covered with nail polish, as this may make it difficult for the pulse ox to read your blood. Very poor blood flow to the body part being detected can obscure the ability to obtain accurate results. Cold hands with poor circulation is an excellent example of this scenario.

If the results are correct, use a thermometer to check if you have a fever. A good rule of thumb is that for every degree in Celsius (1.8 F) that your temperature is above normal, your heart rate will increase about 10 beats per minute [1]. Independent of fever, an elevated heart rate in the setting of feeling ill or unwell might mean that you have signs of a more systemic infection. If you have an elevated heart rate and feel unwell, you should seek attention from a healthcare professional.

However, if you have no symptoms, a heart rate below 50 or above 100 can be benign. Some people get nervous when checking their vital signs and that alone can raise it!

My oxygen level is below 94%, what should I do?
A low oxygen saturation is a marker of either a more serious acute disease (like pneumonia, bronchitis, asthma exacerbation, a clot in your lungs), a chronic disease (like COPD, emphysema), or of being at altitude. If you are not at altitude, this is an indication that you should seek help from a medical professional, because you likely need further medical testing, such as a chest x-ray.

If you are at altitude, a lower saturation is to be expected, and your symptoms will likely notify you of your low oxygen saturation before you even think to check the pulse ox (shortness of breath, light-headedness, headache, trouble concentrating).

A healthy person breathing normally at sea level will almost always have an oxygen saturation of 95-100%.

Photo: Grand Teton National Park. Altitude affects oxygen saturation. Source: National Parks Service

Does altitude affect the pulse ox? Should I use it at altitude?
Yes, dramatically so. At 5,000 feet, if you don’t breathe any faster than normal, your O2 saturation will be about 90%. If you take 5 extra breaths per minute, you can increase this to 95%.

At the extremes of altitude, the changes are even more pronounced. At 10,000 feet, if you breathe at a normal rate, your O2 saturation would be about 80%. At the summit of Mt. Everest, if you breathed at your normal rate, life would not be sustainable. In fact, you would need to breathe twice as fast as normal, and twice as deep as normal, just for your oxygen saturation to be 35%. And that’s if you’re standing still, not expending energy! This is why only 3% of the summits of Mt. Everest are done without oxygen [2].

If you are planning a trip with high altitude climbing, there are things you can do to prepare, including taking acetazolamide, spending time at higher elevation (to acclimatize), and ascending slowly. Discuss your plans with your Duration Health provider if you anticipate climbing at altitude.

Photo credit: pulse oximeter, By Rama - Own work, CeCILL,