Low blood oxygen saturation is a symptom of COVID, so people are researching how to buy a pulse oximeter. These devices are cheap, painless, non-invasive and easy to use. There are only a few features you need to understand to buy the right one for you.
A good pulse oximeter has a Perfusion Index (PI) and an OLED display with large numbers. The PI indicates if you’re getting a bad reading because the pulse is weak. The OLED display is bright and easy to see.
The PI is not the only reason you might get an inaccurate reading. Some pulse oximeters can mitigate “artifact movements” such as you shivering while taking a measurement. Some oximeters include a graph that displays your PI in real time.
Why Your Oximeter Should Include a Perfusion Index Measurement
A good pulse oximeter includes a Perfusion Index (PI). The PI is a measure of how strong the pulse is at the measuring site. If it is weak, you will likely get an inaccurate oxygen reading. Oxygen meters with the ability to measure PI can tell you that the PI is low. This, in turn, tells you that the reading might not be accurate.
Avoid False Perfusion Index Measurements
A finger pulse oximeter measures both your blood oxygen saturation and your pulse rate. Most home-use oximeters clamp onto the index finger for measurement. They rely on contact with your skin and nail. Most if not all oximeter instructions I’ve read advise against wearing dark nail polish while measuring pulse and oxygen at the finger. Nail polish prevents the measuring beam from penetrating through the fingernail. You can get a false reading if you wear nail polish.
A pulse oximeter can give inaccurate readings. Shaking, movement and shivering alter the readings. Darker skin is harder to read than lighter skin. Nail polish interferes with readings. If the perfusion index is low, the meter will get weak signals and give an inaccurate result.
Some oximeters use software algorithms to compensate for some of these problems. These oximeters can cancel out unrelated movements and even low blood perfusion index.
Perfusion Indices Range Among People and Measuring Sites
The perfusion index (PI) is a ratio of two types of blood flow. The value indicates how strong the pulse is at the place where you are measuring blood oxygen. The values range from a weak 0.02% to an strong pulse of 20%.
The PI is not the same in everyone. In fact, it’s not the same in your ear as it is on your wrist. So you want to get a baseline PI for yourself when healthy (if possible), so that you will know when the PI is weak.
Talk with Your Doctor About Basesline Readings
If you want to use your oximeter to measure changes over time, you need to record your readings. For example, if you were to take your pulse right now, and it said “80 bpm,” is that good or bad?
It depends on what your normal pulse rate is, and what you were doing before starting the measurement.
Are you just waking up with an 80 bpm pulse? Or are you just getting off an exercise bike with an 80 bpm pulse?
It’s a good idea to talk with your doctor about good target baseline measurements. You can then use these for comparison should you get unhealthy readings.
The oximeter measures the PI at the monitoring site. If it is too low, the oximeter will be able to convey to you that it cannot get a good reading of your blood oxygen level.
The Plethysmograph Graphs the Perfusion Index
The Plethysmograph is a line graph of the PI’s values as they move from weak to strong and back. If the plethysmograph wave is strong, you can expect an accurate pulse and oxygen reading. Some manufacturers call this the Pulse Bar or Pleth Graph.
How Accurate is an Over the Counter Pulse Oximeter?
The FDA mandates that home pulse oximeters give an accurate oxygen reading to within 2%. Some vendors offer accurate that will vary less than 1% from a hospital oximeter measurement.
OTC Oximeters are Sports, not Medical Devices
Almost if not all consumer oximeter vendors sell their units for sports oxygen measurement. They avoid calling them “medical devices.” This absolves them, presumably, from liability.
It’s probably best to discuss the oximeter baseline measurements with your doctor. While doing that, ask the doctor when you should and should not rely on oximeter readings.
A $25 oximeter can indicate a problem that you should investigate with your doctor. But it can also give you a false positive, as it is a home device. Medical devices at hospitals get calibrations and maintenance that your oximeter will not get. So be proactive with your doctor about your pulse and blood oxygenation concerns.