Eupnea means to breathe air normally without trouble.
Introduction
Let’s talk pulse oximeters. Lately, these nifty little devices have gotten a lot of attention due to COVID-19. How do they work, are they accurate, and should you get one as an OT?
I have been using pulse oximeters for many years now, ever since I was an EMT. First of all, let’s go over how they work. We all know how oxygen is important for human life forms like you and me. We inhale oxygen through the air, which contains about 21% oxygen and 78% nitrogen. Interestingly, hyperbaric oxygen chambers provide oxygen at 100% to patients inside of them.
The oxygen enters the lungs and then our blood via the cardiopulmonary system. So this includes the distal extremities such as our fingertips. Blood helps carry oxygen throughout our body. The main way oxygen is carried is via hemoglobin molecules. A hemoglobin molecule can either bind to oxygen (also called an oxygenated hemoglobin molecule) or not be bound to an oxygen molecule (also called deoxygenated hemoglobin). Hemoglobin actually has 4 binding sites each for oxygen and exhibits what is called positive co-operativity. This means if one oxygen is bound to 1 of the 4 sites of hemoglobin, additional oxygen molecules are more likely to bind to the remaining 3 sites of that same hemoglobin molecule with higher affinity. This comes in handy when you are in situations that require your tells to rapidly take in oxygen to provide you energy such as sports or lifting weights.
Oxygen Saturation
So what does the term oxygen saturation mean? If you take a sample of hemoglobin molecules, say what the pulse oximeter sees on your fingernail, the percentage of hemoglobin molecules that are bound to oxygen is the oxygen saturation. So if all of the hemoglobin molecules are bound to oxygen molecules collectively, you would get an oxygen saturation of 100%. It’s just simple math.
How Pulse Oximeter’s Work
So how does a pulse oximeter determine the oxygen saturation from must being clipped onto a finger, ear lobe, or other body part? As you may have noticed, there is a red light that shines when you look at a pulse oximeter. This red LED light has the wavelength of 660 nanometers. On one end, of the clamp, there is an LED emitting red light. Directly opposite to that on the other side of the clamp, there is a detector made to read the light that was emitted. Did you know that pulse oximeters actually contain 2 lights to function? You cannot see the second LED light with your naked eye because it is infared light and on the 940 nanometer spectrum. There is a detector for the infrared light opposite to it as well.
An easy way to understand how these two red and infrared lights work together is that they have an inverse relationship of how they absorb or pass light through relative to whether hemoglobin is oxygenated or not. In other words, they have inverse effects. When oxygen is bound to hemoglobin, more red light passes through the molecules and is detected by the detector. However, at the same time, less infrared light passes through to the detector in this instance. The opposite also happens. When less or no oxygen is bound to the hemoglobin, LESS red light passes through and MORE infrared light passes through. By nature of how these two lights have an inverse or opposite relationship to each other when passing through the same hemoglobin molecule, it allows you to demonstrate a phenomenon such as oxygen saturation using a mathematical equation. I’m not going to go into the details of the technicals of how pulse ox’s calculate oxygen saturation because I do not fully understand it myself, but if you are really interested in this topic, I’ll post a link to what I found.
OT Practice
When do OTs see and use pulse oximeters? In my experience in acute care and acute rehab, it’s literally every day and everywhere. All the vitals machines have one (whether they are broken or not is a different story as they can be kind of fragile – especially at the stress point of where the cord meets the measuring device). Other places I see is, well – myself. I carry one around in my pocket because it can quickly measure O2 saturation but also heart rate before, during, and after exertion.
Note
A note on personal pulse oximeters. The vitals machines with pulse oximeter’s connected to them that you see in hospital and clinics cost hundreds if not thousands of dollars. They are made to be more reliable and are calibrated to also be accurate. The pulse oximeters you buy such as on Amazon are much cheaper and may or may not be calibrated and likely less accurate – but they can tell the big picture based on high or low readings relative to the same patient. So follow your facility’s guidelines about whether you can document readings from your personal pulse oximeter, most likely not from a legal standpoint.
It is like buying a cheap bathroom scale compared to a more expensive medical grade one and using the cheap scale to document your patient’s weight. Which one would you trust? And as you are documenting vitals, which become part of a patient’s chart and medical record – you should keep this in mind. That’s not to say you should avoid using personal pulse oximeters altogether in clinical practice. They can be a valuable tool, especially for OTs compared to having nothing instead.
Tips for OTs
Here are some tips. You can always use it to take a reading, but not document it. Use it as a tool to provide insight into your patient’s presentation combined with their chief complaint, non-verbal signs and symptoms, and so on. You can always document other findings besides O2 sat from your personal device. If you suspect your pulse oximeter is getting inaccurate, put it on yourself and see if you get a reading close to 100%. Hold your breath and it should drop a little bit. Take your own manual heart rate at the same time you have a pulse oximeter on and it should be fairly close to see if the heart rate is accurate is as well. If you have even more time at work, compare the pulse oximeter to a vitals machine when you hold your breath, with one on each finger. You may look a little funny so if your boss walks by, tell them you are just testing equipment.
Accuracy
I want to go over some points about inaccuracies in pulse oximeters that I learned way back when I was an EMT. First, while they are wonderful little devices, they are not necessarily always correct. That’s not to say you shouldn’t trust the reading, but always always always listen to the patient’s chief complaint and other factors using your professional reasoning. For example, if a patient’s reading from a pulse ox is 100%, regardless if it is an expensive vitals machine or self, but they are complaining of shortness of breath, don’t dismiss the patient’s complaint. I would trust what a patient says over a reading any day. This makes pretty much common sense, but it should be said, because it was drilled into my head in EMT school.
How about the opposite? A patient’s reading is low, say in the 80’s, but they say they are breathing fine. I would keep an eye on this patient and trust the pulse oximeter more. It is better to err on the side of caution in any case than to compromise a patient’s safety.
Second, some factors can affect readings. For example, a patient with carbon monoxide poisoning will have incorrect oxygen saturation readings due to how carbon monoxide has a higher affinity for binding to hemoglobin than oxygen does, thus a reading with a carbon monoxide patient such as with smoke inhalation from a fire will present as if the patient is not hypoxic with say, a 100% reading. But as we have learned about how pulse oximeter’s work, they can’t really differentiate what is bound to the hemoglobin, only how much of them are bound or not bound as a percentage. Interesting isn’t it?
Third, as pulse oximeter’s use red and infrared light to detect oxygen saturation, being outside in bright light or even inside with artificial light sources can interfere with the equipment. Sunlight contains a wide range spectrum of light including infrared that can interfere with a pulse oximeter’s tiny little LED’s (less powered lights) and overpower them. Because I like to take my patient’s outside to get fresh air, do OT outside, or practice community mobility in a dynamic and challenging environment, I also like to take their O2 saturation and heart rates outside. What I do is cover their finger and pulse ox with my own hands, kind of like making a tent to minimize the amount of external light reaching the detector.
Other things like low battery and cold temperatures are known to interfere with readings. Sometimes you may not even get a reading at all. If this happens, well change the battery first, but also try using other fingers. Ear lobes are also a good spot to take a reading. Nail polish can also interfere with readings since it created a barrier that the 2 LEDs need to pass through. Watch for other signs of hypoxia such as using accessory muscles, tripod posture, cyanosis, diaphoresis, lethargy, and confusion.
To bring home the point about devices such as these is that they are just 1 single piece of equipment. OTs should use their professional judgment and reasoning and look at the big picture (top down) and not bottom up all the time, although it is easy to because they can help us quantify our sessions.
COVID-19 and COPD
As we are living in a COVID-19 world, I wanted to mention O2 saturation ranges. Ideally, your patient would have like a 98, 99, 100% O2 saturation. However, with patients who have COPD, their therapeutic ranges are much lower, 88-92%. I don’t have too much experience working with patient’s who have had COVID-19, but I have heard about allied health professionals seeing some crazy low numbers like 50’s, 40’s, 30’s. At this point, how accurate the readings are compared to say 70’s to 90’s , I don’t know, but it is pretty telling of how horrible Sars Cov V2 and COVID-19 is to humans.
If you are a non-professional or even a professional watching this, especially as an OT, it is a good idea to purchase one. I had a family member contract COVID-19 and I lent them my pulse oximeter and it not only gave them some reassurance, but it helped give them additional insurance of when or whether or not to go to the hospital. I’ll end this with a very powerful story.
Case Study
A patient who contracted COVID-19 was in the ICU for I forgot how long, but probably weeks before I worked with them. They were even in a coma for a while and not expected to make it. Remember that many patients were not allowed to have visitors. His wife begged to come visit and talk to him, kind of like how Trinity did to Neo in the Matrix movie. Amazingly, he regained consciousness in a few days and was admitted to our acute rehab program. Keep this mind this is anecdotal. But the patient told me that his wife purchased a pulse oximeter when he began to develop symptoms and tested positive for COVID-19.
One day his reading was in the low 80’s. His wife brought him to the hospital and not long after, he was in respiratory distress. Without the pulse oximeter, he likely would not have known and would have lost some valuable time waiting at home for EMS to arrive to even give him oxygen. Just like how important automatic blood pressure machines have become popular and available for everyone in the home, pulse oximeter’s are the same, there was just more demand than supply. In a post-COVID-19 world, I don’t know, maybe everyone should have one of these in their homes too. If you are a COVID-19 survivor, we still don’t know the long-term effects on our bodies, so I recommend you get one for you and your family. What type or model doesn’t really matter, just get one.
