Let’s Talk Pulse Oximetry
We have all seen them – either the sticker or clip on your finger attached to the vitals machine. The elusive pulse oximeter. There are also small portable ones that are the size of a snack sized snickers bar.
What exactly does a pulse oximeter do?
A pulse oximeter measures 2 different things- heart rate and oxygen saturation in the blood. The heart rate part is pretty straight forward. It picks up on the lub-dub of your heart and in turn shows the number on the screen. The oxygen saturation reading indicates the percentage of hemoglobin molecules in arterial blood which are saturated with oxygen. It works by shining a light through your finger and the sensors detect how much oxygen is in your blood based on the way the light passes passes through your finger. Since the oximeter detects the saturation peripherally on a finger, toe or earlobe, the result is recorded as the peripheral oxygen saturation, known as the SpO2.
Nowadays pulse oximeters are readily available to purchase both online and in most pharmacies to purchase over the counter. Back when I started my career in Respiratory Therapy many years ago I had to show my license in order to be able to purchase one without a prescription. That’s not the case anymore.
Limitations of pulse oximeters
There are limitations to pulse oximeters and the biggest one being that not all pulse oximeters are created equal. Oximeters that are less expensive than the hospital grade ones often times do not have the accuracy as those that you see in the hospital or that your respiratory therapist (such as myself) has in their pockets. There are also many things that can affect the reading and give a false one, such as nail polish, poor circulation to the extremities, movement (or shakiness), cold hands etc which are all things to take into consideration when getting a reading. Also, if your pulse ox has a display that shows a waveform you want to see that the waveform is uniform with the hills & valleys equal as it goes across the screen while giving a reading.
Here is a really good graphic that shows a good reading vs an inaccurate one:
Understanding pulse oximeter
Oxygen saturation is not a good indication of how bad your asthma flare up is. Asthmatics are notoriously good oxygen compensators and can have near normal or completely normal oxygen stats during the midst of a severe asthma attack. Asthma is a disease of the airways, not the little air sacs (alveoli) where the gas exchange itself takes place. So even if your airways are very constricted and tight, often enough oxygen is able to pass through to your blood and therefore show a normal or near normal oxygen saturation. Now if you have been working really hard to breathe for a time (which can be days if you’re in the middle of a bad flare up) your body will eventually tire out as you won’t be able to compensate anymore and your oxygen levels will start to rapidly drop.
If your oxygen levels are in the normal range (textbook normal range for oxygen saturations is 92% to 100%) it definitely does not mean you aren’t suffering nor having a potentially severe asthma attack. It just means that you are still able to oxygenate your body. When an asthmatic’s oxygen levels start to trend downward is when immediate medical attention is required (if you aren’t already at the hospital or doctors office) as it can be a sign of impending respiratory failure.
I want to really stress that symptoms trump numbers. I’ll say it again: SYMPTOMS TRUMP NUMBERS. If you are having difficulty breathing go off that vs the number you see on a screen. I teach new respiratory therapists, nurses and interns etc that you always treat the patient and not the numbers you see.
While pulse oximeters are great & can be a handy little tool, knowing exactly what they do and their limitations can be helpful.