Understanding at Home Pulse Oximetry for Covid-19
What is pulse oximetry?...
How does Covid-19 affect these readings?...
Is it actually a good idea to buy a pulse oximeter?...
At home pulse oximetry has been called a gamechanger for managing this Corona Virus Pandemic by an ER doctor(1). Given that a pulse oximeter is accurate, and the operator can competently use it, at home pulse oximetry could be useful in recognition of potential symptoms. Here we'll briefly discuss the mechanism of pulse oximetry, whether or not they may be useful in detecting early Covid-19 symptoms, and then whether using one at home would actually be worthwhile.
There are two main factors which influence the usefulness of at home pulse oximetry; number one is of course the quality of the instrument being used, and number two is the skill and understanding of the user to interpret the results, and account for any artificial and environmental influences which are discussed further below.
A cardinal sign of Covid-19 beside pyrexia and a dry cough, is shortness of breath (SOB). SOB is caused by Covid-19 pneumonia, and it is also caused by Covid-19-related low SpO2 (a somewhat similar reason patients experiencing an acute myocardial infarction experience SOB). Interestingly though, there are some patients who won’t experience SOB, despite having either Covid-19-related low SpO2 or Covid-19 pneumonia.
SO, does that mean everyone should go and start measuring their SpO2?...
Before we can make that kind of conlcusion, let’s learn a bit more about a pulse oximeter, while bearing in mind that much more information is required to determine the potential for Covid-19 infection aside from accurate pulse oximetry. If there is any suspicion of infection or flu like symptoms at all, call your public health hotline and follow their instructions!
What is a pulse oximeter?
A pulse oximeter measures the proportion of haemoglobin-bound oxygen circulating in your blood – in short, it measures how much oxygen your red blood cells are carrying compared to their maximal capacity.
Light is emitted by a pulse oximeter through your skin, typically your finger. Using a host of complex mathematical algorithms, the device then estimates the level of oxygen saturation by measuring the angles and intensity at which light is reflected. Some smart watches and mobile apps claim to measure user SpO2 but a study actually showed that readings by these types of devices were unreliable (2).
Pulse oximeters themselves are not perfect indicators of a person’s actual SpO2, but they’re much more convenient to use than taking arterial blood gas measurements every time you want to know your SpO2, and they’re still much more accurate than everyday smart devices.
Hence, they’re a fundamental piece of equipment used on just about every patient transported in an ambulance or treated in a hospital. They’re also often used at home for people with some of the chronic health conditions mentioned earlier (COPD etc).
What factors affect SpO2 readings?
It’s no surprise that 100% is the highest saturation you can get and is close to what most healthy people will find their reading to be. Various conditions can affect the accuracy and the validity of SpO2 measurements. These include diseases such as Chronic Obstructive Pulmonary Disease due to blood shunting, diabetes due to peripheral vascular disease(3). In fact, 92% could be a ‘normal’ baseline for a COPD patient! Therefore, someone with a normative baseline of 98% who is now saturating at 93% is much more concerning than someone who’s baseline is 92%.
Other factors such as cold extremities, nail polish and artificial nails, poor circulation (Reynaud’s Syndrome), dyslipidaemia, and even the lighting of the room can all affect SpO2 readings.
Therefore, trends are to be considered more important than individual readings. Recognising consistent trends accounts for any individual peculiar reading due to interfering conditions.
How does Covid-19 affect SpO2?
Our understanding of Covid-19 is constantly in development. Much we once thought we knew about is being challenged as each day brings us new data. Originally low oxygen levels were thought to be a relatively late indicator that a person has Coronavirus. However, increasing reports of cardiovascular abnormalities are making us reconsider this. One paramedic from New York City recounts a case she visited:
“There was someone who was at 34 per cent and still awake and talking to us. We thought, ‘try a different finger, try a different finger’,” says Megan.
“But it actually was her oxygen that low. I’ve never seen a person alive with a number like that.” (4)
While there are numerous theories as to how this may be occurring, the important thing is that we really don’t know. The destruction of vascular endothelial cells resulting in the release of Von Willebrand factor which reacts with free floating Clotting Factor XIII has been plausibly postulated to account for a prevalence of embolic strokes in middle aged Covid-19 patients(5). It is possible that this may mimic peripheral vascular disease which could account for the low SpO2 readings (3,5).
So, is a pulse oximeter useful to buy?
That’s up to you to decide. Although normal oxygen saturation (SpO2) is reassuring, it cannot indicate one’s comprehensive health or respiratory condition; even a ‘healthy’ SpO2 can rapidly deteriorate, and one may have Covid-19 even with an SpO2 in the normal range.
If you do choose to buy one, it’s important to know that some are better than others. There are so many manufacturers, and they all have their own algorithms and accuracy levels. Before buying be sure to check that the listing you are looking at lists the accuracy level and operating parameters as seen here.
1. Johnson K. COVID-19: Home Pulse Oximetry Could Be Game Changer, Says ER Doc [Internet]. Medscape Medical News. 2020 [cited 2020 May 10]. Available from: https://www.medscape.com/viewarticle/929309
2. The Centre for Evidence-Based Medicine. Question: Should smartphone apps be used clinically as oximeters? Answer: No. [Internet]. 2020 [cited 2020 May 10]. Available from: https://www.cebm.net/covid-19/question-should-smartphone-apps-be-used-as-oximeters-answer-no/
3. Kwon J, Lee W. Utility of digital pulse oximetry in the screening of lower extremity arterial disease. 2012;94–100.
4. Vyas K. Behind enemy lines [Internet]. Foreign Correspondent. 2020 [cited 2020 May 10]. Available from: https://www.abc.net.au/news/2020-04-21/coronavirus-paramedics-doctor-on-the-frontline-new-york-covid-19/12139676?nw=0&pfmredir=sm
5. Robert Escher, Neal Breakey BL. Severe COVID-19 infection associated with endothelial activation. Thrombosis. 2020;(January).