What causes false pulse oximeter readings
What causes false pulse oximeter readings ?
Pulse oximeters show limitations and accuracy issues under certain conditions.
Skin complexion may interfere with light beam and dark complexions are thought to be resulting false results.
High skin thickness may also make the same result by obstructing light passage through the finger.
Costumes
applied on fingertip may also affect the accuracy of results by
interacting with light beams. This is why it is advised to remove nail
polish or any other costumes on finger before attaching the oximeter.
Lowered
skin temperature may also cause a risk of inaccuracy. In lower
temperatures blood supply to the fingertip gets minimum. To measure
blood oxygen level through fingertip pulse oximeters it is essential to
have a good blood supply to the capillary bed of the finger.
A recent study claimed that the accuracy of oximeter readings drops when oxygen level drops below 80% in darkly pigmented skins.
All about pulse oximeters
What is a pulse oximeter?
A pulse oximeter is a medical device used in estimating blood oxygen levels. It is designed small and light weight to easily fit into a finger tip. With the pandemic, fingertip pulse oximeters came into household use as general wellness products. Earlier they were been used in medical facilities or in household for prescribed patients.
Although wrong handling techniques and limitations may cause false readings, fingertip oximeters have been proven to be life saving.
Oximeter sales are regulated by medical authorities under the two categories; over counter oximeters and prescription oximeters. Prescription oximeters are tested for accuracy and are intended for medical use. Over the counter, oximeters are not recommended for medical purposes since they are sold as general wellness products. Before buying an oximeter it is always important to check approvals and certifications given by your local health authorities. This is crucial since a black-market is rapidly growing around oximeter sales. Devices with no guaranteed accuracy are readily available in online platforms for couple of dollars.
When
purchasing an oximeter we recommend to purchase from a reputed health
equipment vendor (or pharmacy) that sells goods under the supervision of
local health authorities.
How do pulse oximeters work?
Oximeter uses a visual technique to estimate blood oxygen levels.Hence the device illuminates fingertip with red and infrared light which passes through capillary bed of finger tip.
Hemoglobin is the pigment that carries oxygen in the blood. Blood contains both oxygenated and de-oxygenated hemoglobin. When bound with oxygen hemoglobin absorbs (660nm) red light. De-oxygenated (unbound) hemoglobin absorbs (940nm) infrared light.
Photodetectors in the device measure the affinity of blood capillary bed for both wavelengths.
Red light absorption should proportionally increase when blood oxygen levels are high and vice-versa.When blood is illuminated with the above wavelengths and absorbance is measured an approximate value of blood oxygen level can be calculated using absorption coefficients.
How does a pulse oximeter determine oxygen saturation?
Pulse oximeter transmits a RED and an IR beam through the finger which finally incident on a photodetector. A microcontroller obtains signals from photodetectors and calculates an approximate oxygen concentration level based on relative absorption levels of red and infrared light.
While
light beams passes through the finger, it has to cross blood capillary
bed of fingertip. Hemoglobin in blood of blood capillary bed is
responsible for the major part absorption of light in above frequencies.
Simply it is just like colourimetry where blood oxygen concentration (
spO2 ) is estimated based on color of blood.
How do you read a pulse oximeter?
Manufactures' guidelines should be followed when using the device.
However, a generalized set of procedures will be;
Remove nail polish or any other costumes on the nail and make the finger warm by rubbing over the palm.This was done to dilate blood vessels and to have a good blood supply to the capillary bed of finger tip.
Sit still, keep the finger relaxed and below the heart level.
Attach the oximeter to the finger and wait until the reading is steady. It is important to wait and hold device still on finger until the reading stops changing.
How accurate are pulse oximeter devices?
Generally when a FDA-cleared oximeter registers 90%, the actual value will stay between 86% to 94%.
Because
of the limitations in oximeters they are more ideal to be used to
observe trends over time, rather than observing threshold levels.
However we advise to seek medical help when spO2 percentage drops below
95%
What should be the pulse oximeter reading for a normal person?
The normal value for home pulse oximeter reading is 95% or above. You should contact your healthcare provider if the oximeter registers a value below that. However, in mild respiratory conditions, this may fall up to 90%, and the standard cut-off of WHO is 88%.
Why is my spO2 reading on the pulse oximeter fluctuate?
Fluctuations of oxygen saturation values are normal. But what matters is whether it is associated with a pathological condition. Medical help should be sought if symptoms such as shortness of breath, dry cough, and chest pain also persisted.
Why do cold hands cause false pulse oximeter readings?
Under cold conditions, blood vessels constrict obstructing the relaxed blood flow. Oximeter detects oxygen level by the color of the blood. When blood supply to the finger is poor it causes accuracy issues.
Does a pulse oximeter help identify a coronavirus infection?
Pulse oximeters are not intended to use in covid diagnosis.
If you suspect yourself with covid-19 infection contact a healthcare provider or local health authority for reserving a diagnostic test. You may suspect yourself infected, if you have symptoms or if you had contact with an infected individual.
How useful is a pulse oximeter in detecting coronavirus risk?
Low oxygen levels indicate an early warning that medical care is needed.
Many covid-19 are feeling well, even when their oxygen levels are low. Even though oximeters do not function as a diagnostic tool it may be lifesaving by indicating risk in a terrible situation.
Which finger do you use for a pulse oximeter?
It is normally okay to be used in any finger of both hands.
References
- https://www.fda.gov/medical-devices/safety-communications/pulse-oximeter-accuracy-and-limitations-fda-safety-communication
- http://www.sportscienceresearch.com/IJSEHR_201822_03.pdf
- https://www.health.state.mn.us/diseases/coronavirus/pulseoximeter.html
