The 'Pulse Oximeter' has expanded into the vocab of many with this pandemic.This device is used to measure the oxygenation in your body - which is the concern of physicians as the covid19 virus can inflame the lung bad enough to reduce one's capacity to send enough oxygen to his cells drastically.
So how does this tiny device function ?
The oximeter uses light energy to estimate the oxygen saturation in our blood.As illustrated in the image above, oximeter has a light source from which light is emanated.The photodetector at the opposite side,captures these light particles.
Your finger is sandwiched in a way so that the light emanated passes through the artery in your finger before being detected by the photodetectors.
Why So ?
Oxygen is carried by the blood.The saturation of oxygen is determined by the number of hemoglobins ( stuff in your blood which can bind with oxygen,thus circulating it to all cells ) which are bound to oxygen.
A hemoglobin which carries oxygen with it absorbs light of a particular wavelength.A hemoglobin molecule not carrying oxygen with it on the other hand,absorbs light of another range of wavelength.
You might have noticed an oximeter emitting red light.This is what your eyes perceive.The device actually emits two forms of light energy.
One is red ,and the other - infrared.Humans cannot visually perceive infrared light,but we can feel it as heat.
So if all the hemoglobin in your blood carries oxygen,the oximetry reading : Sp02 would be 100 %,because all of the red light will be absorbed by the blood.
On the other hand,having some amount of non-oxygen bound hemoglobin will show a lower percentage of Sp02,because they would absorb the infrared waves.
In other words,the oximeter shows reading based on how much of red light vs infrared light is being absorbed.
Nail Polish
Now, your polished nail can alter the oximeter reading.Black,blue and green nail polish significantly lower the reading - giving an impression of desaturation.
The varnish in the nail polish apparently absorbs the light emitted in the device.And as we had seen earlier,we know how important the light is in giving an accurate reading !
Arterial Blood Gas ( ABG )
Another accurate way of knowing your oxygen saturation is by taking an arterial blood and sending it to the lab.However,this is an invasive procedure where you explore a needle into an artery.This is a very painful procedure - God knows how many patients have cursed me to damnation for torturing them with ABGS..
Image : Arterial blood is taken with the needle in 45 degree angulation.In a way,it is synonymous to piercing your flesh with needle.
Reference :
(1) Coté CJ, Goldstein EA, Fuchsman WH, Hoaglin DC. The effect of nail polish on pulse oximetry. Anesth Analg. 1988 Jul;67(7):683-6. PMID: 3382042.
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