The science behind the pulse oximetry

☝️️Pulseoximeter measures the percentage of arterial hemoglobin in the blood that is saturated with oxygen
☝️️It consists of 2 LEDs & a photodiode arranged on either side of an adhesive strip and an electronic processor 
☝️️Light from LEDs travel through the patient's body part and is detected by the photodiode
☝️️One LED emits light at 660 nm (red light) and the other at 940 nm (infrared light). 
☝️️Oxyhaemoglobin and deoxyhaemoglobin absorb these wavelengths differently
☝️️Oxyhaemoglobin absorbs more infrared light (940 nm) and allows more red light (660 nm) to pass through. 
☝️️Deoxyhaemoglobin absorbs more red light (660 nm) and allows more infrared light (940 nm) to pass through.

☝️️Isobestic point is at 806 nm

☝️️The LEDs flash in sequence: one on, then the other, then both off (to allow correction for ambient light). This triplet sequence happens 30 times per second

☝️️The amount of light transmitted through the patient at each frequency is detected by the photodiode. 
☝️️The microprocessor corrects for ambient light, and also for the difference between arterial and venous saturations by deducting the minimum transmitted light, during diastole, from the maximum during systole.
☝️️After this, the ratio of oxy to deoxyhaemoglobin is determined and from this the percentage oxygen saturations is determined, using an empirical table derived from healthy volunteers who were exposed to varying degrees of hypoxia.
💅🏽Apart from the common causes like movement, nail varnish, diathermy,  others like 
🔻severe anaemia 

🔻cardiac arrhythmias 

🔻Methaemoglobinaemia (characteristically cause saturations to be measured at around 85%) 

🔻Increased venous pulsation, e.g. severe tricuspid regurgitation 

🔻i.v. methylene blue dye (because it absorbs light in the 660–670 nm range 
also may cause erroneously low readings)
💅🏻Carboxy hemoglobin (CO-Hb has similar absorption spectra as that of oxy-Hb) is detected by normal pulse oximeters as oxy hemoglobin--> erroneous high readings 
💅🏻Cyanide prevents oxygen being utilised in respiration and so its extraction from the blood falls; so in cyanide poisoning, though the value is not inaccurate, it should be interpreted as inappropriately high.
☝️️Fetal haemoglobin and Hb S (sickle) do not affect readings
☝️️The human volunteers used to construct empirical saturation tables did not have their oxygen saturations dropped below approximately 85%; hence readings below this number are extrapolated, not validated.