♈️#PhysicsForAnesthesiologist : Beer-Lambert Law

☢️The #pulseoximeter works based on Beer-Lambert law, which relates the attenuation of light to the properties of the material through which the light is travelling. 

☢️It helps us in the calculation of the absorbance of a solution.

☢️According to the law, the absorbance of a solution depends on:

🖍The concentration of that solution, i.e. the more molecules of a light-absorbing compound there are in the sample, the more light will be absorbed. 

🖍The path-length of light travelling through the solution, i.e. the longer the length of the sample container, the more light will be absorbed because the light will come into contact with more molecules.

🖍A = εlc where 

🔻A is absorbance of light

🔻ε is the molar extinction coefficient(l mol–1 cm–1). It compensates for variance in concentration and the path-length, to allow comparison between solutions. 

🔻l is the length of solution that the light passes through. 

🔻c is the concentration of the compound in solution, expressed in mol L–1

☢️In the pulse oximeter, the concentration and molar extinction coefficient are constant. The only variable becomes the path length, which alters as arterial blood expands the vessels in a pulsatile fashion.

#Anesthesia, #PhysicsAndMedicine , #MedicalExams

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.