Pulse Oximetry Flashcards
Arterial Blood Oxygen Saturation (SaO2)
determined by ration of oxygenated (saturated Hgb) to deoxygenated hgb
What is the infrared that pulse ox absorbs?
Oxyhemoglobin absorbs infrared light 940nm
Deoxyhemoglobin absorbs visible red light 66nm
How does pulse ox work?
the probe contains 2 light emitting diodes; one in the red band and one in the infrared band. They also contain a sensor that detects the ration of red/infrared absorption ratios. Detection of absorption during pulsatile flow several hundred times/sec. Calc is done using an algorithm derived from Beer-Lambert Law
Components of the Beer Lambert Law
transmitted light, incident light, distance through the medium, concentration of solute, extinction coefficient of the solute
lamberts first law
the luminance on a surface illuminated byu light falling on it perpendicularly from a point source is proportional in the inverse square of the distance between the surface and the source.
Lambert’s second law
If the rays meet the surface at an angle, then the luminance is proportional to the cosine of the angle with the normal.
Lambert’s third law
The luminous intensity of light decreases exponentially with distance as it travels through an absorbing medium.
Beer-Lambert and Pulse Ox
the extinction coefficient is specific for a given solute at a given wavelength of light. Therefor, for each wavelength of light used an independent beer-lambert equation can be written, and if the number of equations equal the number of solute, then the concentration for each one can be solved.
How many wavelengths does pulse ox use?
2
What is the Isobestic point?
The absorption wavelength that is the same for both the saturated and desaturated Hgb
Advantages of Pulse Ox
non-invasive, easy to apply, continuous monitoring, earlier detection of desaturation (SaOS of 85%-= PaO@ of 55- no cyanosis…… SaO2 70%= PaO2 40mm- cyanosis)
inexpensive
Disadvantages to Pulse Ox
Prone to artifact, inaccurate at SaO2 values below 70%, rare risk of burns in poor perfusion states
Lag time for ear, finger, toe
Ear= 7-20 secs, Finger= 20-35 secs, Toe= 40-70 secs
What are some reasons for low accuracy?
low flow or low pulsatile states- cardiac bypass, hypotension, BP cuff inflation, tourniquets, vasoconstriction
Pulse ox inaccuracies…
Hgb<10 (underestimate), dyshemoglobins- carboxyhemeglobin (false high) methemeglobin (false high).
Dyes- methylene blue and indigo carmine underestimate SaO2 IC less than MB
Ambient light, seep skin pigment, electrocaudery, motion excessive, fingernail polish
