Pulse Oximetry (II)/Co-Oximetry Flashcards
Two Types of Pulse Oximetry Technology
- Transmission Pulse Oximetry
- Reflectance Light Pulse Oximetry
Transmission Pulse Oximetry
light signal transmitted through tissue bed to photodetector on opposite side of light emitting diode
Reflectance Light Pulse Oximetry
where light signal reflected off tissues back to optical sensor on same side as light emitting diode
Limitations: weaker signals than transmission pulse oximetry, reading may be artificially low if probe over artery or vein, VC - overestimation of SpO2, , probe must eliminate light passed directly to probe or scattered on outer surface of skin
Beer-Lembert Law
Measured absorbance for single compound directly proportional to concentration of compound, length of light path through the sample
Consequences of Hypoxemia
anaerobic metabolism, cell death, insufficient organ function
o Most vulnerable = brain, heart, kidney
MOA Pulse Oximetry
measures HbO2 saturation –> saturation proportional to differential absorbance of red (660nm – deoxygenated), IR light (940nm - oxygenated)
o Requires pulsatile BF
o Oxygen-Dissociation Curve
o Spectrophotometry
Beer Law
Concentration of oxy, deoxy hgb determined from their absorption of the two wavelengths
Beer Portion of Beer Lembert Law
concentration of given solute in a solvent determined by amt of light absorbed by solute at a specific wavelength
Components of Pulse Oximeter
o LED light source
o Photometer
o Control circuit
o User interface with display, alarm functions
Oxygen Dissociation Curve
Same pulmonary processes that determine PaO2 determine SaO2
SaO2 used as surrogate for PaO2, SpO2 used as surrogate for SaO2
Hypoxemia
SaO2/SpO2 <95%, serious hypoxemia <90%
Calculation of SaO2 by Blood Gas Analyzers
SaO2 = calculated value, usually based on normal human dissociation curve
Curve variations btw species, age/individuals within breeds
P50
PO2 at which hgb 50% saturated
Defines position of curve
Higher numbers: curve shifted R
Lower numbers: curve shifted L
Significance of a Left Shifted Oxygen Dissociation Curve
Increased affinity for O2, decreased release from tissues
At the same PaO2, SpO2 increased eg PaO2 60, SpO2 100
At the same SpO2, PaO2 decreased eg SpO2 75, PaO2 ~35
Think: camelids live at high altitude, want to hang onto as much O2 as can
Significance of a Right Shifted Oxygen Dissociation Curve
Decreased affinity for oxygen, increased release to tissues
Think: exercising muscles want more oxygen so hemoglobin delivers it to them
At same PaO2, SpO2 will be decreased eg PaO2 60, SpO2 ~80
At same SpO2, PaO2 will be increased eg SpO2 75 = PaO2 45
Cats P50/Curve Shift
P50 31-36 (34.1), curve shifted R
Dogs P50/Curve Shift
P50 26-36 (29), curve shifted R
Horses P50/curve shift
P50 ~24, curve shifted L
Sheep P50/Curve Shift
30-40mm Hg, curve shifted R
Camelids P50/Curve Shift
curve shifted L, P50 ~17-18
Bovine P50/Curve Shift
P50 25-31, curve shifted L
If accuracy about hemoglobin saturation required…
must measure oxyhemoglobin: absorbance spectrum of dog, cat, horse, cow, pig oxyhemoglobin sufficiently similar to human hgb
Analyzers based on human algorithms of light absorbance satisfactorily accurate
Sites for SpO2 Measurement
tongue, lip, digit/interdigital web, axillary/inguinal fold, pinna, prepuce, vulva, etc
Challenges of Pulse Oximeters
o Most frequent problems: motion artifact, low signal‐to‐noise ratio
o Skin/skin pigment, tissue, venous capillary blood also absorb IR light
o Differences in tissue absorption or scattering of light, different thicknesses of tissue, smaller pulsatile flow patterns and electrical or optical interference may account for some inaccuracies
o Perform poorly in severe situations such as severe hemoglobin desaturation (<70%), severe anemia (PCV <10%) and severe vasoconstriction
Corrections for Ambient Light
strobe at high frequency (400-900Hz)
When LED off, photometer measures absorption of ambient light – subtracts that from signal measured with LEDs on, eliminates contribution of most ambient light
Fetal Hgb, Blood Based Solutions
VERY LITTLE effect on measured hgb saturation
Pulsatile Requirements of SpO2
o Absorption over time signal from arterial blood = pulsatile, signal from venous hemoglobin/tissue is not
o Lembert’s Law: equal parts in same absorbing medium absorb equal fractions of light that enter them
Identification of pulsatile signal
False Readings
o VC: hypothermia, hypovolemia, drug-induced (dexmed)
o Pigment: bilirubinemia, melanin
o Optical interference from ambient light
o Motion
o Duration of Probe
o Carboxyhgb, methgb
Effect of MetHgb
SpO2 85%, biphasic absorption of 660 and 940nm
Effect of Carboxy Hgb
false increases in hgb, absorbs 660nm not 940nm
Methemoglobin
oxidized hgb (Fe3+), cannot bind oxygen
o Imparts brownish to bluish discoloration to MM, SpO2 85%
o Proportionally reduces concentration of oxyhemoglobin
o PaO2 normal bc pulmonary function normal
Advantages of Pulse Oximetry
- Fast Response Time
- Readings not affected by ax agents
- Non-invasive
- Continuous
- Convenient, user friendly
- Compact, light weight, portable
- Cost Effective
Disadvantages of Pulse Oximetry
- Poor function with poor perfusion
- Erratic performance with dysrhythmias
- Cannot distinguish btw O2hgb, COhb
- Errors with metHb
Co-Oximetry
Multi-wavelength spectrophotometry-based technology, incorporates use of 7-12 wavelengths of red, IR light
o Different hemoglobin species absorb light differently
o Co-oximeter then calculates the relative concentration of each of these hemoglobin species from patterns of light absorption
* Continuous hgb determination –> estim of [hgb] based on absorbance characteristics
Uses of Co-Oximetry
Noninvasive, real time continuous measurements
o [Hgb], methemoglobin, carboxyhemoglobin, pulse co-oximeter-derived SaO2, calculated oxygen content, perfusion index, plethysmogram variability index
