Pulse Oximetry (II)/Co-Oximetry

Question 1

Two Types of Pulse Oximetry Technology

Answer 1

1. Transmission Pulse Oximetry
2. Reflectance Light Pulse Oximetry

Question 2

Transmission Pulse Oximetry

Answer 2

light signal transmitted through tissue bed to photodetector on opposite side of light emitting diode

Question 3

Reflectance Light Pulse Oximetry

Answer 3

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

Question 4

Beer-Lembert Law

Answer 4

Measured absorbance for single compound directly proportional to concentration of compound, length of light path through the sample

Question 5

Consequences of Hypoxemia

Answer 5

anaerobic metabolism, cell death, insufficient organ function
o Most vulnerable = brain, heart, kidney

Question 6

MOA Pulse Oximetry

Answer 6

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

Question 7

Beer Portion of Beer Lembert Law

Answer 7

concentration of given solute in a solvent determined by amt of light absorbed by solute at a specific wavelength

Question 8

Components of Pulse Oximeter

Answer 8

o LED light source
o Photometer
o Control circuit
o User interface with display, alarm functions

Question 9

Oxygen Dissociation Curve

Answer 9

Same pulmonary processes that determine PaO2 determine SaO2
 SaO2 used as surrogate for PaO2, SpO2 used as surrogate for SaO2

Question 10

Hypoxemia

Answer 10

SaO2/SpO2 <95%, serious hypoxemia <90%

Question 11

Calculation of SaO2 by Blood Gas Analyzers

Answer 11

SaO2 = calculated value, usually based on normal human dissociation curve
 Curve variations btw species, age/individuals within breeds

Question 12

P50

Answer 12

PO2 at which hgb 50% saturated
 Defines position of curve
 Higher numbers: curve shifted R
 Lower numbers: curve shifted L

Question 13

Significance of a Left Shifted Oxygen Dissociation Curve

Answer 13

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

Question 14

Significance of a Right Shifted Oxygen Dissociation Curve

Answer 14

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

Question 15

Cats P50/Curve Shift

Answer 15

P50 31-36 (34.1), curve shifted R

Question 16

Dogs P50/Curve Shift

Answer 16

P50 26-36 (29), curve shifted R

Question 17

Horses P50/curve shift

Answer 17

P50 ~24, curve shifted L

Question 18

Sheep P50/Curve Shift

Answer 18

30-40mm Hg, curve shifted R

Question 19

Camelids P50/Curve Shift

Answer 19

curve shifted L, P50 ~17-18

Question 20

Bovine P50/Curve Shift

Answer 20

P50 25-31, curve shifted L

Question 21

If accuracy about hemoglobin saturation required…

Answer 21

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

Question 22

Sites for SpO2 Measurement

Answer 22

tongue, lip, digit/interdigital web, axillary/inguinal fold, pinna, prepuce, vulva, etc

Question 23

Challenges of Pulse Oximeters

Answer 23

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

Question 24

Corrections for Ambient Light

Answer 24

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

Question 25

Fetal Hgb, Blood Based Solutions

Answer 25

VERY LITTLE effect on measured hgb saturation

Question 26

Pulsatile Requirements of SpO2

Answer 26

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

Question 27

False Readings

Answer 27

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

Question 28

Effect of MetHgb

Answer 28

SpO2 85%, biphasic absorption of 660 and 940nm

Question 29

Effect of Carboxy Hgb

Answer 29

false increases in hgb, absorbs 660nm not 940nm

Question 30

Methemoglobin

Answer 30

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

Question 31

Advantages of Pulse Oximetry

Answer 31

• Fast Response Time
• Readings not affected by ax agents
• Non-invasive
• Continuous
• Convenient, user friendly
• Compact, light weight, portable
• Cost Effective

Question 32

Disadvantages of Pulse Oximetry

Answer 32

• Poor function with poor perfusion
• Erratic performance with dysrhythmias
• Cannot distinguish btw O2hgb, COhb
• Errors with metHb

Question 33

Co-Oximetry

Answer 33

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

Question 34

Uses of Co-Oximetry

Answer 34

Noninvasive, real time continuous measurements
o [Hgb], methemoglobin, carboxyhemoglobin, pulse co-oximeter-derived SaO2, calculated oxygen content, perfusion index, plethysmogram variability index