11A SpO2 & SctO2

Question 1

Electromagnetic Specrum

Answer 1

Visible light
380-800nm

Infrared

Question 2

Colors

Answer 2

Wavelengths reflected or transmitted to eyes
White - reflect all visible light wavelengths
Black - absorbs all wavelengths

Question 3

Red Light

Answer 3

620-750nm
Any object that reflects red part of spectrum will appear red
Red - molecules absorb all light wavelengths except red spectrum (660nm)

Question 4

Spectrophotometry

Answer 4

Method to measure how much a chemical substance absorbs light by measuring the intensity of light that has passed through a sample solution

Question 5

Absorbance

Answer 5

Measures quantity of light absorbed by sample
Light passes through sample and none absorbed
Absorbance = 0 and Transmission 100%
Therefore solution would be pure solvent
Inverse relationship to transmission

Question 6

Deoxygenated Hemoglobin

Answer 6

Absorbs light at 660nm (red wavelength)
Reflects less red to the eye
Venous blood appears more blue

Question 7

Oxygenated Hemoglobin

Answer 7

Arterial blood appears more red
Poor 660nm absorption
Absorbs 940nm (infrared)

Question 8

Pulse Oximetry (SpO2)

Answer 8

Measure difference between how much 660 and 940nm absorption
Calculates proportion oxygenated to deoxygenated Hgb
SpO2 denotes oxygen saturation from pulse oximetry
Measured at peripheral capillary
Non-invasive SaO2 estimate
SpO2 reflects SaO2 (100% arterial supply)
Exception: Carbon monoxide poisoning

Question 9

Pulsatile Blood Flow Significance

Answer 9

Wider capillary during systole d/t increased blood volume in the vessel ↑ volume ↑ diameter or path length
Diastole - relaxation decreased absorption
Beer-Lambert law application
↑ path length ↑ absorption
↓ path length ↓ absorption
Systole - diastole = only blood

Question 10

Pox Limitations

Answer 10

Calibration based on light skin
Normal operating range minimal to no change b/w light and dark skin
Low SaO2/SpO2 levels less accurate
Consider how devices calibrate - on who & what conditions?

Question 11

Methemoglobin

Answer 11

Hemoglobin in metalloprotein form - iron in the heme group in the Fe3+ (ferric) state not Fe2+ (ferrous)
CanNOT bind oxygen ჻ cannot carry/transport O2 to tissues
Most cases acquired rather than inborn
Exposure to certain oxidizing substances may lead to conversion from normal Hgb to methemoglobin
Known toxins can cause methemoglobinemia - aniline dyes, nitrates or nitrites, medications (i.e. local anesthetics)
Pox reading 85%
↑ O2 binding affinity normal Hgb resulting in ↓ unloading O2 to the tissues

Question 12

Carbon monoxide

Answer 12

SpO2 not accurate

Actual SaO2 decreased % oxygenated Hgb

Question 13

SctO2

Answer 13

Cerebral tissue oxygenation or cerebral oximetry
Based on near infrared spectroscopy (NIRS)
Normal values 60-80%
Assess baseline values in awake patients & monitor trends
Adequate cerebral oxygenation dependent on CBF and oxygen content

Question 14

Cerebral Oximeter Components

Answer 14

1. Light source
   Produces NIR light w/ known wavelengths & intensity passes through skin, skull, & cerebral tissue
2. Light detector
   Measures light intensity exiting the cerebral tissue reflected back to the detector
3. Computer
   Coverts light intensity exiting the cerebral tissue into amount O2Hgb and Hgb - calculations to determine O2 saturation

Question 15

SctO2 Measures:

Answer 15

Non-pulsatile blood flow
Venous & arterial O2Hgb & Hgb
How much O2 delivered & returned
Supply & demand

Question 16

Cerebral Oximetry Anatomy

Answer 16

Light source emits
Two sensors/detectors
Light returns in elliptical shape (bends) d/t skull curvature
Total (skull + brain) - skull transmission = brain absorption or cerebral oxygenation
Superficial reading/measurement

Question 17

SctO2 Depth

Answer 17

Approximately 1.5cm or 15mm

1/3 distance b/w light emitter and detector

Question 18

Cerebral Oxygen Saturation Calculation

Answer 18

Based on SaO2 & SjbO2 (jugular bulb)
Use arterial:venous ratio 30:70%
SavO2 = 0.3 x SaO2 + 0.7 x SjbO2
Strong correlation b/w calculated & measured (SctO2)
Limited value d/t 3-5cm depth

Question 19

Supply Factors

Answer 19

↑O2
BP
CO ↑CBF
PaO2/SaO2 = CaO2
Cerebral vasodilation
Hgb/Hct

Question 20

Demand Factors

Answer 20

Cerebral metabolic rate determines O2 utilization & return
↑ metabolism (not matched by autoregulation vasodilation ↑CBF)
Stroke patients lose or ↓ autoregulation
Compromised blood flow impairs autoregulation
↑ temperature
Anesthetics ↓ cerebral metabolic rate ↓ CBF

Question 21

SctO2 Considerations

Answer 21

Supply - CO/BP (hemorrhage or embolism)
Oxygen - CO2/FiO2
Unload - arterial pH
Metabolic rate - temperature
Autoregulation - local blood flow
CaO2 - Hgb concentration
Pre-existing disease
Position changes - neck rotation compresses carotid
↓ SctO2 ↑ cerebral metabolic rate (demand) OR ↓ CBF (supply)

Question 22

Advantages

Answer 22

Non-invasive and no specialized training
Real time oxygenation status from that brain region
Provides information about oxygen supply and demand balance

Question 23

Disadvantages

Answer 23

Does not measure global oxygenation
Limited penetration depth (large part brain not monitored)
Measures only intravascular oxygenation - not true reflection intracellular oxygen availability
Cannot differentiate neurologic dysfunction cause
Electrocautery can interfere w/ cerebral oximetry

Question 24

Intraoperative Interventions

Answer 24

Adjust head position
↑ anesthetic depth
↓ temperature
↑ FiO2
↑ PaCO2 (minute ventilation)
↑ CO/MAP
↑ Hct
All r/t supply & demand

Question 25

LED

Answer 25

Somanetics INVOS
Uses LED light at 2 wavelengths
Measures change in regional oxygen saturation (rSO2)

Question 26

LASER

Answer 26

CASMED Fore-Sight
Uses LASER light at 4 wavelengths
Measures actual cerebral tissue oxygenation saturation (SctO2)