11A SpO2 & SctO2 Flashcards Preview

AS - N927 Chem/Physics > 11A SpO2 & SctO2 > Flashcards

Flashcards in 11A SpO2 & SctO2 Deck (26)
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1

Electromagnetic Specrum

Visible light
380-800nm

Infrared

2

Colors

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

3

Red Light

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

4

Spectrophotometry

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

5

Absorbance

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

6

Deoxygenated Hemoglobin

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

7

Oxygenated Hemoglobin

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

8

Pulse Oximetry (SpO2)

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

9

Pulsatile Blood Flow Significance

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

10

Pox Limitations

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?

11

Methemoglobin

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

12

Carbon monoxide

SpO2 not accurate
Actual SaO2 decreased % oxygenated Hgb

13

SctO2

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

14

Cerebral Oximeter Components

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

15

SctO2 Measures:

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

16

Cerebral Oximetry Anatomy

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

17

SctO2 Depth

Approximately 1.5cm or 15mm
1/3 distance b/w light emitter and detector

18

Cerebral Oxygen Saturation Calculation

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

19

Supply Factors

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

20

Demand Factors

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

21

SctO2 Considerations

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)

22

Advantages

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

23

Disadvantages

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

24

Intraoperative Interventions

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

25

LED

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

26

LASER

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