11A SpO2 & SctO2

Visible light 380-800nm Infrared

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

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

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

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

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

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

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

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

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?

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

SpO2 not accurate Actual SaO2 decreased % oxygenated Hgb

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

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

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

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

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

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

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

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

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)

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

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

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