Cerebral Oximetry

Wesam Farid MousaAssist prof Anesthesia &

ICUDammam University

Experts have created helmets to protect the brain of motorcyclists

Under general Anesthesia, the human brain can be in danger and the anesthesiologists are

sometimes faced with neuronal dysfunction/death during anesthesia

NEURODETECTION & NEUROPROTECTION

• No “real time” bioassays to detect the onset of brain injury

•CT, ECHO, EEG & MRI are late indicators of brain injury

• The need is critical: Cerebral ischemia is the

leading cause of compromised neurocognitive outcomes

The duration of reduced oxygenation has a direct impact on brain function

• THE EVIDENCES Approximately 465,000

cardiopulmonary bypass surgeries performed in the U.S. each year

Cerebral oxygen desaturation occurs in 17%-23% of cardiopulmonary bypass surgeries (or about 79,000-107,000 cases per year) Most desaturation events cannot be detected by common monitoring devices

• THE EVIDENCES• Adverse neurologic outcomes have been

reported in 6% to 53% of patients undergoing cardiopulmonary bypass surgeries

• Neurological complications vary in

severity; some are transient and some are permanent

• Adverse cerebral outcomes include focal injury, stupor, coma, seizures, memory deficit or deterioration in intellectual function

Incidence of Cerebral Desaturation Events

WHY CEREBRAL OXIMETRY?

• NIRS cerebral oximetry is non-invasive and continuous “real-time” detection that guides clinicians in their interventions

WHY CEREBRAL OXIMETRY?

• The brain: Complex and fragile system The brain weighs about 1200 to 1500

grams, accounts for only 2% of body weight, but receives 15% of total cardiac output and utilizes 20% of the oxygen

Elapsed time is critical in desaturation events

Why is Cerebral Oximetry different from other parameters?

° Cerebral Oximetry (SctO2) measures regionalmetabolism and the balance of local

tissue oxygen supply and demand.

° SctO2 is a combined saturation measurementof arterial blood and venous blood.

° other monitoring modalities are invasive

Cerebral Oxygen Saturation Consensus Level

From Pulse Oximetry to Cerebral Oximetry•Both are based on Beer-Lambert’s Law that describes light absorption

• Pulse Oximetry: Only Two wavelengths! Using light loss (Systole – Diastole) to calculate only signals due to pulsation of arterial blood to derive arterial O2 saturation.

• Cerebral Oximetry: Looking into entire nonpulsating field to derive tissue O2 saturation. It requires precise light sources and more wavelengths to account for light lost from other elements -as melatonin- in addition to Oxy- and Deoxy-Hemoglobin.

Light lost due to pulsation of arterial blood

Light lost due to non-pulsating arterial blood

Light lost due to venous blood

PulseOximetry(2 λ)

CerebralOximetry(multiple λ)

Light lost from the Background:• Light lost due to other molecules• Light scattering lost

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Arterial Pulse Oximetry

• Tells how much oxygen blood is carrying in arterial system (oxygenated Hb)

• Unable to determine how much oxygen is used (venous system)

• Unable to determine how different organs use oxygen (global vs regional) Accuracy about 2% (not accurate below 85%)

SpO2 is100% arterial blood

i.e., all pulsing

SctO2 is mix of:• 30% arterial • 70% venous

• no pulse needed

VeinsArteries

Tissue (Arterioles, capillaries, venules)

Pulse Oximetry SpO2 : Arterial

Oxygen Saturation Normal range:

90-100%

Cerebral OximetrySctO2: Regional TissueOxygen SaturationNormal Range: 60-80%

SjvO2: Jugular VenousOxygen SaturationNormal Range: 50-70%

sats95%

VBAG65%

NIRS

The Scientific Basis For CerebralOximetry NIRS

The Spectrum of Light The NIRS “Window” From 650nm to 900nm The Modified Beer-Lambert Law The Elliptical Photon Pathway In The Brain Chromophores In Cerebral Tissue Random Background Scattering Of Light More Wavelengths = Greater Accuracy Depth Of Penetration

The Spectrum of Light

• The term “spectrum” for the colors produced by a prism,

• colors in sunlight can be recombined to make white light.

Sir Isaac Newton1642-1727

Microwave Infrared Near-infrared Visible Ultraviolet X-ray Gamma ray

Window for NIRS Between 650 and 950 Nanometers

Fundamental Principle of Oximetry: Beer – Lambert Law

 Emitter-detector pairs showing the banana-shaped paths of light.

How Does Light Penetrate Bone?

Near infrared light and some wavelengths of visible red light easily penetrate human tissue,including bone Light is used to noninvasively interrogate gray matter in the frontal portion of the cerebral cortexThe depth of the light path is based on the separation of light source to detector and the strength of the light source

Chromophores In Cerebral Tissue

NIRS is based on specific chromophores absorption of near infra red light for various wavelenghths. In case of cerebral tissues, these chromophores are: oxygenated hemoglobin, deoxygenated hemoglobin and oxidized cytochrome (Caa3)

Other Chromophores

• There are a number of chromophores which absorb light in the same region as oxy-Hb and deoxy-Hb

• Interference caused by these chromophores must be detected and accounted for to get accurate SctO2 measurements

wavelengths for CO-, Met-, total-Hb, Skin pigmentation

CADAVERS: NIRS Oxygenated Hb Showed a 90% DecreaseWithin 7h of Death, Continued Decrease in from 7 to 96 h.

Absorption spectra for oxygenated haemoglobin (HbO2), deoxygenated haemoglobin (Hb), Caa3, melanin, and water (H2O) over wavelengths in NIR range. Note the relatively low peak for Caa3.

Commercial cerebral NIRS devices currently utilize wavelengths in the 700–850 nm range to maximize separation between Hb and HbO2. The presence of melanin as found in human hair can significantly attenuate Hb, HbO2, and Caa3 signals.

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Light Absorbance of Hemoglobin

Arterial Pulse Oximetry

• Coronary artery bypass grafting (CABG) on/off pump

• Isolated valve replacement (AVR, MVR)• Combined valve replacement and CABG• Aortic arch replacement with DHCA• Ventricular assist device (LVAD, RVAD,

BiVAD)• Cardiac transplantation

Clinical Applications Of Cerebral Oximetry

• Liver transplantation• ECMO• Carotid endarterectomy• “Beach Chair” positioning surgery (e.g.,

shoulder surgery)• Neonatal cyanotic heart defects• High-frequency ventilation• Bowel Ischemia (Neonates)

Sizes are: Neonate, pediatric, adult

Somatic Oximetry

Brain Areas Accessible by NIRSLIMITATIONS: What Part of the Brain is Involved

LIMITATIONS: What Part of the Brain is Involved

LIMITATIONS: What is the cause

Know how Different Organs Behave Under Different Conditons

Ischemia (Poor Flow)

Vs

Hypoxia (No Oxygen)

Vs

Venous Congestion

Old Technology uses two wave lengths, with no absolute normal values and theirAlgorithms are only designated for adults

AccurateImprecise AccuratePrecise InaccuratePrecise InaccurateImprecise

Precision and Accuracy

Standard Finger Oxygen Sat =2%Old technology NIRS: Two channels 9.72%Recent technology NIRS : Five channels 3%

Thank You