Cerebral Oximetry and Near Infrared Spectroscopy

Introduction

• Maintaining oxygen delivery to tissues and organs throughout the body is paramount to successful anesthesia.
• Traditionally, upper-extremity pulse oximetry is used to assess global tissue oxygenation.
• Cerebral oximetry was first described in 1977 by Frans Jöbsis, who found that brain tissue transparency was adequate to measure hemoglobin oxygenation in the near infrared range (650-1000 nm).¹
• The Beer-Lambert law is the basis for the analysis of NIRS. These principles enable determination of regional oxygen saturation by quantifying both oxygenated and deoxygenated hemoglobin.^1,2
• Infrared light from the device enters the body, where it scatters within tissues; some is absorbed, and some is reflected. The light that is returned to the skin is then analyzed.
• Similar to pulse oximetry, NIRS measures the percentage of oxygenated hemoglobin (at 920 nm) relative to total hemoglobin (at 760 nm).
• Unlike pulse oximetry, NIRS does not require pulsatile flow because it measures all the blood in tissues, including arterial, venous, and capillary blood. Cerebral oximetry values reflect a balance between oxygen consumption and oxygen delivery to the brain.^2,3
• Cerebral oximetry values reflect a balance between oxygen consumption and oxygen delivery to the brain.^2,3
• The use of NIRS has expanded since its initial discovery to provide not only information on cerebral oxygenation and perfusion but also on systemic perfusion.

Principles of NIRS

• The combination of Beer’s and Lambert’s law proves that the absorbance of a solution is directly proportional to both the length and concentration of the solution. This theory can be applied to hemoglobin absorption characteristics, allowing for different absorbances between oxyhemoglobin, deoxygenated hemoglobin, and myoglobin.
• The classic representation of Beer’s law (Figure 3) assumes that a source (in this case, light) traveling through a medium is either absorbed or passes through unchanged and detected.
• NIRS modifies Beer’s law since light in tissue can be absorbed either fully, partially, and/or scattered.
• By adding variables for differential pathlength factor and the effects of scattering (G), the calculated values from the NIRS monitor are more physiologically accurate.
• NIRS provides an extensive amount of raw data, and there are emerging applications using different algorithms and calculations. One of these is the ability to process data and estimate hemoglobin in real time.
• Figure 4 below shows the correlation between measured blood gas hemoglobin (x-axis) and estimated hemoglobin from NIRS data (y-axis). While still in early development, this shows the inherent value of NIRS data. Future use of these calculations could provide extremely useful for intraoperative monitoring.

• A is the absorbance, a dimensionless quantity that indicates the extent to which light is absorbed by the sample.
• ꞓ (epsilon) is the molar absorptivity, which characterizes how strongly a chemical species absorbs light at a given wavelength.
• c is the concentration of the absorbing species within the medium.
• l is the path length that light travels through the sample.

Anesthetic Considerations

• NIRS and cerebral oximetry are versatile and have played important roles in cardiac surgery, pediatrics, and during resuscitation from cardiac arrest.
• Baseline values should be obtained on an awake patient prior to administration of supplemental oxygen or anesthetic agents.
• Baseline values typically range from 60% to 80%; however, values below 50% are not uncommon in certain disease states.¹
• The primary use is for changes related to baseline, and changes of 20-25% are considered significant and can indicate systemic hypoperfusion.¹

Specific Applications

Cardiac Surgery

• Use of NIRS during adult cardiac surgery is associated with decreased major organ morbidity; absolute mortality reduction is unclear with NIRS use.^4,5
• Prolonged desaturations while utilizing cerebral oximetry during cardiac surgery have been associated with increased morbidity and mortality.¹
• Decreases in cerebral oximetry values of greater than 20% are associated with increased cognitive dysfunction after coronary artery bypass graft surgery.⁵
• The use of NIRS during deep hypothermic circulatory arrest remains controversial, and changes in oximetry values are unclear in their significance.⁴
• In the absence of pulsatile blood flow, such as during extracorporeal membrane oxygenation and ventricular assist devices, cerebral oximetry remains a viable measure of brain perfusion and oxygenation. In these non-pulsatile states, conventional pulse oximetry frequently fails.¹

Vascular Surgery

• Cerebral ischemia during carotid endarterectomy may be identified by intraoperative electroencephalogram, somatosensory evoked potentials, and transcranial Doppler.
• Use of NIRS during cerebral endarterectomy is a relatively new practice, and data indicate that a baseline reduction of more than 12% is sensitive and specific for acute brain ischemia.¹
• Case reports have shown that during thoracic aortic surgery, placing NIRS pads over the thoracic spine can facilitate early detection of spinal cord ischemia.¹

Pediatrics

• In most intraoperative instances, NIRS is a reliable monitor for pediatric patients.
• Use of cerebral oximetry in extremely preterm infants (less than 28 weeks’ gestational age) is not associated with decreases in mortality or severe brain injury.⁶

Organ Perfusion

• NIRS placement over various sites is being investigated as a marker for organ perfusion, including renal and hepatic tissue.⁷

Neuromonitoring

• Recent developments have suggested NIRS as a tool for investigating real-time cerebral metabolism; however, it remains unrefined and is not currently used.⁷

Cardiac Arrest

• Use of NIRS during cardiac arrest and subsequent resuscitation is a new area of research.
• Higher cerebral oximetry and NIRS values during cardiac arrest and resuscitation are associated with better outcomes and an increased incidence of return of spontaneous circulation.⁵

Limitations

• Cerebral oximetry measures regional (frontal lobe) cerebral oxygen saturation and therefore does not provide information about middle and posterior cerebral circulations.⁷
• Hypothermia and high-dose vasoconstrictors can affect signals and ultimately values
• Cerebral oximetry is more prone to interference from electrocautery use than pulse oximetry.
• Operating room lights, particularly when a transparent drape is in place, can affect NIRS and cerebral oximetry values.
• Accurate readings depend on adequate sensor-to-patient contact; poor skin contact with the sensor can lead to falsely low readings.
• Individual patient factors can affect the accuracy of readings. For example, bilirubin reduces measured cerebral oxygenation at 733 and 809 nm because it absorbs near-infrared light.⁸