Chapter 17. Cerebral Oximetry

Introduction

Cerebral oximetry is a noninvasive measurement of brain tissue oxygenation using near infrared spectroscopy. It can be used to show local brain tissue hypoxia or asymmetry in saturation between the two sides of the brain potentially indicating unilateral blood flow compromise.

Definitions and Terms

• ▪  Near infrared spectroscopy: Light of multiple wavelengths (690-1100 nm) is passed through multiple tissue layers (skin, bone, brain) and reflected photons are measured by a receiver.
• ▪  Chromophores: Transmitted photons at certain wavelengths are absorbed by chromophores (ie, hemoglobin) in the tissue and the absorption will be a function of the degree of oxygen saturation of the hemoglobin.

Techniques

• ▪  Low-intensity, near infrared light is transmitted through a “source” applied (typically) to the forehead of a patient.
• ▪  Reflected/Transmitted light is measured at a point, approximately, 4 cm from the transmitter (Figure 17-1).
• ▪  Transmitter-receiver pairs are typically applied to each side of the forehead allowing comparison between left and right frontal lobe perfusion (Figure 17-2).

Clinical Pearls and Pitfalls

• ▪  Transmitters and receivers should not be placed over skin with hair follicles (even shaven) because the follicles can absorb and alter light.
• ▪  Transcranial oximetry readings do not measure blood flow or oxygen uptake, and normal values may occur in the presence of active pathology if the pathology is not perfusion related.
• ▪  Cerebral oximetry shares many of the principals and applications of pulse oximetry.
• ▪  Electrocautery, movement, lead placement problems, and ambient light can all interfere with cerebral oximetry.

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