CEREBRAL OXIMETRY AND OTHER MONITORS OF THE BRAIN
Cerebral oximetry uses near infrared spectros-copy (NIRS). Using reflectance spectroscopy near infrared light is emitted by a probe on the scalp (Figure 6–12). Receptors are likewise positioned to detect the reflected light from both deep and super-ficial structures. As with pulse oximetry, oxygenated and deoxygenated hemoglobin absorb light at dif-ferent frequencies. Likewise, cytochrome absorbs infrared light in the mitochondria. The NIRS satura-tion largely reflects the absorption of venous hemo-globin, as it does not have the ability to identify the pulsatile arterial component. Regional saturations of less than 40% on NIRS measures, or changes of greater than 25% of baseline measures, may herald neurological events secondary to decreased cerebral oxygenation.Measurements of jugular venous bulb satura-tion can also provide estimates of cerebral tissue oxygen extraction/decreased cerebral oxygen deliv-ery. Reduced saturations may indicate poor out-comes. Direct tissue oxygen monitoring of the brain is accomplished by placement of a probe to deter-mine the oxygen tension in the brain tissue. In addi-tion to maintaining a cerebral perfusion pressure that is greater than 60 mm Hg and an intracranial pressure that is less than 20 mm Hg, neuroanesthe-siologists/intensivists attempt to preserve brain tis-sue oxygenation by intervening when oxygen tissue tension is less than 20 mm Hg. Such interventions center upon improving oxygen delivery by increas-ing Fio2, augmenting hemoglobin, adjusting cardiac output, or decreasing oxygen demand.
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