Neuro ICU Procedure Atlas by Jack I. Jallo David F. Slottje

Author:Jack I. Jallo, David F. Slottje
Language: eng
Format: epub, pdf
Publisher: Thieme
Published: 2021-05-15T00:00:00+00:00

Fig. 7.1 Inferior skull base venous anatomy.

The arteriovenous oxygen content difference of the brain (AVDO2) is calculated according to the following formula:

AVDO2 = Hgb × 1:34(SaO2 – SjO2 + 0:003(PaO2 – PjO2)2

Cerebral oxygen extraction (CO2E), a simplified representation of AVDO2, is calculated according to the following formula:

CO2E = SaO2 – SjO2

Saturation of arterial blood (SaO2) is measured by continuous pulse oximetry, and saturation of jugular bulb blood (SjO2) is measured continuously by the fiber-optic jugular bulb catheter.

Any disturbance that increases cerebral oxygen consumption or decreases oxygen delivery may decrease SjO2. Current guidelines define SjO2 < 50% as pathologic and harmful.1 SjO2 > 75% is also correlated with adverse outcomes in head injury.3 Thus, the brain tissue of a healthy person extracts between 25 and 50% of arterial oxyhemoglobin, yielding a normal SjO2 of 50 to 75%.

One prospective study showed mortality benefit by maintaining CO2E between 24 and 42%.

High CO2E suggests low cerebral blood flow relative to metabolic demand. This could be caused by decreased oxygen supply due to anemia, hypotension, or hypoxemia, or due to increased demand due to agitation, fever, seizures, or pain. This state has been described as “oligemic cerebral hypoxia,” and can lead to ischemia.

Low CO2E suggests that there is excess blood flow relative to metabolic demand, due to excessive cardiac output, or due to processes such as infarction, deep coma, or hypothermia causing decreased metabolism. This state of “luxury perfusion,” can cause intracranial hypertension or hemorrhage.

Limitations of SjO2 monitoring include erroneous values due to the monitor coming into contact with the vessel wall, thrombosis on the catheter tip, and the fact that it is a global monitor and may not detect regional ischemia or hyperemia.

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