Parenchymal Brain Oxygen Monitoring in the Neurocritical Care Unit

Patients admitted to the neurocritical care unit (NCCU) often have serious conditions that can be associated with high morbidity and mortality. Pharmacologic agents or neuroprotectants have disappointed in the clinical environment. Current NCCU management therefore is directed toward identification, prevention, and treatment of secondary cerebral insults that evolve over time and are known to aggravate outcome. This strategy is based on a variety of monitoring techniques including use of intraparenchymal monitors. This article reviews parenchymal brain oxygen monitors, including the available technologies, practical aspects of use, the physiologic rationale behind their use, and patient management based on brain oxygen.

Key points

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Parenchymal brain tissue oxygen (PbtO ₂) monitoring is a safe and reliable technique for continuous bedside evaluation of patients with severe brain injury.
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Two techniques, a modified Clark electrode that uses the electrochemical properties of noble metals or optical fluorescence technology, can be used to measure PbtO ₂.
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PbtO ₂indicates the balance between regional oxygen supply and cellular oxygen consumption and may be described by the equation PbtO ₂= CBF × AVT o ₂, where CBF is cerebral blood flow, Pv o ₂is partial oxygen pressure in mixed venous blood, and AVT o ₂is Pa o ₂− Pv o ₂.
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PbtO ₂values less than 20 mm Hg are considered worth treating and values less than 15 mm Hg are consistent with brain hypoxia or ischemia.
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Reduced PbtO ₂is associated with worse outcome in acute brain injury in adults and children, although the strength of this relationship may depend on probe location.
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When severe traumatic brain injury care is based on data from both a PbtO ₂and intracranial pressure (ICP) monitor, some (but not all) observational series suggest that outcome is better than when just ICP-based care is provided.