Neurocritical Care and Management after Traumatic Brain Injury: Devices for Monitoring Intracranial Pressure

The first priority in severe TBI is to establish complete and rapid physiologic resuscitation, which includes a secure airway and maintenance of O₂ saturation of greater than 90% and arterial systolic pressure greater than 90 mm Hg. If not already performed, endotrachial intubation should be undertaken in any patient with a GCS less than 9 or one who remains hypoxic despite supplemental oxygen. It is routine to place an arterial line for continuous blood pressure (BP) recording. Central or Swan Ganz lines may be helpful in guiding fluid resuscitation.

The optimal resuscitation and maintenance intravenous fluids have been discussed earlier.

Once the patient is medically and surgically stabilized, the next priority is to establish intracranial pressure (ICP) monitoring in patients with a GCS less than 9 who have abnormal computed tomography (CT) scans. ICP monitoring is also important in comatose patients with normal CT scans if two of the following are present: age greater than 40 years, systolic BP less than 90 mm Hg, or there is unilateral or bilateral motor posturing.

There are various devices for ICP monitoring. The intraventricular catheter is considered the gold standard and also allows for the drainage of cerebrospinal fluid (CSF) when ICP is elevated. However, placement of a ventricular catheter may be difficult in the swollen and/or shifted brain, and a variety of parenchymal monitors may serve as reasonable substitutes.

Brain oxygen saturations (PBO₂) are being increasingly monitored in severe TBI; however, their true usefulness is unclear. Early information suggests that PBO₂ less than 20 mm Hg may be associated with worse outcomes, but it is unclear whether this represents the severity of the underlying brain injury or a potentially treatable “secondary injury.”

Less commonly used is cerebral microdialysis, which can measure a variety of neurotransmitters and metabolites, such as glutamate, aspartate, and lactate. It is likewise unclear how information obtained in this way will play a role in the institution of a specific therapy.

The central tenant of severe TBI management is control of ICP and, by extension, cerebral perfusion pressure (CPP). CPP is the mean arterial pressure minus the ICP and is the driver of cerebral blood flow. As has been noted, persistently elevated ICP (>20 mm Hg) is associated with significant mortality. An optimal CPP is generally in the range of 60 to 70 mm Hg.

First-line therapies for ICP control include sedation, paralysis, head-of-bed elevation to 20 to 30 degrees, and avoidance of hyperthermia (>38.5° C).

Mannitol is the most commonly used pharmacologic agent to lower ICP. The primary action of mannitol is in inducing an osmotic gradient between plasma and cells, thus drawing edema fluid from the brain into the circulation. This causes an expansion of blood volume and a potential elevation in blood pressure but ultimately results in a diuresis that may lower blood pressure. Secondary effects of mannitol include a reduction in blood viscosity, which increases cerebral blood and cerebral oxygen delivery. Effective doses of mannitol range from 0.25 g/kg to 1 g/kg and lead to ICP reduction within 15 to 30 minutes.

Because mannitol is an osmotic diuretic, it is excreted entirely by the kidneys and thus should be used with caution in patients with renal failure. It is important to follow serum sodium and osmolality and limit use if serum sodium is elevated to greater than 155 mEq/L or osmolality is greater than 320 mOsm/L.

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