49 New Trends in Neurotrauma Monitoring

Case 49 New Trends in Neurotrauma Monitoring

Judith Marcoux and Abdulrazag Ajlan

Fig. 49.1 Computed tomography scan of the head showing a very small intraparenchymal hematoma in the right hippocampal area.

Clinical Presentation

A 20-year-old man is involved in a high-speed motor vehicle accident. He was the driver and was wearing his seatbelt.

He is hemodynamically stable and his Glasgow Coma Score (GCS) before intubation is 8.

A computed tomography (CT) scan of his head is obtained and shown in Fig. 49.1.
His GCS remains 8 without sedation and he has an intraventricular drain placed to monitor his intracranial pressure (ICP). The opening pressure is 17 mm Hg.
He is kept normothermic, normocapnic, and with a cerebral perfusion pressure (CPP) above 70 mm Hg.
Over the course of the next 3 days, his ICP rises and he needs heavier sedation, cerebrospinal fluid (CSF) drainage, and hyperosmolar therapy.
A repeat CT scan shows diffuse cerebral edema with effacement of the subarachnoid spaces.
Transcranial Doppler (TCD) measurements reveal high velocities in both carotid arteries as well as the middle cerebral arteries.

Questions

What other tools can help you optimize his ICP management?
What is the role of continuous electroencephalography (EEG)?
What is the role of microdialysis?

Answers

What other tools can help you optimize his ICP management?

Jugular venous oxygen saturation (SjvO₂):

The oxygen saturation in the brain draining veins will depend on the cerebral blood flow (CBF), the metabolic rate of oxygen consumption, as well as the hemoglobin concentration. Under normal circumstances the saturation will be between 55–70%.

However, if the cerebral tissue does not receive enough oxygen for its metabolic demands (due to low flow or low transportation of oxygen from low hemoglobin), the SjvO₂ will drop, reflecting an increased extraction of oxygen from the brain.

On the other hand, if the metabolic rate of oxygen in the brain is decreased, or the blood flow is high, the SjvO₂ will increase. Therefore, a supranormal value of SjvO₂ could mean a state of hyperemia. But it could also reflect an impaired extraction capacity of the brain, which can be seen following a traumatic brain injury. SjvO₂ will detect global changes, but may miss regional abnormalities.

SjvO₂ clinical use is not simple; however, it does have some applications. Hyperventilation is very effective in lowering the ICP, but it does so by reducing the cerebral blood flow. Because brain tissue is very sensitive to ischemia following an acute traumatic brain injury, one must be careful not to overuse hyperventilation. One way to monitor the effect of hyperventilation on cerebral perfusion is by measuring the SjvO₂.

If the hyperventilation is excessive, the SjvO₂ will drop and the ventilator settings can be quickly adjusted (level III recommendation by the Brain Trauma Foundation).1

Low SjvO₂ has also been linked with poor outcome.²

Brain tissue partial pressure of oxygen (PO₂):

Human brain is dependent on a good oxygenation for its metabolism and a decrease in tissue oxygenation will increase the risk of ischemic cell damage, especially following a traumatic brain injury. Brain tissue oxygenation is dependent on cerebral perfusion pressure (CPP), but local hypoxia can happen despite normal CPP, intracranial pressure, and mean arterial pressure.

Monitoring brain tissue PO₂ can help detect an area at risk of ongoing damage. Recent evidences seem to show that aggressively treating the PO₂ will improve the outcome.³ One must bear in mind, however, that it reflects the metabolic state of a small area of the brain only and other areas may differ significantly.

Cerebral blood flow (CBF)

Monitoring CBF when the ICP is elevated may help differentiate an ischemic state from a hyperemic state and dramatically change the therapeutic approach to the traumatic brain injury (TBI) patient.⁴

There are different ways to measure CBF; the most commonly used being the TCD, which is a bedside noninvasive method, but is usually noncontinuous.

In the current case, the increased velocities of TCD reflect hyperemia. Lowering the CPP to 60 mm Hg helped control the current patient’s ICP.
What is the role of continuous EEG?
- Posttraumatic seizures complicate a sizable percentage of TBIs. The risk will increase with the severity of the trauma.
- In patients with moderate and severe TBI, 22.3% had electrical seizures on continuous EEG monitoring. More than half of which had no clinical manifestation of their seizures.⁴
- In critically ill comatose patients, the duration of nonconvulsive status epilepticus and the delay to diagnosis was associated with increased mortality.⁵
- Continuous EEG monitoring may help diagnose and treat nonconvulsive seizures, and thus prevent secondary injury to the brain. Furthermore, continuous EEG is of primary importance in the titration of barbiturate-induced coma to treat raised ICP.
What is the role of microdialysis?
- Microdialysis gives a unique insight into the brain’s metabolism by measuring tissue biochemistry.
- It can give a warning regarding impending hypoxia/ischemia or the occurrence of secondary damage. But its measure is local and there is a great variability in the range of the values collected. Its use is still limited to research. However, a growing number of clinicians are using it. In 2004, a consensus⁶ was made for its use and some of the major points are as follows:
  - The use of microdialysis should be reserved for patients with a severe TBI requiring ICP monitoring.
  - The probe should be placed in the right frontal area in patients with diffuse lesions or placed in the pericontusional area, if there is a focal lesion.
  - The best marker for secondary damage is the lactate/pyruvate ratio; glucose, glutamate, and glycerol can also be used as markers of ischemia.