Cerebral monitoring of patients with acute intracranial disorders generally focuses on intracranial pressure and cerebral perfusion pressure monitoring. Recently, several new techniques have become available for more detailed routine monitoring of cerebral function, oxygenation and metabolism.
Elevated Intracranial pressure has been recognized as one of the most important factors affecting morbidity and mortality rates in patients who have had TBI; therefore, ICP monitoring has become routine in the management of severe head injuries. Normal ICP is 7-15 mm Hg adult. It is considered abnormal if exceeds 15 mm Hg.
Hemodynamic monitoring in the care of the critically ill neuroscience patient provides information that assists the clinician in minimizing secondary neuronal injury.
Cerebral perfusion pressure (CPP) is the pressure gradient across the cerebral vessel, the difference between arterial inflow and venous outflow. CPP is measured clinically as the mean arterial pressure minus the intracranial pressure.
Reduced CPP, due to intracranial hypertension or inadequate blood pressure, contributes to a fall in Cerebral Blood Flow CBF in patients with severe TBI.
EEG investigates the spontaneous electrical activity of the cerebral cortex. EEG is the summation and recording of postsynaptic potentials from the pyramidal cells of the cerebral cortex. The electrical signal is amplified, filtered, and displayed to give an accurate representation of electrical activity throughout the cortex. Changes in neuronal perfusion and oxygen supply lead to changes in the EEG recording.
Bispectral Index is a continuous EEG reading. The calculations obtained from the EEG indicate the sedation level in the patient. It was designed for use during general anesthesia to measure sedation but has since crossed over to the ICU.
Evoked potentials are the electrical responses generated in the nervous system in response to a stimulus. Evoked potentials are event-related. They are pathway-specific. They have much lower amplitude than the normal EEG activity.
The severity of altered consciousness is evaluated with the Glasgow Coma Scale. The scale consists of eye opening, verbal communication, and motor response to verbal or noxious stimulation. The Glasgow Coma Score is the sum of the three components, and has a range from 3 to 15.
Cerebral microdialysis is a relatively new technique for measuring the levels of brain extracellular chemicals, which to date has predominantly been used as a research tool. Brain glucose and lactate levels can be monitored with the help of imaging modalities such as magnetic resonance and PET scanning. Recently, however, the ability to measure the concentration of metabolites in the extracellular space directly and continuously, has only become feasible with the application of cerebral microdialysis
Jugular venous oxygen saturation and brain tissue PO2 have been used as measures of cerebral oxygenation in place of quantitative CBF measurement. Because they have been found to be very good indicator of the adequacy of CBF in relation to cerebral metabolic requirements. If the brain is hypoperfused, oxygen extraction will be increased, and SjvO2 will be reduced. On the other hand, if CBF is adequate for the brain's metabolic need, then SjvO2 will remain normal. This information is more often useful clinically than the absolute CBF values
Cerebral near-infrared spectroscopy is a noninvasive technique to monitor brain oxygenation by measuring regional cerebral venous oxygen saturation.
Brain tissue oxygen pressure measurement is increasingly being used for evaluation of cerebral oxygenation. It is used in patients with multiple extracranial injuries and TBI, aneurysmal subarachnoid hemorrhage, during aneurysm surgery, during surgery of cerebral arterio-venous malformations, during cerebral angiography, and during cardiopulmonary resuscitation. Two monitoring devices are currently available, "Licox" and "Neurotrend" technology. The Neurotrend probe uses optical sensors while The Licox probe uses a polarographic cell. |
