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Neurological Assessment and Monitoring 439

have been developed to assist nurses when caring for evaluating cerebral circulation and haemodynamics.
patients with jugular bulb oximetry. 71 Pulses of ultrasound are directed using a handheld trans-
ducer towards the vascular formations in the base of the
Partial Brain Tissue Oxygenation Monitoring skull. Velocities from the cerebral arteries, the internal
Changes in ICP values alone do not accurately depict carotids, the basilar and the vertebral arteries can be
poor cerebral blood flow or oxygenation deficits to brain sampled by altering transducer location, angle and the
tissue. Consequently brain tissue hypoxaemia is often instrument’s depth setting. The commonest windows in
observed during the first 24 hours after injury despite the cranium are located in the orbit (of the eye), and in
controlled brain pressures. Monitoring partial pressure of the temporal and suboccipital regions. TCD measures
oxygen in brain tissue (PbtO 2 ) can be used to collect systolic, diastolic and mean middle cerebral artery (MCA)
more accurate and timely information about cerebral flow velocities and a derived value, the pulsatility index
oxygen delivery and demand than ICP allows. A tissue (PI). Changes in the PI can be used to identify the thresh-
oxygen value of less than 10 mmHg for more than 10 old of autoregulation or cerebral perfusion pressure break
minutes carries a higher risk of death. Normal brain point in individual patients. In subarachnoid haemor-
oxygen levels (PbtO 2 between 20 and 25 mmHg) emerge rhage (SAH) and TBI this may be due to vasospasm, or
as a critical determinant of outcome, with values below impaired autoregulation or abnormal intracranial com-
20 mmHg carrying a higher risk. 69 pliance. TCD is a simple, portable and non-invasive tool,
well suited to serial monitoring, that can be used at the
Regardless of ICP, brain tissue oxygenation falls with a bedside to detect relative changes in CBF in brain-injured
decrease in cerebral blood flow below an ischaemic patients. 76
threshold of 18 mL/100 g/min. ICP may respond to the
changes but often several hours later when the damage Continuous Electroencephalography
can not be reversed. Alterations in cerebral metabolic rate Electroencephalography (EEG) is the recording of electri-
can also change tissue oxygen levels. Reducing the cal activity by sensors along the scalp produced by the
patient’s energy consumption via reduced noise and/or firing of neurons within the brain. Continuous EEG
distractions, and increasing their protein caloric intake to (cEEG) has the advantage of being continuous, noninva-
complement their increased stress state can improve sive and carrying the potential to detect alterations in
tissue oxygenation. 72 brain physiology at a reversible stage, which may trigger
treatment before permanent brain injury occurs. The
Microdialysis invention of digital EEG has made cEEG monitoring fea-
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Cerebral microdialysis (using a catheter ideally placed in sible for ICU patients. Currently, the main applications
the frontal lobe) is a tool for investigating the metabolic of cEEG are diagnosing nonconvulsive status epilepticus,
status of the injured brain and is part of multimodal monitoring and guiding the treatment of status epilepti-
monitoring. The microdialysis probe is inserted into the cus and detecting delayed cerebral ischaemia from vaso-
cerebral tissue where substances in the extracellular fluid spasm in subarachnoid haemorrhage patients. Other
surround the semipermeable membrane at the tip of the applications may include monitoring of reperfusion after
catheter. Following equilibration of the tissue metabolites tissue plasminogen activator in acute stroke patients and
with the perfusion fluid, the dialysate can be analysed for detection of intracranial hypertension. Clinically unrec-
concentrations of products of energy metabolism (glucose, ognised electrographic seizures and periodic epileptiform
lactate, pyruvate) as indicators of hypoxia and ischaemia. discharges have been shown to be frequent and associ-
In addition, interstitial glycerol can be determined, which ated with poor outcome in patients with severe brain
is a parameter of lipolysis and/or cell membrane damage. injury from different aetiologies, including TBI, ischaemic
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In theory, the microdialysis catheter acts like a blood and haemorrhagic strokes and CNS infection. The EEG
73
capillary. Thereby, it is proposed that microdialysis pro- becomes substantially abnormal (suppressed) when cere-
vides information regarding events that take place in the bral blood flow declines to 20–30 mL/100 g/min. More
tissue before any chemical events are reflected by changes subtle abnormalities accompany lesser degrees of hypo-
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in systemic blood levels of indicator substances. These perfusion, including initial loss of beta activity, slowing
molecules diffuse across the membrane part of the cath- to the theta range, and then to the delta range. Irreversible
eter and equilibrate with the perfusion fluid, which is injury to brain tissue occurs at cerebral flows of about
pumped through the probe at very low rates of flow. 10–12 mL/100 g/min. Thus, the EEG sensitivity to isch-
Changes in the concentration of a substrate in the sur- aemia allows its use in situations where cerebral perfu-
79
rounding milieu are reflected by subsequent changes in sion is at risk. To facilitate interpretation, digital EEG
75
the dialysate. Rather than inserting an instrument into data can be transformed into power spectra by fast Fourier
the tissue, microdialysate is extracted and later analysed transformation. Changes over time in these quantitative
in the laboratory or clinically at the patient’s bedside. EEG (qEEG) parameters can trigger remote reading,
focused neurological examination, imaging studies and
NON-INVASIVE ASSESSMENT early treatment. Subtle EEG changes may be difficult to
Transcranial Doppler interpret in isolation, but may be better understood when
interpreted in concert with other components of a mul-
Transcranial Doppler (TCD) ultrasound has proven to be timodality monitoring paradigm, which may include
a safe, reliable and relatively inexpensive technology microdialysis, brain tissue oxygen and cerebral perfusion
for measuring cerebrovascular blood velocities and pressure.