Indirect monitoring consequences of raised intracranial pressure

Cerebral venous oxygen saturation

The cerebral arteriovenous oxygen content difference should normally be 5-7 ml/dl. Values below 4 ml/dl indicate cerebral hyperaemia, whereas values above 9 ml/dl indicate global cerebral ischaemia. Jugular bulb oxygen saturation may be monitored, preferably continuously, with an indwelling catheter. Single measurements of jugular venous oxygen are of little value given the many fluctuations during the day. Overenthusiastic treatment, which on occasion may induce cerebral ischaemia, may be monitored with this technique. Hyperventilation and barbiturate induced falls in cerebral perfusion pressure have been shown in individual patients to be counterproductive. An index of regional oxygen metabolism is required. Transcutaneous, transcranial near infrared spectroscopy is completely non-invasive.60 This is a promising technique, however the scope for technological refinement is still huge.

Intraparenchymal probes

The oxygen content of cerebral tissue is reactive to high ICP.61 However low oxygen tension may be provoked by other factors such as hyperventilation, low arterial blood pressure, microvascular problems, mitochondrial dysfunction, etc. Therefore, specificity of the method is low. Moreover, the measurement covers only a very small area of the brain.

Similar criticism can be raised in case of microdialysis. The lactate/pyruvate ratio is an accepted marker of brain ischaemia. Similarly, glutamate is supposed to be a marker of neuronal disintegration. Both markers have, however, non-established relationship to ICP. Recently, concerns were raised that ICP may modulate microdialysis recovery rate, as it acts as enviromental pressure.

Recent studies have explored other tracers of intracranial hypertension such as potassium ions.62,63

Cerebral electrical activity

The compressed EEG (cerebral function monitoring) is helpful in deciding whether cerebral metabolic depressants may be indicated in the treatment of intracranial hypertension.21 Such drugs will obviously not be helpful if the EEG is flat or greatly reduced in amplitude.

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