However, it is associated with a higher risk of infection compared with the fiberoptic intra-parenchymal catheters. Subarachnoid monitors should be placed on the same side as the lesion in order to avoid inaccuracy due to pressure differential between the two hemispheres. Computerized recording and display of the transduced ICP pressure wave is now standard with most multimodal bedside patient monitors: the 'real-time' pressure wave and analysis of any trend in pressure may be viewed and compared with other monitored signs such as systemic blood pressure or central venous pressure (CVP).
Indications for ICP Monitoring An ICP monitor may be useful in the management of any neurosurgical patient in whom a raised ICP is suspected, although it is in the management of severe head injury that its utility is most established. In those patients with moderate head injury in whom non-neurosurgical surgery is essential, an ICP monitor may also provide the only indication of deterioration of the patient whilst anaesthetized. Maintenance of an adequate CPP is only possible with knowledge of the ICP and systemic blood pressure in order to avoid secondary ischemic insults. ICP measurement also facilitates the ability to gauge response to therapeutic measures and gives early warning of the expansion of mass lesions. In general, ICP monitoring is indicated in all patients who are comatose with brain injury, and in patients with deteriorating neurological status with an abnormal CT scan. As mentioned above, many consider ICP monitoring desirable in patients with moderate head injury requiring prolonged surgical procedure under general anesthesia. In addition, routine post-operative ICP monitoring following major neurosurgical procedures is performed in some centers. The only absolute contraindication for ICP monitoring is the presence of uncorrected coagulopathy.
The nature and characteristics of ICP waves were extensively described in 1960 by Lundberg from his observation of ICP monitoring in neurosurgical patients. He described three wave-types: A-waves or plateau waves, B-waves and C-waves.
A-waves, termed "plateau waves" for their characteristic shape, are associated with both an increase in CBV as a result of vasodilation and a decrease in CBF. They manifest in an abrupt rise in the ICP to levels of 60-80 mmHg for a duration of 5-20 minutes and are an indicator of poor prognosis (Fig. 5.3).
Rosner and Becker showed that plateau waves in cats with mild brain trauma are preceded by a decrease in systemic blood pressure to approximately 70-80 mmHg and that CBV increases exponentially with this decrease. In association with poor intracranial compliance, this increase in CBV is accompanied by an exponential rise in ICP, seen as the plateau wave. Plateau waves may be abolished with an increase in CPP or with maneuvers to improve intracranial compliance.
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