An adult has approximately 150ml of cerebrospinal fluid (CSF) and produces about 500ml per day. Therefore the CSF turns over about 3x per day (continually produced and reabsorbed). Interference with the normal reabsorption (for example by clogging of arachnoid granulations with subarachnoid hemorrhage) causes communicating hydrocephalus. Interference with drainage of CSF from any or all of the ventricles (two lateral, 3rd ventricle, 4th ventricle), where CSF is produced by the choroid plexuses, causes obstructive hydrocephalus.
Both communicating and obstructive hydrocephalus cause an increase in intracranial pressure and are treated acutely with CSF drainage. In communicating hydrocephalus, lumbar drainage is generally safe. In obstructive hydrocephalus the CSF must be drained directly from the ventricular system intracranially (with an external ventricular drain). Lumbar drainage in obstructive hydrocephalus can cause a cerebral herniation syndrome.
Measuring intracranial pressure (ICP): Intraoperatively, the brain is most vulnerable to hypoperfusion injury when the head is elevated. The safest approach is to monitor blood pressure at the level of the head (ie, the tragus on the external ear). Furthermore, in order to accurately calculate cerebral perfusion pressure (CPP= MAP-ICP), the mean arterial pressure and intracranial pressure need to be measured at the same height. In a fully supine patient this is irrelevant. In a partially upright patient, both transducers need to be at the head (tragus).
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