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Raised ICP,the hyperosmolar therapy..Part1..Pathophysiology


Raised intracranial pressure (ICP) appears to be quite lethal: in traumatic brain injury patients, those with ICP > 40 mm Hg had a mortality of 56%, compared to 18% for those with ICP < 20 mm Hg. Most traumatic brain injuries causing long-term disability also initially presented with raised intracranial pressure.
As volume increases inside the skull, intracranial pressure exponentially rises after it passes an inflection point of ~20-25 mm Hg. As ICP passes 50-60 mm Hg and approaches arterial pressure, global brain ischemia and eventual brain death result. The brain is 80% water, so using hyperosmolar agents to create an osmolar gradient between the inside of the brain and the systemic circulation has strong theoretical appeal. Hypertonic saline and mannitol are effective because they do not cross the blood-brain barrier (much), and thereby draw cerebrospinal fluid out of the cranium and fluid out of the injured brain, reducing pressure and further injury.
In brain injuries that include disruption of the blood-brain barrier, hyperosmolar therapy may be less effective.
There is no definitive evidence from prospective randomized trials that reducing intracranial pressure with hyperosmolar therapy saves lives or prevents disability. The theoretical evidence for its benefit is so persuasive, though, that placebo-controlled trials will not be performed.Post hoc analyses of randomized trials of brain injured patients, along with observational trials, suggest that reducing intracranial pressure does improve outcome.
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