A critical component of traumatic brain injury (TBI) is brain swelling. Contents inside of the skull include the brain matter itself, spinal fluid (around the brain and inside ventricles), and the blood in vessels. Because the skull is a rigid container, a change in volume of any one of these three components can dramatically alter intracranial pressure.
Under normal conditions the brain’s blood vessels are partially constricted. Immediately following a blow to the head, the normal control of these vessels is lost. As a result, the vessels dilate, causing an increase in intracranial pressure. The CO2 content of blood affects vessels; high CO2 dilates them whereas low CO2 constricts them. To lower intracranial pressure neurosurgeons hyperventilate the patient; over-breathing quickly lowers the blood’s CO2 content, which constricts the vessels.
Brain tissue has a great deal of water in it. So sucking water out of brain can also lower pressure. This is accomplished by injecting a non-metabolized sugar into the blood stream. The sugar molecules (such as mannitol) are too large to pass from blood into brain tissue, but their presence pulls water from the brain into the blood by osmosis. The water then excreted by the kidneys. For obvious reasons, these sugars are termed osmotic diuretics.
Finally, intracranial pressure can be lowered by carefully draining small amounts of fluid from the chambers in the brain. Severely head injured patients however may have already collapsed these spaces, making this option impossible.
Why is controlling intracranial pressure so important? Because the higher the pressure the harder it is to push blood through the brain. When the pressure inside the skull exceeds arterial pressure blood flow to the brain stops. If this happens, massive brain damage results. To help manage a patient, pressure sensors are commonly placed directly inside the skull.