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one of the cornerstones of intracranial hypertension treatment for many years until the deleterious effects of low tension CO2 on morbidity and mortality were worked out in the 1980's. This deleterious effect is caused by the vasoconstriction that occurs during falling levels of CO2 caused by severe prolonged hyperventilation. This vasoconstriction causes decreased blood flow to the brain and therefore enlarges areas of the ischemic penumbra, extending areas of ischemic damage. Current treatment of cerebral vasospasms is to optimize the perfusion pressure to the brain by increasing blood pressure and thus blood flow.
Vasospasm is a phenomenon that occurs following subarachnoid hemorrhage (SAH) due to an aneurysm or other source of arterial bleeding. The proximal vessels of the cerebral circulation seem to be the most sensitive structures in the cerebral vasculature to this type of aberration. Vasospasm causes local, discrete areas of spasm that decreases the diameter of the vessel and therefore the total flow of blood through the constricted areas. The constricted areas also cause an increase in the velocity of the blood flowing through the vessel in an attempt to keep total blood flow pumped through the vessel constant. Vasospasm is a variant of normal cerebral blood flow (CBF), and has been shown in rabbit and dog models of SAH, that although CBF is reduced vascular reactivity does remain intact (1,3) being able to once again change as the situation changes. However, some cat SAH studies demonstrate that the reactivity to hypercarbia is blunted (2), possibly allowing the blood vessel to stay dilated longer, improving vasospasms even more.
Transcranial Doppler (TCD) is used to monitor the blood velocities through major vessels in the brain that are undergoing spasm. The velocity of the blood is directly proportional to the amount of spasm, the diameter, occurring in the vessel once corrected for the blood pressure. The velocities in the Internal Carotid Artery (ICA) are used to correct for the differences in the velocities caused by changes in blood pressure. When the velocity of the blood in the cerebral vessels is corrected for the velocity of blood in the ICA the Lindegaard ratio is obtained.
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