Clinical management and cerebral perfusion pressure

The brain tolerates ischemic conditions poorly. Frequently, the need to maintain cerebral perfusion pressure necessitates the use of vasopressors directed by the patient's cardiac condition. Elevations in intracranial pressure may be treated directly with mannitol and mild hyperventilation if refractory to ventricular drainage. Hypocapnia is a potent vasoconstrictor and controls elevations in intracranial pressure by limiting cerebral blood flow. Unfortunately, at PaCO2 levels below 25 mmHg (3.5 kPa), it may promote ischemic injury. Thus, although it is effective in reducing intracranial pressure, hyperventilation should not be allowed to reduce PaCO2 below 25 mmHg (3.5 kPa). Mannitol is also effective in reducing intracranial pressure. Its immediate effect on intracranial pressure has been ascribed to a rheological property. It also functions as a volume expander to improve perfusion pressure and as an osmotic diuretic which removes extravascular water from the brain. Bolus doses of 0.25 g/kg are effective in reducing intracranial pressure and may be repeated as needed. Care needs to be taken to monitor serum osmolality and to prevent levels from increasing above 320 mosmol. It is important to maintain a euvolemic state with the use of mannitol. Dehydration may compromise cerebral perfusion and could have other deleterious consequences. Fluid management is directed towards maintaining an adequate intravascular volume and avoiding a hyposmolar state, i.e. hypotension and hyponatremia must be avoided.

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