Little substantial human data exists concerning the impact of fluids on the brain, which may guide rational IV Fluid Replacement in neurosurgical patients.
Therefore, recommendations arise from known factors that influence water movement into the brain.
IV Fluid Replacement in neurosurgical patients can alter three properties of the blood, viz., osmolality, colloid oncotic pressure and haematocrit.
Osmolality describes the molar number of osmotically active particles per kilogram of solvent and is determined by the total number of dissolved “particles” in a solution, regardless of their size.
Cerebral perfusion pressure should be maintained in the high-normal range because autoregulation is impaired in the damaged brain, with a baseline increase in cerebrovascular resistance. Avoidance of intracranial hypertension, the other determinant of cerebral perfusion pressure, is crucial to preserve blood flow to compromised but viable brain tissue.
Osmotic pressure is the hydrostatic force acting to equalize the concentration of water on both sides of the membrane that is impermeable to substances dissolved in that water.
Colloid osmotic pressure is the osmotic pressure generated by large molecules (e.g., albumin, hydroxy ethylstarch, dextran).
This property is an important determinant of IV Fluid Replacement in neurosurgical patients in biological systems where vascular membranes are permeable to small ions, but not to large molecules.
Cerebral edema formation is related to capillary pressure, COP, and permeability. The capillary pressure is between the arterial and venous pressures. The damaged capillary bed becomes excessively permeable, with conductivity being greatest for the smallest molecules, but less for colloids.
IV Fluid Replacement in neurosurgical patients with cerebral edema is directed at maintaining cerebral perfusion pressure, avoiding elevations of cerebral venous pressure and hypertension, preventing large changes in plasma osmolality (particularly depression of plasma osmolality), and avoiding hyperglycemia.