Fluid Dynamics of the Intracranial Compartment

Intracranial volume is limited by the confines of the adult calvarium. An increase in the volume of one fluid compartment results in compensatory decrease in the others. Displacement of CSF and venous blood compensates for pressure differentials transmitted between compartments. These volumetric changes in response to pressure differences define the brain's compliance. Under normal physiologic conditions, changes in the intracranial volume are associated with minimal or insignificant fluctuations in the intracranial pressure (ICP), but considerable increases in ICP may occur as the intracranial volume increases and brain compliance decreases. The relationship between intracranial volume and ICP is illustrated in Fig. 1. ICP is commonly evaluated by measuring the opening pressure of the CSF in the lateral decubitus position or with a ventriculostomy or subdural monitor in the neurointensive care unit. ICP normally ranges from 15 to 20 mmHg. Focal elevations in ICP may lead to displacement of brain tissue with herniation, precipitating neurologic deficits that may be fatal. Neuro-pathologic correlates of herniation are illustrated in Fig. 2.

Volume

Figure 1 Intracranial volume and ICP correlation, demonstrating that relatively larger pressure increments result from decreasing compliance with volume increases.

Changes in ICP may affect cerebral perfusion pressure (CPP). The CPP is equal to the difference between the mean arterial pressure and the ICP. In normal circumstances, autoregulation of vasomotor tone in the cerebral circulation allows for maintenance of an adequate CPP, coupling brain metabolism with cerebral blood flow. Autoregulation is predominantly mediated at the arteriolar and capillary level under the influence of vasoactive factors and direct innervation from the sympathetic nervous system. Cerebral capillary function is regulated by many substances, including adenosine, vasoactive intestinal peptide, and arachidonic acid metabolites. Numerous disease states including cerebral ischemia and head trauma interfere with this protective mechanism. As the ICP exceeds a critical threshold of20-25 mmHg, CPP decreases, with significant decrements resulting in cerebral ischemia.

A simplistic approach to fluid dynamics in the intracranial compartment is derived from the application of Starling's law, which states that fluid flux is dictated by a balance between hydrostatic forces and osmotic gradients. The complex structural and functional properties of the BBB result in a highly modified form of Starling's law with dynamic homeostasis influenced by local and systemic mediators, although the concept ofbalanced hydrostatic and osmotic forces is preserved. Plasma flow across the BBB is driven both by hydrostatic forces determined by the relatively constant CPP and by osmotic forces principally determined by the concentration gradient of sodium ions. Under pathologic conditions, the osmotic forces are modified by the generation of idiogenic osmoles, substances produced by brain parenchyma to prevent

Figure 2 Neuropathology features of herniation syndromes demonstrated in gross brain specimens. (a) Subfalcine midline shift due to a frontal lobe glioma. (b) Uncal herniation of the temporal lobe over the tentorial edge due to a traumatic hematoma. (c) Compression of the cerebellar tonsils following elevated ICP. (Courtesy of Harry V. Vinters, M.D.)

Figure 2 Neuropathology features of herniation syndromes demonstrated in gross brain specimens. (a) Subfalcine midline shift due to a frontal lobe glioma. (b) Uncal herniation of the temporal lobe over the tentorial edge due to a traumatic hematoma. (c) Compression of the cerebellar tonsils following elevated ICP. (Courtesy of Harry V. Vinters, M.D.)

rapid fluid loss with resultant cellular shrinkage. These idiogenic osmoles included the following: alanine, aspartate, betaine, choline, GABA, glutamate, gluta-mine, glycerophosphorylcholine, glycine, lysine, myo-inositol, phosphocreatine, serine, taurine, and threo-nine.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

Get My Free Ebook


Post a comment