[Hydrostatic pressure and neurosurgical pathology].

Hydrostatic pressure is a force applied by a liquid on the surface of an immersed body. Inside the circulatory system it depends on the weight of the blood column between the heart and a given level. In neurosurgical patients, the hydrostatic pressure plays an important role in cerebral perfusion, transcapillary fluid movements and venous air embolism. The mean arterial pressure (Pa), the CSF pressure (CSFP) and the pressure in the jugular vein (JVP) are the hydrostatic determiners of the cerebral perfusion pressure (CPP). The hydrostatic pressure gradient associated with head raising decreases Pa, cerebral venous pressure and JVP, decreases or increases intracranial pressure (ICP) and decreases CPP. The consequences of the resulting hypoperfusion depend on the status of autoregulation. When the skull is open, the pressure under the cerebral retractors determines the transmural pressure and cerebral perfusion. The transcapillary fluid movements depend on a permeability coefficient, a hydrostatic pressure gradient and an osmotic pressure gradient. In case of a rupture of blood brain barrier (BBB), the increase of Pa and the decrease of ICP (craniotomies) favour the development of vasogenic oedema. When the Pa stands higher than the upper limit of autoregulation, the hydrostatic capillary pressure increases. It results in a vasodilation, an increase of cerebral blood flow and oedema, the lesion of BBB being initiated by an increase of the amount and the activity of the pinocytotic vesicles in endothelial cells. The syndrome of BBB rupture at a normal CPP is the consequence of an increase in hydrostatic pressure in the dilated capillary territory and can make more difficult the surgical treatment of arterio-venous malformations.(ABSTRACT TRUNCATED AT 250 WORDS)