Facebook
Twitter
LinkedIn
Email
 |
Checking for direct PDF access through Ovid | |
Excerpt
Summary: The factors that influence water movement into the brain will be examined, in order to provide reasonable recommendations for perioperative isotonic fluid management in patients with brain pathology.
Introduction: Restrictive fluid management, in patients with brain pathology, arises as a result of fear that fluid administration can enhance cerebral oedema. However, fluid restriction leads to hypovolaemia, haemodynamic instability, and may adversely influence cerebral perfusion [1].
Factors that affect water movement into the brain:
These include the following:
• Osmotic pressure-The hydrostatic force acting to equalize the concentration of water on both sides of the membrane, which is impermeable to substances dissolved in that water.
• Osmolarity and osmolality-For most dilute salt solutions, osmolality (molar number of osmotically active particles per kilogram of solvent) is equal to or slightly less than osmolarity (molar number of osmotically active particles per litre of solution).
• Colloid oncotic pressure. (COP)-Osmolarity/osmolality is determined by the total number of dissolved 'particles' in a solution, regardless of their size. COP is the proportion of the total osmolality produced by large molecules.
• Starling's equation-this describes the forces driving water across vascular membranes. In a simplified fashion, fluid movement is proportional to the hydrostatic pressure gradient minus the osmotic plus oncotic gradient. In the periphery, the capillary endothelium has a pore size of 6.5 nm and is freely permeable to small molecules and ions (Na+, Cl−), but not to large molecules, such as proteins. The brain, however, is very different, making it necessary to re-examine Starling's equation.
• Blood-brain barrier (BBB)-The endothelial cells in the brain are joined together by continuous tight junctions. The effective pore size is only 0.7 nm, making this unique structure impermeable to large molecules, relatively impermeable to small ions, and fairly permeable to water. This makes the brain an exquisitely sensitive osmometer: brain water content can be altered by small changes in osmolality, but not by clinically achievable changes in COP [2-4]. If the BBB is open it is impossible to maintain any form of osmotic or oncotic gradient. Oedema forms secondary to the lesion, and alterations in COP have no added effect [5-7].
