Читать книгу Manual of Equine Anesthesia and Analgesia - Группа авторов - Страница 332

 CSF is continuously formed by the choroid plexuses in the lateral and third ventricles and has a specific gravity of 1.002–1.009. The fluid passes into the fourth ventricle, and then into the subarachnoid space around the brain and spinal cord through three foramina.

 Most absorption occurs through the subarachnoid villi which protrude into the venous sinuses of the cranium.

 The subarachnoid space does not communicate with the subdural space, but is continuous with the ventricles of the brain through medial and lateral connections in the roof of the fourth ventricle. Separations of the arachnoid and pia mater form the cerebro‐medullary cistern or cisterna magna.Samples of CSF can be obtained from lumbar puncture or through the cisterna magna, although the latter technique carries a higher risk.

 CSF is exchanged every four hours and absorption helps maintain pressure at a constant level.

 CSF acts as a cushion and support for the brain.

 Changes in brain volume can be offset by CSF production and absorption.

 The composition of CSF is tightly regulated. CSF pH remains about 7.33 even with wide changes in plasma pH. Compared to other extra‐cellular fluids, CSF contains 7% higher sodium and chloride ion, and 30% and 40% lower glucose and potassium concentrations, respectively.

 A CSF pH decrease of 0.05 units rapidly results in a fourfold increase in ventilation and reflects the ability of lipid soluble CO2 to cross the blood–brain‐barrier, not hydrogen ions. Active transport of bicarbonate ions returns CSF pH to normal with chronic changes in arterial pH.

 A potential difference between CSF and blood of about +5 mV is the result of an active transport system.

 O2, CO2, barbiturates, glucose, and lipophilic (anesthetic and sedative) substances effectively cross the blood–brain barrier.

 Inorganic ions, highly dissociated compounds, amino acids, and sucrose cross very slowly.

 CSF pressure changes with body position, and expiratory efforts such as coughing or straining can sharply increase the pressure.