Reduction of Cerebrospinal Fluid Pressure by Hypocapnia: Changes in Cerebral Blood Volume,Cerebrospinal Fluid Volume,and Brain Tissue Water and Electrolytes

The study examined the role of cerebral blood volume (CBV), cerebrospinal fluid (CSF) volume, and brain tissue water and electrolytes on CSF pressure during 4 h of hypocapnia in dogs, Group I (n = 6) was examined during hypocapnia (P_aCO₂ 20 mm Hg), with no intracranial mass being present Group II (n = 6) was examined with an intracranial mass present (epidural balloon, CSF pressure 35 cm H₂O), but no hypocapnia. In group III (n = 6), an intracranial mass was present, and hypocapnia was used to lower CSF pressure. In group I, hypocapnia initially reduced CBV from 3.4 to 2.4 ml. With continued hypocapnia, CBV reexpanded to 3.4 ml by 4 h. CSF volume changed reciprocally, so that intracranial CSF pressure remained constant. In group II, CBV remained steady (2.7 ml), and CSF volume fell only slightly, so that CSF pressure remained elevated. In group III, hypocapnia initially reduced CBV from 2.8 to 2.2 ml, and CSF pressure fell from 35 to 19 cm H₂O. With continued hypocapnia, CBV rose to 2.8 ml by 4 h, but CSF volume fell from 6.1 to 5.0 ml, so that CSF pressure remained low. Net intracranial absorption of CSF did not exceed net intracranial CSF production, suggesting that CSF volume fell because hypocapnia improved access of intracranial CSF to spinal sites of CSF reabsorption. Brain tissue composition was not different among groups. The indicate that hypocapnia lowers elevated CSF pressure initially by lowering CBV. This CSF pressure-lowering effect is sustained (despite reexpansion of CBV) by a further reduction of CSF volume. In this model, brain tissue water or electrolytes did not contribute to changes in CSF pressure.