OTHER EFFECTS OF ANESTHETICS ON CEREBRAL PHYSIOLOGY
Cerebrospinal Fluid Dynamics
Adult humans have approximately 150 mL of CSF, half within the
cranium and half in the spinal CSF space. CSF,
*Hypocapnia
only.
which is formed in the choroid plexuses and to a lesser extent by transependymal
diffusion from the brain's interstitium into the ventricular system, is replaced
about three times per day.[393]
It functions both
as a cushion for the CNS and as an excretory pathway. Anesthetics have been shown
to influence both the rate of formation and the rate of reabsorption of CSF. Table
21-4
provides nonquantitative information regarding the direction of influences
of common anesthetics. All the information has been derived from animals[394]
[395]
[396]
[397]
[398]
[399]
[400]
[401]
because these processes have not been examined
in humans. They may be of relevance when a prolonged closed-cranium procedure is
to be performed in a patient with poor intracranial compliance. The most deleterious
potential combination of effects in the setting of poor intracranial compliance is
increased CSF production and decreased reabsorption. In the dog, this pattern occurs
with enflurane, which is perhaps another reason (in addition to the potential for
epileptogenesis in the presence of cerebral injury and hypocapnia) for omission of
enflurane in this circumstance.
The time course of these effects on CSF dynamics is slow. In
clinical situations in which the CSF space is to be opened (most craniotomies) or
in which a ventriculostomy is in place, they are probably of relatively little clinical
importance.
