Concentration-related changes in the rate of CSF formation and resistance to reabsorption of CSF during enflurane and isoflurane anesthesia in dogs receiving nitrous oxide.
The rate of cerebrospinal fluid (CSF) formation (Vf) and resistance to reabsorption of CSF (Ra) were determined at four concentrations of enflurane (0.9, 1.8, 2.0, and 3.5% end-expired) or isoflurane (0.6, 1.1, 1.7, and 2.2% end-expired) in dogs receiving nitrous oxide (66%) in oxygen. At the two higher concentrations of enflurane, Vf was about 27% greater (38-40% greater when corrected for the effects of time) than at the two lower concentrations of enflurane. At the two lower concentrations of enflurane and at all doses of isoflurane, Vf did not differ between or within anesthetic groups (0.029-0.033 ml/min). The pattern of Ra data was more varied. Ra was elevated with the two lower concentrations of enflurane (by about 68%) and with isoflurane 1.1% (by about 33%), and reduced with the two higher concentrations of isoflurane (by about 14-17%). It is concluded that the effects of enflurane and isoflurane on CSF dynamics are concentration-related. All concentrations of enflurane studied favor CSF volume expansion, while high concentrations of isoflurane favor CSF volume contraction.
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Department of Anesthesiology, School of Medicine, University of Washington, Seattle, Washington, USA.