The relative effects on cerebrospinal fluid pressure (CSFP) of equipotent concentrations of isoflurane and N20 were compared in 20 patients with brain tumors who had lumbar subarachnoid catheters in place. Patients were randomly assigned to receive one of two anesthetic sequences: group 1, 0.7% end-tidal isoflurane in 02, which was changed to 70% N20 in O2; or group 2, 70% N20 in 02, which was changed to 0.7% end-tidal isoflurane in Oz. End-tidal Pco2 and percent end-tidal N2O and isoflurane were monitored by mass spectrometry from just before changing anesthetics (time = 0 min) until the end of a 20-min observation period (time = 20 min). Ventilation was held constant at Paco2 = 36 ± 1 mm Hg (mean ± SE). The patients in group 1 sustained an increase in CSFP that reached a maximum of 33% above the value at 0 min, despite a 3-mm Hg decrease in Paco2 (P < 0.05). By contrast, CSFP remained unchanged in group 2. Although the absolute increase in CSFP after replacement of isoflurane/02 by N20/02 anesthesia was relatively small (9 ± 1 to 12 ± 2 mm Hg; P < 0.05), the absence of a similar effect in patients where N20 was replaced by isoflurane suggests that replacement of isoflurane by an equipotent concentration of N2O is more likely to lead to an increase in CSFP in patients with altered intracranial dynamics than is replacement of N20 by isoflurane.
Address correspondence to Dr. Marx, Anesthesiology Service, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021.
© 1992 International Anesthesia Research Society