Isoflurane and nitrous oxide: comparative impact on cerebrospinal fluid pressure in patients with brain tumors

The relative effects on cerebrospinal fluid pressure (CSFP) of equipotent concentrations of isoflurane and N2O 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 O2, which was changed to 70% N2O in O2; or group 2, 70% N2O in O2, which was changed to 0.7% end-tidal isoflurane in O2. 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/O2 by N2O/O2 anesthesia was relatively small (9 +/- 1 to 12 +/- 2 mm Hg; P < 0.05), the absence of a similar effect in patients where N2O 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 N2O by isoflurane.