[Is the combination of nitrous oxide and hyperventilation in elective neurosurgical operations useful?]

The use of nitrous oxide (N2O) and hyperventilation (HV) in elective neurosurgery is controversially discussed. The emphasis of the study was to show the effects of N2O and/or moderate hyperventilation (paCO2 31.0 +/- 1.2 mmHg) on parameters of cerebral metabolism: jugularvenous oxygen saturation (SjVO2), cerebral extraction of oxygen (CEO2), arterial jugularvenous difference of oxygen contents (AJDO2), arterial jugularvenous difference of lactate (AJDL) and glucose (AJDGL) and lactate-oxygen index (LOI). The study was approved by the Ethics Committee of the University of Leipzig. Forty patients undergoing an elective craniotomy for brain tumour resection were divided into four groups: group 1: n = 10, N2O + normoventilation (NV), group 2: n = 10, N2O + hyperventilation (HV), group 3: n = 10, O2/air + NV, group 4: n = 10, O2/air + HV. N2O + HV led to a significant decrease in SjVO2 from 68.1 +/- 10.7% to 49.7 +/- 5.6%. O2/Air + HV produced a drop from 67.1 +/- 11.1% to 49.8 +/- 7.7%. CEO2 increased significantly in the group N2O + HV from 30.6 +/- 10.6% to 49.6 +/- 5.5% and in the group O2/Air + HV from 31.7 +/- 11.1% to 50.0 +/- 7.8%. AJDO2 increased significantly in the group N2O + HV from 5.79 +/- 1.54 ml% to maximal 8.49 +/- 1.10 ml% and in the group O2/Air + HV from 5.29 +/- 1.76 ml% to maximal 8.03 +/- 1.76 ml%. In the normoventilation-groups 1 and 3, no significant changes in SjVO2, CEO2 and AJDO2 were observed between MP2 and 4. The parameters AJDL, AJDGL and LOI did not show any significant changes in any of the four groups. The described data represent a reduction of cerebral oxygenation, but deleterious effects caused by cerebral ischaemia could not be observed. Based on our data, hyperventilation and its combination with N2O should not be used routinely in neuroanaesthesia.