Nitrous oxide increases cerebral blood flow velocity during pharmacologically induced EEG silence in humans

We examined the effect of nitrous oxide on cerebral blood flow velocity (Vmca), arteriovenous oxygen content difference and cerebral use of glucose during propofol-induced electrical silence of the electroencephalogram (EEG) in 10 patients undergoing anesthesia for nonneurosurgical procedures. Anesthesia was induced with propofol 2.5 mg/kg, fentanyl 3 micrograms/kg (followed by an infusion of 2 micrograms/kg/h), vecuronium 0.1 mg/kg, and maintained with a propofol infusion (250-300 micrograms/kg/min) sufficient to induce EEG silence. A transcranial Doppler was used to measure the Vmca and a jugular bulb catheter was inserted for oxygen saturation and glucose use measurements. After a 15-period of isoelectric EEG and normocapnia (PaCO2 38 +/- 1 mm Hg), baseline arterial and jugular bulb venous blood gases were drawn, and mean arterial pressure (MAP), heart rate (HR), and Vmca were recorded. Nitrous oxide was then introduced and equilibrated to an end-tidal concentration of 70% for 15 min, after which MAP, HR, Vmca, arterial and jugular bulb venous blood gases were measured again. Nitrous oxide increased Vmca (29 +/- 4 to 35 +/- 4 cm/s, p < 0.01), cerebral use of oxygen (166 +/- 13 to 190 +/- 12 vol%-cm/s, p < 0.05) and glucose (245 +/- 38 to 290 +/- 48 g%-cm/s, p < 0.05) by approximately 20%. Occasional bursts of EEG activity were observed in eight patients studied during the N2O stage. We conclude that in patients with propofol-induced isoelectric EEG, the increase seen in Vmca with the introduction of N2O is mainly due to cerebral stimulation and increase in cerebral metabolic rate.