title: Nitrous oxide diffusion and the second gas effect on emergence from anesthesia.
abstract: BACKGROUND
  Rapid elimination of nitrous oxide from the lungs at the end of inhalational anesthesia dilutes alveolar oxygen, producing "diffusion hypoxia." A similar dilutional effect on accompanying volatile anesthetic agent has not been evaluated and may impact the speed of emergence.
  
  
  METHODS
  Twenty patients undergoing surgery were randomly assigned to receive an anesthetic maintenance gas mixture of sevoflurane adjusted to bispectral index, in air-oxygen (control group) versus a 2:1 mixture of nitrous oxide-oxygen (nitrous oxide group). After surgery, baseline arterial and tidal gas samples were taken. Patients were ventilated with oxygen, and arterial and tidal gas sampling was repeated at 2 and 5 min. Arterial sampling was repeated 30 min after surgery. Sevoflurane partial pressure was measured in blood by the double headspace equilibration technique and in tidal gas using a calibrated infrared gas analyzer. Time to eye opening and time extubation were recorded. The primary endpoint was the reduction in sevoflurane partial pressures in blood at 2 and 5 min.
  
  
  RESULTS
  Relative to baseline, arterial sevoflurane partial pressure was 39% higher at 5 min in the control group (P < 0.04) versus the nitrous oxide group. At 30 min the difference was not statistically significant. Time to eye opening (8.7 vs. 10.1 min) and time to extubation (11.0 vs.13.2 min) were shorter in the nitrous oxide group versus the control group (P < 0.04).
  
  
  CONCLUSIONS
  Elimination of nitrous oxide at the end of anesthesia produces a clinically significant acceleration in the reduction of concentrations of the accompanying volatile agents, contributing to the speed of emergence observed after inhalational nitrous oxide anesthetic.
authors.affiliation: Department of Anesthesia, Austin Hospital, Melbourne, Australia. phil.peyton@austin.org.au
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Peyton
authors.forename: Ian
authors.initials: I
authors.lastname: Chao
authors.forename: Laurence
authors.initials: L
authors.lastname: Weinberg
authors.forename: Gavin J B
authors.initials: GJ
authors.lastname: Robinson
authors.forename: Bruce R
authors.initials: BR
authors.lastname: Thompson
mesh_terms: D000368:Aged; D000762:Anesthesia Recovery Period; D000769:Anesthesia, Inhalation; D018685:Anesthetics, Inhalation / Q000097:blood / Q000493:pharmacokinetics*; D001784:Blood Gas Analysis; D002245:Carbon Dioxide / Q000097:blood; D056805:Consciousness Monitors; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D023381:Endpoint Determination; D005260:Female; D006801:Humans; D008297:Male; D008738:Methyl Ethers / Q000097:blood / Q000493:pharmacokinetics; D008875:Middle Aged; D009609:Nitrous Oxide / Q000097:blood / Q000493:pharmacokinetics*; D000077149:Sevoflurane; D012907:Smoking / Q000378:metabolism
chemical_list: D018685:Anesthetics, Inhalation; D008738:Methyl Ethers; D002245:Carbon Dioxide; D000077149:Sevoflurane; D009609:Nitrous Oxide