title: Nitrous oxide for monitoring fluid absorption in volunteers.
abstract: BACKGROUND
  We studied whether nitrous oxide (N(2)O) added to a fluid allows the infused volume to be quantified by measuring N(2)O in the expired air during normal breathing. If so, N(2)O might serve as a tracer of fluid absorption during endoscopic surgery.
  
  
  METHODS
  Twelve male volunteers received continuous and intermittent i.v. infusions (5-45 min) of fluid containing 40 ml litre(-1) of N(2)O. Breath N(2)O and CO(2) concentrations were measured every second via a flared nasal cannula, a standard nasal cannula, or a Hudson mask.
  
  
  RESULTS
  An expression for the amount of infused fluid was obtained by calculating the area under the N(2)O concentration-time curve for samples representative for exhalation (CO(2)>median) and then dividing this area by the median CO(2) for the remaining samples. The N(2)O method then estimated fluid volumes of between 50 and 1400 ml within a 95% prediction interval of +/-200 ml. There were differences of up to 14% in results between the airway devices tested, but the volunteers preferred the flared nasal cannula. N(2)O showed a distinctly higher 3 min variability during intermittent infusion, which could indicate whether fluid absorption is directly intravascular or extravascular. No adverse effects were seen.
  
  
  CONCLUSIONS
  N(2)O method does not require forced end-expiratory breath sampling but still predicts an administered fluid volume with high precision. N(2)O variability can probably be used to distinguish immediately between intravascular and perivesical fluid absorption during surgery.
authors.affiliation: Department of Anesthesiology, Karolinska Institute at South Hospital, Stockholm, Sweden.
authors.forename: D
authors.initials: D
authors.lastname: Piros
authors.forename: D
authors.initials: D
authors.lastname: Drobin
authors.forename: R G
authors.initials: RG
authors.lastname: Hahn
mesh_terms: D000042:Absorption; D000328:Adult; D001944:Breath Tests / Q000379:methods; D002245:Carbon Dioxide / Q000032:analysis; D005440:Fluid Therapy / Q000379:methods*; D006801:Humans; D007262:Infusions, Intravenous; D007552:Isotonic Solutions / Q000493:pharmacokinetics*; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000379:methods; D009609:Nitrous Oxide; D000077325:Ringer's Lactate; D007507:Therapeutic Irrigation
chemical_list: D007552:Isotonic Solutions; D000077325:Ringer's Lactate; D002245:Carbon Dioxide; D009609:Nitrous Oxide