title: Nitrous oxide as a marker for irrigating fluid absorption--an experimental study in the pig.
abstract: OBJECTIVE
  To study whether tracer amounts of a gas, nitrous oxide (N(2)O), might be a suitable marker for absorption of irrigating fluid during endoscopic procedures.
  
  
  METHODS
  N(2)O was dissolved to a concentration of 10% in a commercially available irrigating fluid containing 3% mannitol and 1% ethanol. Seven experiments involving administration of the fluid via either the i.v. or i.p. route were performed in three pigs under general anesthesia. The N(2)O concentration and, in one pig, the ethanol concentration, were measured in the expired breath.
  
  
  RESULTS
  The N(2)O method readily detected i.v. bolus injections of 2, 10 and 20 ml quantities of irrigating fluid. Continuous administration of the fluid over a 20-min period showed that the N(2)O concentration increased and decreased very rapidly when the infusions were turned on and off, while a quasi-steady state was reached after approximately 2 min. The maximum breath N(2)O concentration during a 600-ml infusion was 0.05%. The breath ethanol concentration rose as fast as the N(2)O level but decreased more slowly when the infusion was turned off. As with ethanol, the N(2)O responses were much slower when the fluid was given by i.p. infusion, and the maximum concentration was only one-third of that during i.v. infusion.
  
  
  CONCLUSIONS
  Dissolving N(2)O in irrigating fluid offers the possibility of detecting an administered fluid volume in the exhaled breath in the same way as for ethanol. The advantages of N(2)O include better resolution, lower toxicity and rapid elimination.
authors.affiliation: Karolinska Institute, Stockhom, Sweden. Robert.Holm@sos.sll.se
authors.forename: Robert G
authors.initials: RG
authors.lastname: Hahn
mesh_terms: D000042:Absorption; D000818:Animals; D015415:Biomarkers; D000431:Ethanol / Q000378:metabolism*; D008353:Mannitol / Q000493:pharmacokinetics*; D009609:Nitrous Oxide / Q000378:metabolism*; D012997:Solvents / Q000493:pharmacokinetics*; D013552:Swine; D007507:Therapeutic Irrigation / Q000379:methods*; D014867:Water / Q000737:chemistry / Q000378:metabolism*
chemical_list: D015415:Biomarkers; D012997:Solvents; D014867:Water; D000431:Ethanol; D008353:Mannitol; D009609:Nitrous Oxide