Physiological Effects of Nitrous Oxide on Anaesthesia
Condition: Anesthesia
Intervention: sevoflurane on air/O2; sevoflurane in N2O/O2
Intervention description: sevoflurane on air/O2; sevoflurane in N2O/O2
Intervention MeSH terms: Sevoflurane
Phase: Phase 4
Study type: Interventional
Primary purpose: Treatment
Allocation: Randomized
Intervention model: Parallel Assignment
Masking: Single (Participant)
Enrollment: 20.0
Sex: All
Minimum age: 18 Years
Healthy volunteers: Accepts Healthy Volunteers
Keywords: Nitrous Oxide; washin; washout; second gas effect; sevoflurane; induction; emergence

Summary:

      Nitrous oxide is the oldest anaesthetic agent still in routine use today. Despite huge
      changes in the pharmacology of volatile anaesthetic agents and intravenous anaesthetics, the
      unique properties of nitrous oxide have maintained its place in modern practice, where it is
      used in combination with other, more powerful inhaled agents, such as sevoflurane. It has
      useful analgesic properties, unlike the other agents used today, and its inclusion reduces
      the concentration of other agents required to maintain an adequate depth of anaesthesia for
      surgery.

      In particular, its low solubility in body tissues gives it a unique pharmacokinetic profile,
      with rapid washin and washout from the body. It has been shown to have a similar effect on
      the speed of uptake of accompanying agents like sevoflurane (the "second gas effect"), which
      have much slower pharmacokinetics. A recent study by us suggested that this promotes faster
      and smoother onset of anaesthesia, as measured using the standard monitor of depth of
      anaesthesia (the BIS monitor). This finding requires confirmation prospectively in a larger
      group of patients. The investigators further hypothesise that a similar effect also exists on
      washout of sevoflurane at the end of the procedure, promoting quicker recovery (emergence)
      from anaesthesia. This has never been previously demonstrated. This information will help
      better define the place of nitrous oxide in achieving optimal outcomes in modern anaesthetic
      practice. The investigators propose to conduct a simple study to measure the effects of
      nitrous oxide washin and washout on exhaled concentrations of accompanying sevoflurane during
      both induction of anaesthesia and emergence, and identify any accompanying effect on the rate
      of change in depth of anaesthesia using BIS. The investigators hypothesise that the rate of
      fall of exhaled sevoflurane concentration at the end of anaesthesia will be more rapid in the
      group of patients breathing a gas mixture containing nitrous oxide, and that the rate of fall
      of BIS on induction and the rate of rise of BIS on emergence will be faster in the nitrous
      oxide group.
    

Eligibility criteria:

        Inclusion Criteria:

          -  Adult patients undergoing elective general or orthopaedic surgery under relaxant
             general anaesthesia anticipated to take >1 hour

        Exclusion Criteria:

          -  Age under 18 years

          -  Morbid obesity BMI > 35

          -  Severe or moderately severe lung disease (FEV1 < 1.0L, FEV1/FVC < 50%)

          -  Past history of severe post-operative nausea and vomiting

          -  Pregnancy