Physiological Effects of Nitrous Oxide on Anaesthesia
Status:
Completed
Trial end date:
2010-10-01
Target enrollment:
Participant gender:
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.