Nitrous oxide has been in use for many decades in parts of Europe and there
is massive information about its effects.  The main reason it fell into
disfavor here is that it seemed to pose a reproductive hazard for staff who
were exposed to it while working labor ward, but I suspect it was
bad-mouthed by anesthetists who were trying to get a foothold in labor wards
to provide epidurals, too.  Most labor wards here had systems without any
way to evacuate the exhaled gas or the leaked gas from the room so staff
could be exposed to low levels of it over long long periods, unlike the
sporadic short term exposure laboring women got.  The gas itself can be
delivered in any mix with oxygen so you can end up getting almost pure
oxygen or almost pure nitrous.  We used a lowish concentration of nitrous
and never more than 50%.  It is rapid-acting and rapidly excreted, barely
enters the blood and its effects, such as they are, disappear within 15-30
seconds of taking the mask away from your face.  We would instruct women to
start inhaling it deeply at the first sign of a contraction, and then
to take the mask off as soon as the contraction peaked, and they were quite
coherent and 'present' between contractions then.   In the concentrations we
used, the main complaints I heard were from women who objected to the
claustrophobic feeling of breathing into a mask.  They didn't get out of
their minds on it but it seemed to help them get through the peaks of
contractions.  It doesn't cause respiratory depression in newborns.  Since
its use was in a decline when the first research on effects of analgesia on
the behavior of newborns and mothers was getting started, I don't think
there is much systematic knowledge about it, but I haven't done a search for
it myself.  But the pharmacokinetics would suggest that it doesn't
accumulate in the fetus.  We never used it in second stage because it made
it too difficult to badger the woman into directed pushing with prolonged
breath-holding, which was, and I fear remains, the rule rather than the
exception here for how midwives deal with second stage, esp in primips.  The
silver lining in that cloud is that there would be almost no possibility of
residues in the mother in the last 15-30 minutes before the baby emerged.

A more serious objection to using it is to be found in the doctoral research
of the late Karin Nyberg, a Swedish midwife who did research on labor
analgesia and substance abuse prevalence in the children exposed to maternal
analgesia.  Use of nitrous oxide in labor at the concentrations used in
Sweden in the 1950's and 1960's was associated with increased likelihood of
amphetamine abuse in the children when they were older. She also found a
correlation between opiate use in labor and opiate abuse in the offspring.
 Her research was so controversial (and upsetting to midwives) that it has
mainly been ignored, at least here in Scandinavia.  She replicated some of
her work from Sweden in the US, when she was a visiting scholar in Rhode
Island, I think in the late 1980's or early 1990's.  I don't know what the
concentrations were and I think Karin extracted data from labor records with
all the pitfalls inherent in that kind of data collection.

Rachel Myr,
Kristiansand, Norway

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