History of Nitrous Oxide
Nitrous oxide was first produced by the English chemist and
Presbyterian minister, Joseph Priestley, in 1772 and further
investigated by Humphrey Davy in 1800 at the Pneumatic Medical
Institution in Bristol. In his book on nitrous oxide, Davy
recorded that breathing the gas helped to relieve toothache
- from which he was suffering at the time - and suggested:
'it may probably be used with advantage in surgical operations'.
But the pain-relieving properties of nitrous oxide were not
explored any further until nearly fifty years later. Initially,
society was more interested in nitrous oxide as a source of
amusement and entertainment. It is for this reason, no doubt,
that nitrous oxide was commonly called 'Laughing Gas'. (This
term has always seemed rather odd to the author, because in
his experience, laughter is rarely observed!)
Although nitrous oxide was the first anaesthetic ever to be
used, it was soon replaced by ether and chloroform. This was
because the latter were more potent and convenient to use.
Because they were more potent, however, ether and chloroform
were more dangerous - especially if consciousness was lost.
It was soon appreciated that they were not safe for use during
labour except when given by someone experienced in anaesthesia:
even then, tragedies sometimes occurred.
It was in response to this challenge that Dr Minnitt invented
his Gas and Air machine in 1933. It was designed to deliver
a mixture of nitrous oxide and room air in suffficient concentrations
to relieve pain - but not, loss of consciousness. Minnitt's
machine proved to be very effective and soon became available
for midwives to use during labour. Gas and Air remained popular
in Britain for many years - until it was replaced by machines
which delivered nitrous oxide in oxygen - instead of air.
How Nitrous Oxide Works
Like other anaesthetics that we breathe, nitrous oxide depresses
the normal function of the brain - depending on its dose or
concentration. But exactly how nitrous oxide produces analgesia
remains a mystery. Whatever the mechanism may be, it is necessary
for the gas to reach the brain in a concentration which is
sufficient to relieve pain. This is achieved via the lungs
When nitrous oxide is inhaled, it mixes with the air already
in our lungs and then passes into the circulation. This transfer
to the bloodstream occurs easily and quickly. Once it enters
the blood, incidentally, the gas goes into solution - it does
not form bubbles! From the lungs, nitrous oxide starts to
reach the brain (and other organs) within 15 seconds. The
actual amount of gas which reaches the brain depends upon
the concentration that is inhaled and how long it is breathed.
When used in labour, the concentration of nitrous oxide reaching
the brain rises rapidly.
How is Nitrous Oxide Given?
As already mentioned, nitrous oxide is a relatively weak anaesthetic.
A concentration of at least 50 per cent is required to produce
worthwhile analgesia. For this reason, nitrous oxide is always
mixed with oxygen instead of air. Two types of apparatus are
The first mixes the two gases (supplied separately) before
delivery to the patient. This apparatus is usually fixed permanently
to the wall, and the concentration of nitrous oxide can be
adjusted within the range of 0-50 per cent or 0-70 per cent
(depending on national regulations). These upper limits ensure
that patients will not lose consciousness and can never receive
less oxygen than exists in room air (21 per cent). This type
of apparatus is popular in Australasia. Being a wall fixture,
it is not portable and so cannot be used for a home delivery.
The other kind ot apparatus which is commonly used to supply
nitrous oxide is called 'Entonox'. In this case, a 50:50 mixture
of nitrous oxide and oxygen is contained in a single cylinder
. A special valve at the top of the cylinder reduces the pressure
to safe levels (so there is absolutely no danger of being
'blown up'!). It is not possible to alter the concentration
of nitrous oxide when using Entonox, but in practice, this
does not seem to be important. One advantage of Entonox is
that it is portable, and can therefore be used for home deliveries
(in Britain, but not in Australia). Entonox is also used in
many countries by paramedical personnel to provide pain relief
at accident sites and in ambulances.
What Happens To Nitrous Oxide?
Like all drugs that enter the bloodstream, nitrous oxide is
distributed throughout the body. It also passes very easily
across the placenta and is distributed likewise throughout
the baby's body. Unlike opioid drugs, however, nitrous oxide
is excreted from the body very quickly - and entirely - by
the lungs. It does not have to be broken down (or metabolised)
first by the liver and so there are no 'by-products'. This
rapid elimination of nitrous oxide also applies in the case
of the baby: within five minutes of birth it cannot be detected
in the baby's breath at all. Because of its rapid elimination,
it doesn't really matter how long nitrous oxide is used; the
gas does not 'build up' or accumulate to any degree whether
it is used for five minutes or five hours.
How Effective Is Nitrous Oxide?
Most large surveys have come to similar conclusions regarding
the effectiveness of nitrous oxide in labour when it is used
properly. Rather less than 50 per cent of women claim satisfactory
relief: 20 per cent obtain some relief for some of the time,
and approximately 30 per cent find it completely ineffective.
Nitrous oxide very rarely relieves contraction pain altogether.
It is not as effective during labour, therefore, as it is
with other painful conditions, such as bone fractures and
burns. This is because uterine contractions are obviously
How To Use Nitrous Oxide
With both types of apparatus, the gas mixture is only delivered
when the machine detects that someone is attempting to take
a breath in. (It does this by opening a valve when a negative
pressure is applied to the breathing tube.) For this to occur,
an airtight seal must exist and so the mask (or mouthpiece)
must be applied firmly to the face (Figure 6.4), otherwise
the machine switches off automatically. This not only reduces
wastage and pollution but prevents everyone else in the room
from becoming 'under the influence' as well! When the machine
is being used properly, you will hear the valve clicking on
and off with each breath. It is not necessary to take the
mask or mouthpiece away from your face every time you breathe
out because another valve, situated close to the mask, allows
the expired breath to pass easily into the atmosphere. This
valve makes a quiet hissing sound and confirms that you are
using the machine properly.
The speed of entry into the lungs and bloodstream depends
on how deeply you breathe: slow deep breaths are better than
rapid shallow ones. Remember that there is some time lag before
the concentration starts to build up sufficiently in the brain
to have an effect . So it is important to start breathing
from the machine as soon as you feel the contraction coming
on. Don't wait until it hurts, or the gas will not have time
to help you at all. Once the contraction starts to feel easier
again, then you can take the mask away from your face until
you begin to feel the next one coming on.
Getting The Most From Nitrous Oxide
In order to get the most benefit from nitrous oxide, it needs
to be used properly. Your midwife will teach you how to use
the gas most effectively. If the pain relief is not adequate,
and you are using a machine where the strength can be altered,
then you should ask your midwife to make it stronger. Remember
nitrous oxide is perfectly safe and you will not lose consciousness
- even when the machine is adjusted to its strongest setting.
However, it is important that you have control over the mask
or face-piece yourself. Sometimes partners want to be helpful
and hold the mask over your face for you. Always resist this
suggestion (politely, of course!). If is far better (and safer)
that you decide when you want to use the gas. Otherwise, it
can be frightening if someone is holding a mask over your
face when you do not want them to. Some people feel claustrophobic,
or as if they are suffocating. Only you can determine how
you are feeling and only you, therefore, should hold the mask
During the pushing stage of labour, the contractions reach
their peak more quickly and intensely. The nitrous oxide concentration
in the brain may then lag so far behind the peak of the contraction
that it seems ineffective. The urge to bear down is often
so overwhelming during this stage that it is difficult to
use the gas and concentrate on pushing at the same time. For
these reasons, it is best to start breathing the gas before
the contraction begins. At this stage during the labour, the
contractions usually occur at regular intervals and so it
is possible to predict to some extent when the next one will
begin. Your midwife or partner can time the interval between
your contractions and encourage you to start breathing the
gas half a minute or so before the next contraction is due.
You will then be able to put the gas aside and concentrate
on pushing. In this way, the gas will have had an opportunity
to achieve an effect at the beginning of a contraction - yet
leave you free to push, unencumbered. The relationship between
the pain of the contraction and the blood level of nitrous
oxide is shown in the graph.
There are other occasions when nitrous oxide can be useful.
Some women find vaginal examinations or an induction of labour
more comfortable if they breathe the gas for a few minutes
beforehand and during the procedure. It is also helpful in
relieving stretching pain at the vaginal outlet just before
the babys head is crowned. For this reason, nitrous oxide
is often useful as a supplement to epidural analgesia (especially
low-dose epidurals) if perineal pain persists during this
final stage. It may also make delivery of the placenta more
Side Effects Of Nitrous Oxide
Nitrous oxide has no smell and is not unpleasant to breathe.
The mask may smell a little rubbery but this is not usually
unpleasant. Some people have a 'phobia' for face masks and
feel as though they are suffocating. (Such fears often go
back to a previous unpleasant experience; for example, in
the dentist's chair! ) If so, many people prefer to breathe
through the mouth piece instead Not everyone likes the effects
of nitrous oxide. Some people feel that is makes them feels
nauseated (although this occurs commonly in labour anyway).
Others feel confused or disoriented, floating, or a bit drunk.
These feelings are pleasant for some, but unpleasant for others.
The important thing to remember is that all of these effects
will quickly disappear once you stop using it. If they don't
- then something else must be to blame, such as stress or
fatigue. There is absolutely no risk of becoming dependent
or addicted to nitrous oxide when using it during childbirth.
If it is used in high concentrations for a very long time,
nitrous oxide can depress the bone marrow and lead to temporary
anaemia. For this to occur, the exposure period needs to be
at least eight hours (and at concentrations sufficient to
produce anaesthesia). There is no evidence that this complication
has ever occurred -or even could occur - using nitrous oxide
Effects On The Baby
Nitrous oxide does not cause any abnormalities or malformations.
Nor does it interfere with the contractions or have any effect
on the duration of labour. Although it passes easily to the
baby, as we have already seen, it is very rapidly eliminated
as soon as the baby cries and starts to breathe. It does not
have any effect on the fetal heart rate or circulation and
does not depress the baby's respiration at birth. In other
words, nitrous oxide is perfectly safe for the baby.
Studies on the neurobehaviour of infants who have been exposed
to nitrous oxide during labour have detected no influence
of the gas whatsoever - even in the first few hours of life.
Similarly, other studies have confirmed that nitrous oxide
has no effects on the infant's ability to suckle.
On 7 April,1853, Dr John Snow administered chloroform during
labour to Queen Victoria for the birth of Prince Leopold:
The inhalation lasted fifty three minutes. The chloroform
was given on a handkerchief in fifteen minim doses, and the
Queen expressed herself as greatly relieved by the administration.
It was not, however revealed to the public that the Queen
had received chloroform. This may have been because her medical
advisors disapproved of chloroform and that it was used only
on the insistance of the Queen. It is said that Her Majesty
declared 'It is we who are having the baby; and we will have
Diagram to show how nitrous oxide reaches the brain. Note
that the concentration rises quite quickly but there is still
some time lag in reaching the maximum concentration - because
the gas is diluted by the blood and is also taken up by other
tissues. Hence the importance of starting to breathe gas from
the very beginning of a contraction if it is to have optimal
effect. This is obviously beneficial because it means that
it takes effect quickly. Likewise, once the face mask is removed,
the gas is eliminated from the body equally quickly. This
relationship is shown in the graph below
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