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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 and bloodstream.

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 used.
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 more painful.

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 or mouth-piece.
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 comfortable.

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 during childbirth.

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 chloroform!'.

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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