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527
ANALGESIA IN OBSTETRICSBy P. J. HELLrWELL, M.B., CH.B.,
D.A.,
andA. MICHAEI HUTTQN, M.R.C.S., L.R.C.P., D.A.
Department of Anaesthetics, Guy's Hospital
During the past few years the problem of thecontrol of pain in
obstetrics has received muchattention from obstetricians,
anaesthetists andgeneral practitioners, and the methods
employedhave undergone a profound change. More elabor-ate methods
have been made possible by thegrowing tendency for the mother to
have herconfinement in, an institution rather than at homeand it is
felt that as more beds become availablethis will occur more and
more frequently. Inspite of this, today, the majority of
confinementsstill take place at home where the mother isattended by
a domiciliary midwife acting alone;and for these patients also new
methods havebecome available.
It is now no longer considered satisfactorymerely to give
analgesic drugs when the labourpains are severe, as although there
are nowinnumerable drugs available for this purpose mostauthorities
realize that one of the most importantfactors contributing to a
pain free labour lies inthe mental attitude with which the
motherapproaches her confinement. This correct mentalapproach can
only be obtained by careful educa-tion in the antenatal period and
therefore themodern technique of treatment falls into twomain
divisions:
(A) Antenatal instruction and explanation.(B) Methods employed
during the labour itself.These methods can be grouped as
follows:(a) Drug therapy by the oral, hypodermic,
intravenous or rectal routes.(b) Inhalational agents employing
an apparatus.(c) Local or regional analgesia.Normally a combination
of two of these methods
is used.
Antenatal InstructionFrom the earliest days of the patient's
antenatal
supervision it is essential for the obstetrician orpractitioner
to explain to her the normal processesof pregnancy and labour.
Under no circumstancesmust the mother be allowed to commence
herlabour in a state of terror arising from the fear ofthe unknown,
or from the exaggerated stories ofwell-meaning but unwise friends
and relations.
The patient must be confident that she will receivehelp from her
attendants whenever it is requiredand know that she will not suffer
the agonies whicha less fortunate woman tends to expect.
Clinicalexperience has shown that better results may beexpected
from a willing and co-operative patientwhatever type of analgesia
is used, and towardsthe end of her antenatal period a decision
shouldbe made on broad lines as to what method ofanalgesia will be
employed in her case. ;If anyinhalational apparatus is to be used,
at one of herfinal antenatal visits she should be shown
theapparatus and given instruction in its use. In thecase of gas
and air Minnittl maintains that thepatient must realize that the
analgesic is selfadministered and that she will not become
uncon-scious.
In stressing the importance of this, Minnittfurther considers
that in his opinion one of themain causes of failure of gas and air
analgesia froma self-administered machine is the
inadequateeducation of the patient in its use. These remarksapply
equally well to any other form of auto-inhalational apparatus. In
the same way otherauthorities writing on caudal block have
statedhow important it is that the patient should befully
conversant with the implications of themethod which is being
employed.
In an institution it is most desirable that bothobstetrician and
anaesthetist should work togetheras a team in this matter.
Methods employed during Labour itself(a) DRUG THERAPY
Before describing in detail the various analgesicdrugs used it
is necessary to consider the require-ments of an ideal analgesic,
which are
(a) It must (produce an immediate and satis-factory relief of
pain, leaving the motherconscious and co-operative.
(b) It must not depress the foetal respiratorycentre, thereby
increasing the incidence ofasphyxia neonatorum.
(c) It must not inhibit the uterine contractionsor it will cause
delay in labour and predisposeto post-partum haemorrhage.
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528 POST GRADUATE MEDICAL JOURNAL October I948
(d) The drug must be non-toxic and safe forboth the mother and
the child.
It has often been stated that there is no idealsingle analgesic
drug, and although it is possibleto conduct the whole labour using
only one or onegroup of non-volatile narcotics, better results
aregenerally obtained by restricting their use to theearly Ist
stage, and following on with eitherinhalational or local
analgesics. This combinationhas a synergistic effect and therefore
an increasedefficiency.By this means undue depression of the
infant's
respiratory centres at birth and excessive sleepinessand
unco-operation on the part of the mothergiving rise to delay in the
2nd stage is more likelyto be avoided as the inhalational
analgesics aremore controllable and more rapidly excreted thanthe
drugs given by other routes.
Simple sedativesDuring the early part of the first stage these
are
usually all that are required and in this country amixture which
has attained almost universal popu-larity is one of potassium
bromide 20-30 gr. andchloral hydrate 15-20 gr. Tincture of
opium10-I5 minims is frequently added if a more potenteffect is
required. The mixture should be welldiluted with water to prevent
nausea and may berepeated in four hours if necessary. To avoid
thisnausea the mixture may be administered perrectum as described
by Marston2. The CentralMidwives Board allow these drugs to be
usedby midwives acting alone.' This type ofsedation enables the
patient to sleep between herpains, and frequently tides her over
until thecommencement of inhalational analgesia at a
laterperiod.
The Opium alkaloidsMorphia, omnopon and heroin have all been
used because of their potent analgesic properties,in doses of
morphine J-j grain, omnopon i-igrain, heroin 2- grain. They all
possess the samedisadvantage in that they cause profound
res-piratory depression particularly of the foetus andwill if used
injudiciously increase the rate ofasphyxia neonatorum. This foetal
depression ismaximal about 2 hours after hypodermic injectionand
therefore they should never be administeredwithin three hours of
the expected time of delivery.They are most useful however in the
first stage oflabour but should not be repeated. The type ofcase in
which they are most strongly indicated isthe exhausted patient
whose labour is not pro-gressing and for whom a complete rest is
essential.
Hyoscine Hydrobromide (Scopolamtne)This drug has been used both
alone and in
combination with morphia, omnopon, nembutaland pethidine. If
used alone in doses of -11grain its main virtue is the production
of amnesia,occasionally however it does produce restlessness.In
combination with morphia or omnopon itenjoyed a considerable vogue
for the production of' Twilight Sleep.' According to Hewer3
thismethod has been abandoned almost entirely inthis country
because of frequent failure of anal-gesia, prolonged labours,
occasional violent excite-ment and narcotized babies.
Pethidine (Dolantin, Germany. Demerol, U.S.A.)This is
undoubtedly the most important
addition to the range of non-volatile drugs madeavailable in
recent years, as it is an excellentanalgesic, has few toxic effects
and has no delayingaction on labour. It is the hydrochloride of
theethyl ester of i-methyl-4-phenyl-piperidine-4-carboxylic acid
and was first described by Eisleband Schaumann in 19394.
In addition to possessing an analgesic potencymidway between
morphine and codeine it has ananti-spasmodic action due to
depression of theparasympathetic nerve endings similar to
atropine.This latter property is of value in accelerating
thedilatation of the cervix. Toxic effects are slight,notably
vertigo and nausea but some cases of drugaddiction have been
reported. From an analysisof 500 cases Barnes5 considered that the
drugproduces a high degree of analgesia, has little orno amnesic
effect, but may contribute to respira-tory depression in the foetus
in a few cases.DOSAGE-The usual initial dose is 1OO mgm. by
intramuscular injection when labour is well estab-lished, the
effect is rapid. usually taking place afterabout 15 minutes. This
dose may be repeated athourly intervals if necessary but 400 mgm.
isusually stated to be the maximum required dose,it may be
administered by mouth but its effect isless certain. More recently
it has been shown thata dose of i50 mgm. intramuscularly gives
relieffrom pain without any respiratory depression inthe infant,
but any increase in this dose tends tocause such depression.
Pethidine is excellentwhen used before an inhalational technique
withwhich it can be conveniently combined.
The BarbituratesAlmost all the derivatives of barbituric
acid
have been tried in labour but in this country theyhave never
achieved any universal popularity asthey do not in any way fulfil
the requirements ofthe ideal analgesic already described.
Nembutalis the most commonly used the initial dose being3 gr. by
mouth with additional doses of 4I gr.every 3-4 hours to a maximum
of 71-9 gr. in IZhours. It has been combined both with chloral
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October 1948 HELLIWELL and HUTTON: Analgesia in Obstetrics
529
jiydrate 20 gr. and with scopolamine T1, _( gr.but has the
disadvantage of producing respiratorydepression and occasionally
restlessness withoutany high degree of analgesia or amnesia.
Pentothalsodium and pentothal acid have also been used ina similar
manner by mouth.
Drugs given by the rectal routeThe technique of the
administration of drugs by
this route is the same in principle for them all.For the best
results to be.obtained the lower bowelshould be emptied by an enema
given at the onsetof labour, or failing this the cleansing enema
mustbe given at least one hour before the drug itself
isadministered. To introduce the drug a rubbercatheter is inserted
into the rectum between pains,and the tip of it guided above the
presenting partby means of a finger inside the rectum. The
fluiditself should be introduced in a pain free intervaland if a
pain occurs during the injection thebuttocks are compressed
together until it is over.
(a) ParaldehydeThe main advantages of this drug are its
stability
and the fact that in therapeutic doses there is noclinical
depression of the respiratory centre. Theanalgesia obtained is poor
when compared withthe true analgesic drugs, and nowadays its
mostfrequent use is to obtain a refreshing sleep in awoman
undergoing a tedious labour such as maybe expected in an elderly
primipara. It can beused with complete safety to both mother
andchild and although the infant may be somewhatdrowsy respiration
commences without delay.The dose is from -iI drachm per stone of
bodyweight to a maximum of 8 drachms and is made upto form a io per
cent. solution with normal saline.
Ether-in-oilThis method is used very much more extensively
in America than in this country but has beenfavourably reported
on by Milne and Younger6.It developed as a modification of the work
of J. T.Gwathmy who used ether by the rectum for theproduction of
anaesthesia. The advantagesclaimed for it are its complete safety,
almostuniversal application and ease of control by thepractitioner
working single-handed. In many partsof America it takes the place
of gas and air in thiscountry.The technique of administration as
described
by Lull and Hingson7 is as follows:A cleansing enema of io per
cent. sodium
bicarbonate is given as mentioned previously.As soon as the
patient's pains become uncom-fortable she is given nembutal 3-41
gr. bymouth; when the effect of this is wearing offit may be
repeated or in the case of a protracted
labour morphine gr. I - may be substituted.The ether enema,
which may be either of thefollowing:
(a) Ether 2 OZ. olive oil ii oz. or(b) Ether 24 oz. paraldehyde
2 drachms,
olive oil 9s. 4 Oz.is given when once again the pains
becomeuncomfortable.The analgesic effect is maximal after about
40
minutes and lasts from 2-6 hours; the ethermay be repeated as
required, the averagenullipara requiring 2-3 instillations, the
averagemultipara l-2. In case of need during the crown-ing of the
head the analgesia may be augmentedby some form of inhalational
anaesthetic.
Bromethol (Avertin)This drug is occasionally used in the same
way
as described for paraldehyde, it has however theadded
disadvantages of causing respiratory depres-sion and is reported to
depress the uterine con-tractions. The dose is 75 mgm. per kilo of
bodyweight less 6, kilos for the weight of the uterus andits
contents. Under no circumstances must thisdosage be increased, if
the degree of analgesiaproduced is insufficient gas and air or gas
andoxygen by inhalation should be superimposed.
(b) THE INHALATIONAL ANALGESICSThe importance of antenatal
instruction in the
use of any type of apparatus for this method hasalready been
stressed and it must be ensured thatthe patient is conversant with
the followingpoints:
(i) She must learn how to apply the face-pieceto obtain an
air-tight fit.
(2) The object of the finger-hole safety deviceif present must
be understood and how it isoccluded.
(3) For analgesia to be effective inhalation mustbe conmenced
immediately she feels that a painis imminent. This enables her to
hold her breathand bear down during the second stage whilst thepain
is at its height.
Nitrous oxideNitrous oxide possesses many advantages for
analgesia in obstetrics. It will produce a rapid andsatisfactory
analgesia, will not interfere withuterine contractions and in the
absence of anoxiais completely non-toxic to both mother and
child.It is however a weak anaesthetic and for anadequate effect to
be obtained the oxygen intakeof the patient when using any machine
is reducedto a level bordering on the physiological
minimum.Therefore as a method it is contra-indicated in apatient
who cannot tolerate a reduced oxygenblood tension because of the
effect, of anoxia on a
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530 POST GRADUATE MEDICAL JOURNAL * October 1948
, pre iously damaged myocardium. There is com-plete absence of
any foetal depression, and labouris frequently hastened. Because it
does not inter-fere in any way with liver function it is the
methodof choice in the toxaemias of pregnancy.
Nitrous oxide and oxyg-nThis is probably the best method
available and
a mixture of from 10-I5 per cent. oxygen and90-85 per cent.
nitrous oxide is the one usually em-ployed by an anaesthetist
supervising the analgesiaand using an intermittent flow machine.
Threeto four breaths at the onset of a pain are requiredbefore
analgesia is established. The patient cancontrol her own analgesia
in this way until thelate second stage, when if necessary the
anaesthetistcan take over the machine and produce a
fullanaesthesia.
Special types of apparatus have been designedfor the production
of an intermittent flow, this isessential to avoid wastage
of.expensive material.The McKesson ^-hid Portanest machines are
themost popular but recently Chassar Moir hasreported favourably on
a machine which has beenused in Denmark for some years.
Nitrous oxide and airIn an attempt to reproduce the
satisfactory
conditions obtainable with gas and oxygen, with-out constant
supervision being necessary Minnittintroduced his gas and air
machine in I9331. Atpresent this is the only method available for
useby the domiciliary midwife. The original machinewas constructed
to deliver approximately 45 percent. of nitrous oxide and 55 per
cent. of air.Although in a few cases there is failure to
obtainperfect analgesia many thousands of patients haveobtained
satisfactory relief without ill effect toeither mother or child. It
is well recognized thatmost of the failures occur in the
hysterical, unco-operative type of patient. The principle
governingthe design of this machine was to produce anapparatus
which would be completely safe for thepatient and' yet would
require the minimumamount of supervision. To attain these ends
itwas necessary tr) sacrifice a certain amount ofefficiency.When a
constant gas-air mixture is all that is
available it is difficult to obtain analgesia withsufficient
rapidity from the moment when thepatient feels that a pain is
imminent to the timewhen the pain is at its height. The
experiencedmidwife overcomes this difficulty by anticipatingthe
onset of a pain, but this entails constant andalert supervision. To
accelerate the onset of anal-gesia other workers have attacked the
problemfrom a different angle. Moir8 has designed umachine in which
a limited quantity of pure
nitrous oxide (Q gallon) flows into a reservoir bagfrom which
the patient breathes, as soon as thishas become empty only air is
available. Theapparatus is so designed that the bag takes oneminute
to refill and no more nitrous oxide can beobtained in this period.
In the Wellhouseapparatus designed by Elam9 this principle hasbeen
developed so that following one breath ofpure nitrous oxide the
ordinary mixture of gasand air is available. This was thought to be
neces-sary as the period of analgesia produced by theMoir apparatus
is frequently insufficient to coverthe whole pain. An attachment is
now availablewhich can be fitted to the Minnitt's apparatus
toproduce the same sequence of events.
Trichlorethylene (Trilene)Trichlorethylene is an unsaturated
chlorinated
hydrocarbon, which is a colourless transparentfluid with a sweet
odour. Both the liquid and itsvapour are stable and non-inflammable
and forthis reason it can be safely used in domiciliarypractice.
The vapour is non-irritant to the res-piratory tract at low
concentrations, is rapidlyabsorbed through the lungs and has a
powerfulnarcotic action but it is somewhat slowly excreted.The drug
is relatively non-toxic to the healthyliver and kidneys and has
very little effect onthe blood chemistry and for this reason it has
beenused without ill effect in the toxaemias of preg-nancy.
Trichlorethylene is produced under thetrade name of Trilene which
is coloured blue todistinguish it from chloroform. Because of its
lowvolatility the use of an open mask is unsatisfactory,and an
apparatus for the administration of aTrichlorethylene vapour/air
mixture was firstdescribed by Freedman'0. This is designed
forself-administration in a manner similar to theMinnitt's machine
and is claimed to produce aconstant vapour strength. The finger
hole safetydevice is incorporated as in the gas and airmachines in
order that analgesia alone and notanaesthesia will be obtained.A
modified version of the original Freedman's
machine is now being used with increasingfrequency both in
hospitals and domiciliarypractice. Very satisfactory results as
regards reliefof pain are obtained and in this respect it
appearsslightly superior to gas and air. Clinically thereare no
toxic effects beyond the occurrence ofoccasional cardiac arrythmias
and it does notappear to influence the course of labour. The
onlyserious drawback to the method is the tendencyof a very few
patients to pass beyond the stage ofanalgesia into one of
unco-operative stupor.When this is observed the withdrawal of
theinhaler rectifies the condition and there seem to beno ill
effects, but for complete safety constant
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October 1948 HELLIWELL and HUTTON: Analgesia in Obstetrics
531
supervision of the patient is essential. Trichlor-ethylene has
been used frequently without illeffects on cases of cardiac
insufficiency.
This method is most satisfactorily combinedwith pethidine given
in the early stages of labour,the only combination of drugs which
must beavoided is that a cyclopropane anaesthetic in a
closedcircuit system must not be superimposed on a
tri-chlorethylene analgesia, owing to the danger oftoxiceffects,
due to decomposition in the soda-lime.
Several different types of apparatus have beenconstructed with
which analgesia may easily andspeedily be deepened into anaesthesia
if requiredin the second stage, of these the best known arethose of
Marrett" and Hyatt12. Recently anextremely portable apparatus for
the production ofanalgesia alone has been described by
Hayward-Buttl3.
Trichlorethylene i& a suitable alternative to gasand air.
For domiciliary practice the apparatushas the advantage of being
more easily Portableand also the cost per case is very small.
ChloroformIt is now just over ioo years since Simpson first
used chloroform in obstetric practice and it stillremains one of
the most commonly used analgesicsin domiciliary practice at the
present time. Thereasons why it has retained its popularity for
suchan immense period of time are the rapid andefficient analgesia
produced, the simplicity of itsadministration, without any
complicated equip-ment, and the fact that it is both
non-inflammableand economical to use. It will be noted that
thesesame advantages can be claimed for trichlorethy-lene.
Chloroform however is by no means without itsdisadvantages, it
depresses contraction of theuterine musculature and therefore tends
to prolonglabour and predispose to post partum haemorrhage.It is
toxic to both mother and child and althoughit is sometimes claimed
that the pregnant motheris immune to the risk of primary cardiac
failure,that she is not immune to all the dangers of chloro-form
was well shown in the Investigation under-taken by the Royal
College of Obstetricians andGynaecologists in I934 when there were
twodeaths attributable to this drug. Chloroform hasbeen the subject
of much bitter controversy. Itis probable that its opponents have
over emphas-ized its dangers, it is equally certain that
itsadherents have minimized them. Since there are.now so many
undoubtedly safer drugs availableit is hard to produce a convincing
argument forthe retention of this drug.
Chloroform may be administered intermittentlyon an open mask
using a drop bottle or chloroformcapsules. Various inhalers may be
used instead,
probably the safest of which is Mennell's modi-fication of
Junker's bottle.
In view of the fact that trichlorethylene pos-sesses all the
virtues of chloroform without itsdisastrous drawbacks it is tending
to replacechloroform in domiciliary practice among
manypractitioners.Other inhalational methodsThe irritant properties
of ether vapour preclude
its use to any great extent for intermittent anal-gesia, but
when the requisite closed-circuit equip-ment is available 5 per
cent. cyclopropane inoxygen may be used and in this concentration
ithas little or no depressant effect on the foetus.This has the
advantage that the mother is breathingan excess of oxygen and is
probably the method ofchoice in a patient whose cardiac
insufficiency isgiving rise to anxiety. Cyclopropane may also
beused to advantage in toxaemias.(c) LOCAL AND REGIONAL
TECHNIQUESThe main attraction of any of the following
methods is the complete absence of any toxiceffects on the child
from the analgesic used;the main drawback is the specialized
knowledgeand skill required for their administration and thefact
that few are suitable for use outside aninstitution.
(i) Subarachnoid blockThis has no place in a straightforward
labour.
Under certain circumstances however a low spinalblock to Sacral
I may be used for a low forcepsdelivery. In these cases it must
never be used ifthe mother is suffering from shock or if the headis
not on the perineum.
(2) Caudal block (Extra-dural block)The dural sac terminates in
the sacral canal at
the level of the lower border of the second sacralvertebra. Any
fluid injected through the sacralhiatus into the sacral canal will
spread upwardsextra-durally and come into contact with thespinal
nerves where they pass out from themeninges. The level to which
this fluid will risedepends almost entirely on the volume of
fluidwhich is injected. The level of analgesia thereforewhich is to
be obtained when a local analgesicsolution is injected, through the
sacral hiatus canbe accurately determined.
It has been shown that the sensory nerve path-ways from the body
of the uterus enter the spinalcord through the iith and I2th dorsal
segments.Sensory nerve pathways from the lower birthcanal enter the
cord via the sacral segments.Motor nerves to the uterus on the
other handcome from the upper thoracic segments; if thenerve
pathways below the ioth dorsal segment are
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532 POST GRADUATE MEDICAL JOURNAL October I948
blocked, the pain of labour is entirely abolishedwithout
interfering with the uterine contractions.It is on these anatomical
facts that the success ofthis method depends.The disadvantage of
caudal block is in the
limitations imposed upon its use. Because of therigid asepsis
required it can only be employedsafely in an hospitalized patient,
and she must bein the labour theatre at least one hour
beforedelivery is due. For the complete success of themethod the
possibility of its use should have beenenvisaged before the onset
of labour so that themother is fully conversant with the principle,
andonly a co-operative and temperamentally suitablepatient should
be chosen. It is said by some that20 per cent. of patients have
sacral abnormalitieswhich physically preclude the method and
furtherthe technique can be made extremely difficult byobesity.
Local infection at the site of injection isa complete
contraindication and it must only beused with circumspection in
cases of profoundanaemia and dehydration. Caudal block is
alsocontra-indicated in certain obstetric conditions,placenta
praevia, premature separation of theplacenta and disproportion. One
further pre-caution must be observed namely a history ofsensitivity
to the analgesic agent.
This method of analgesia has achieved a muchgreater popularity
in the U.S.A. than in thiscountry. Originally a single dose method
wasused but as the period of analgesia was ofteninsufficient
continuous techniques have beenintroduced in which a needle is left
in positionin the caudal canal. As the needle was liable tobreakage
from movement of the patient Lull andHingson describe the use of a
malleable needle orthe introduction of a ureteric catheter through
alarge bore needle which is then withdrawn leavingthe catheter in
position. The drug advocated is4i per cent. metycaine, using an
initial dose of 30cc. Maintenance doses depending on the rate
ofmetabolism of the drug by the patient are of 20 CC.every half to
one hour. The indication for arepeat dose is given by a dropping of
the level ofthe skin analgesia. A continuous drip techniquehas also
been described.
Great care must be taken when using any ofthese techniques to
avoid either intrathecal orintravenous injection of the drug, as an
addedprecaution when aspiration tests prove negative,5 cc. of the
initial injection should be given and apause of 5 minutes made
before injecting theremainder. In this way intrathecal injection
willbe shown by loss of movement in the toes.
Local InfiltrationThe injection of a local analgesic into the
vulva
is frequently used for an episiotomy or for the
repair of perineal lacerations after birth. In anemergency some
relief may be obtained during anormal delivery to ease the passage
of the headthrough the perineum by infiltrating with i percent.
procaine aroundc the vaginal orifice andbetween the vaginal wall
and the rectum.Injections should also be made into the levatorani
muscles and into the ischio-rectal fossae oneither side, from a
point midway between the anusand the tuberosity of each ischium-to
a depth oftwQ inches.
Intravenous ProcaineA continuous infusion of intravenous
procaine
has been used recently in the U.S.A. to produceobstetrical
analgesia. This method although onlysuitable for hospital use
possesses the advantageof simplicity when compared with caudal
analgesia.The concentKation of procaine used in the infusionis
usually I.o per cent. made up in an aqueous5 per cent. glucose
solution. The only apparentdanger is the threat of convulsions from
overdosewhich is avoided by pre-medication with a barbi-turate
usually nembutal 4 to 3 gr. by mouth priorto commencing the
infusion and by the anaesthetistconstantly watching the patient for
any twitchingof the facial or other muscles in which case thedrip
is immediately stopped14. As however thismethod is still in an
expermental stage its valuecannot yet be assessed.Whatever method
of analgesia is to be employed
it is of paramount importance to bear, in mirnd theeffect of the
analgesic agent on the foetus, as anymethod of pain relief however
effective whichmarkedly raises the incidence of asphyxia
neona-torum is unsuitable for general use. As the non-volatile
narcotics have a more prolonged depres-sant action in this respect
than the volatile inhala-tional agents particularly
trichlorethylene andgas and air, in most cases the best results
areobtained by using the former drugs for the earlierstages of
labour and reserving the inhalationalagents for the latter part of
the first stage andsecond stage.
REFERENCES
I. MINNITT, R. J.(I044), 'Gas and Air Analgesia.' 2nd
Edition,London. Bailliere, Tindall and Cox.
2. MARSTON, A. D. (1946), The Practitioner, 157, 373.3. HEWER,
C. L. (I944), 'Recent Advances in Anaesthesia and
Analgesia,' sth Edition, 292. Churchill, London.4. EISLEB, O.,
and SCHAUMANN, 0. (I939), Dtsch. Med.
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