NITROUS OXIDE.
Special Physiology.
Upon the nervous system, nitrous oxide acts like other anæsthetics,
but the stages of anæsthesia are passed through so rapidly that a
second stage can hardly be distinguished. It is rare for struggling or
excitement to be manifest, unless air or oxygen be admitted at the same
time, when the effect which led Humphrey Davy more than a century ago
to apply to nitrous oxide the name of “laughing gas” is very evident
indeed.
Upon the other systems of the body, nitrous oxide has little if any
effect in itself. The essential point to remember in connection with
nitrous oxide administered unmixed with air or oxygen is that there
is an _inevitable element of asphyxia_. The larger part of the
oxygen normally carried by the red blood corpuscles is eliminated
and replaced by N_{2}O: oxygen starvation is therefore of necessity
present. In other words, the “vicious circle of asphyxia” (_see_
Fig. 5) is entered, and muscular spasm is bound ultimately to appear.
Moreover, the blood pressure rises very materially as a result of the
lack of oxygen. That no harm results to the normal healthy patient
from this rise is due to the fact that the gas does not in itself
poison the heart muscle, which can therefore stand up to the extra
strain of working against higher resistance, so long as the process
is not carried to extremes. A heart muscle weakened by the action of
chloroform would give out at once if exposed to such a test.
Apparatus.
Nitrous oxide is supplied by the makers as a fluid condensed in iron
bottles or cylinders, and only becomes gaseous upon being released from
them. In passing from the fluid to the gaseous state, heat is of course
lost, and it will be noticed that the end of the cylinder which is in
use becomes rapidly crusted over with frost. Ice, moreover, forms in
the small channel at the head of a cylinder, and is apt from time to
time to block it.
The cylinders are of various sizes and designated after the numbers
of gallons of gas which they will supply: 25 is the smallest size, 50
or 100 are more usual; anything up to 500 is occasionally met with in
hospital practice. Moreover the cylinders are of two types, called
respectively _vertical_ (for use in the upright position), and
_angle_ for use in the horizontal position (_see_ Fig. 13).
[Illustration: FIG. 12.--Frame for adapting vertical
cylinders to foot use.]
[Illustration:
FIG. 13A. FIG. 13B.
Two types of N_{2}O cylinders.
A. Vertical (or ordinary). B. Angle.]
The cylinders are fixed in frames of various types of which examples
are seen in Figs. 13 and 14.
Upon each cylinder, of whatever size or type, will be found a label
stating its weight when full and empty, the difference representing
the weight of the contents when the bottle is full. For instance, in
the case of the 50-gallon cylinder the weight of its full charge is 15
ounces. Weighing the cylinder is the only certain means which we have
to estimate how much of the charge remains. The student will readily
appreciate therefore that once a cylinder has been used at all there
is always a risk of the supply of gas from it running out during an
administration. It is for this reason that cylinders are habitually
used in couples, one of which is always supposed to be quite full. To
this one it is well to attach a label marked “full,” and care must be
taken to replace at once a cylinder known to be empty. In this way we
always have upon the frame one cylinder partly and another entirely
full.
[Illustration: FIG. 14.--Complete N_{2}O apparatus,
showing twin cylinders, supply pipe, 2-gallon bag, 3-way tap,
and face-piece.]
By whatever makers the cylinder is supplied it will be found that the
thread upon the outlet pipe is the same, and the metal nipple figured
in Fig. 14 will fit it. To the distal end of the nipple, a rubber tube
is attached which leads to a rubber bag usually of 2 gallons capacity.
The remainder of the apparatus may be of several types.
Fig. 15 shows the ordinary _Barth three-way_ tap with facepiece;
the indicator on the tap has three possible positions designated on
the dial as “Air,” “Valves,” and “No Valves.” If the tap is pointed
backwards towards the bag at the position marked “air” the end of the
bag is closed and the patient is breathing air only. With the tap in
the middle position of “valves,” the inspiration of the patient will
draw gas from the bag, but the expiration closes the valve which is now
in operation at the orifice of the bag, and will open the expiratory
valve which conducts the expired air into the general atmosphere. In
the third position of “no valves” the patient breathes both in and out
of the bag.
[Illustration: FIG. 15.--Barth 3-way N_{2}O tap.]
[Illustration: FIG. 16.--Hewitt’s wide-bore gas
valves.]
Fig. 16 shows the Hewitt type of inhaler. The calibre of the orifices
through which respiration takes place is greater than in the Barth
three-way tap, and to that extent this type of valve is to be
preferred. Although differently arranged exactly the same possibilities
are present in it.
The _facepiece_ is sometimes made of celluloid with an inflatable
rubber edging. The object of this type of facepiece is that the
colour of the lips may be appreciated by the anæsthetist during the
administration. The preferable plan is to make the whole facepiece of
rubber with an inflatable border. Such a facepiece made by a good maker
will last many years, and is much more stable and reliable than its
celluloid competitor.
The Care of the Nitrous Oxide Apparatus.
This is a matter of considerable moment, particularly to those who do
not use their apparatus every day. After use, the valves, facepiece,
and bag should be disconnected from each other, all moisture wiped away
from the bright parts, and the bag hung up with its open end downwards,
and preferably in a warm room. If re-breathing has been extensively
practiced it is well to wash the bag out with some carbolic lotion
before hanging it up. The rubber valves in the valvepiece are liable
to lose their elasticity, particularly if kept in a cold place after
becoming damp. From time to time the valve piece should be taken to
pieces, the valves carefully dried in front of a warm fire and powdered
over with a little talc.
Administration.
Most commonly nitrous oxide is administered to a patient sitting in a
chair. Care should be taken that the respiration of the patient shall
not be obstructed by tight clothing round the throat or chest, and
that the head and neck are neither unduly flexed nor extended upon the
shoulders. The patient should not have any solid food for two hours
before the anæsthetic. At the last moment he should be instructed to
empty his bladder. Artificial dentures if present should be removed,
and, if the anæsthetic is being given for the purpose of the extraction
of a tooth, it will be necessary before applying the facepiece to
insert between the teeth a dental prop. (See Fig. 6.) Standing to the
left and slightly behind the patient the anæsthetist’s first step is to
secure good apposition between facepiece and face. This is best done by
working from above downwards: that is to say, secure first a good fit
at the bridge of the nose, and then approximate the remainder of the
rim of the facepiece to the cheeks and lower jaw. During this stage the
indicator of the tap is kept at “Air.”
Working with the left foot the administrator now opens the head of one
of the cylinders with the foot key. It is wise first to have loosened
this with a hand key, and leave it just “on the swing,” otherwise
one’s boots are apt to suffer! Gas is allowed to flow into the bag
until it is partially, but by no means tightly distended. The patient
is instructed to breathe naturally and easily, and during the whole
process the anæsthetist should converse with him in a quiet easy way.
The tap is now turned to “valves” and the patient begins to inspire
the gas, a supply of which is allowed to flow steadily from cylinder
into bag. After a few breaths of the gas, when the sensibilities
of the patient are a little dulled, it is wise to allow the gas to
flow a little more freely and to distend the bag. This exercises
upon the patient a slight _positive pressure_, which has been proved
both experimentally and practically to increase the rapidity of the
absorption.[1] The valves are left in operation for some thirty or
forty seconds, after which time the supply of gas should be cut off
and the tap be pushed over to the position of “no valves” for further
twenty seconds. This should be ample to secure full gas anæsthesia.
_The phenomena_ seen in nitrous oxide anæsthesia are so different from
those of any other that a few words must be said about them. Within a
few seconds of the inhalation beginning, the colour of the patient
shows evidence of the presence of the gas in his blood. The normal
complexion changes first to a dull pink, and very rapidly to the
definite blue of cyanosis. The _eye symptoms_ are of the utmost value.
Very early the pupil begins to dilate, and the eyeball tends during the
first twenty or thirty seconds to rotate as if the patient were looking
for some object in his field of vision. In full anæsthesia the eyeball,
however, comes to rest, usually pointed downwards. The pupil is widely
dilated, the conjunctival reflex is almost or even entirely abolished,
but the corneal reflex is still brisk. _The respiration_ tends to
become steadily deeper and more frequent, and in the later stages
stertor at least, if not stridor usually develops. _The muscles_ under
ordinary nitrous oxide anæsthesia are rarely entirely relaxed, but the
limbs hang motionless, and it is only if an attempt be made to move
them into some abnormal position, that one appreciates the persistence
of muscular tone.
A phenomenon peculiar to nitrous oxide anæsthesia is observed in its
deepest stage. Designated as _jactitation_, it consists in a tremor
beginning in the limbs, but spreading from them to the trunk if its
development is allowed to proceed. It is a finer movement than that
described under “ether tremor” (page 38), and wholly different in type
from the athetosis referred to on page 38. Jactitation is almost wholly
an asphyxial phenomenon, and is therefore definitely an indication
that the process of oxygen starvation has been carried as far as is
permissible.
_The signs of fully developed nitrous oxide anæsthesia_ then are:--
1. Deep regular snoring respirations.
2. Dilated pupils.
3. Rotation of the eyeball downwards.
4. Loss of conjunctival but persistence of corneal reflex.
5. A colour of the skin definitely blue, but not blackish blue.
6. The commencement of jactitations.
_The signs of overdose_ are:--
1. An enormously dilated pupil not re-acting to light.
2. Loss of corneal reflex.
3. A blackish blue colour.
4. Jactitations fully developed.
5. Failing respirations.
The final arrest of respiration in nitrous oxide anæsthesia is usually
painfully sudden. Upon the respiratory side the only warning is one or
two gasps, and even that is sometimes absent. The paralysed pupil and
the jactitations are the most useful signs of overdose.
The above, then, may be taken as an account of what one expects to see
in a normal gas anæsthesia during the induction stage. For the great
majority of cases, nitrous oxide is given for the purpose of rendering
painless the extraction of a tooth, and it is, in this large class of
case, the induction stage only which need be considered. It requires
only some fifty to sixty seconds to bring the patient to the stage
described under the heading “fully developed anæsthesia” and when that
has been attained, the mask may be removed and the operation begun.
From the moment of removal of the mask, however, it must be noted that
the patient begins to breathe fresh air and to eliminate the N_{2}O.
The period of anæsthesia available to the surgeon or dentist during
which he must perform the operation, is therefore very small. In thirty
seconds the patient has frequently recovered sufficiently to begin
to feel pain, and it is rare to secure more than forty-five or fifty
seconds by the use of a single dose of nitrous oxide.
Nitrous Oxide and Air.
If the nature of the operation does not necessitate the removal of the
mask from the face, it is possible to maintain nitrous oxide anæsthesia
for some considerable time. The exact length of that time varies a good
deal with two factors--the type of patient and the experience of the
administrator. Heavily built muscular patients are not easily dealt
with by prolonged gas anæsthesia (unless with admixture of oxygen as
explained in chapter viii.) Of far greater importance, however, is the
other factor. The student can easily be taught to give a single dose
of gas for the extraction of a tooth, or the momentary incision of an
abscess. He will, however, be wise to secure a good deal of practice in
that class of work before attempting to prolong gas anæsthesia for more
than a minute or two.
With reasonable skill and experience and the utmost care, it is,
however, perfectly possible to prolong nitrous oxide anæsthesia for
periods of five, ten, or even fifteen minutes in the average healthy
patient. As soon as the signs of full anæsthesia appear, the valve
tap is pushed back to “air” for the space of one inspiration and
one expiration, and then at once pushed back to “no valves.” By
this manœuvre, one inspiration of air is permitted to the patient,
whose colour at once shows amelioration, or at any rate no further
progression of cyanosis. The admission of air is repeated every third,
fourth, or at most fifth respiration. After the first minute or so
of this cycle of events, it is obvious that the contents of the bag
will be composed of a mixture of nitrous oxide, air, and CO_{2} in
proportions quite impossible to calculate. It is therefore best to
push the indicator to “valves,” and allow the bag to be emptied by the
suction of the patient’s inspirations. The cylinder head is then opened
by the turning of the foot-key, and the bag filled again with gas. The
cycle of “air” and “no valves” is then begun again for another minute
or so.
It must be understood that by this process, it is not to be expected
that an ideal anæsthesia can be produced. Some movement of the patient
will not improbably take place when sensitive structures are cut or
handled by the surgeon, and at no time will the muscles be entirely
relaxed. Such an anæsthesia is therefore only suitable for a limited
class of case, but does admirably for, say, opening an abscess,
exploring its interior, and removing from it a sequestrum or an easily
found foreign body. During his service in Macedonia the author had not
at his disposal any of the appliances later to be described under the
heading of nitrous oxide and oxygen, and found “gas and air” a most
useful form of anæsthesia for the requirements of military surgery as
seen in a Base Hospital, under active service conditions.
Contra-indications to the Use of Pure Nitrous Oxide Gas.
In the healthy subject, there is no safer anæsthetic than nitrous oxide
when administered properly and limited to its proper province.
From the account given of the physiological action of the gas it will,
however, be obvious to the student that in a limited class of case its
use is not permissible. Such cases fall into two categories.
_Firstly, cases in which an asphyxial element already exists will
have their condition greatly aggravated by the substitution of N_{2}O
for the oxygen in their blood._--Already caught in the vicious circle
of asphyxia, nitrous oxide would but push them deeper into the
vortex. Examples of such cases are patients suffering from tumours or
inflammatory swellings in the neck which are pressing upon the air
passages. In passing, one may note that it might be the desire of the
surgeon to submit an individual case falling into this group to the
operation of tracheotomy for the immediate relief of the condition. The
short space of time required for this little operation might well tempt
the unwary to choose nitrous oxide as the anæsthetic, and in point of
fact such an error of judgment has more than once been made with fatal
results.
_Secondly, no patient suffering from any condition which will be
aggravated by a sudden rise of blood pressure, should be submitted to
nitrous oxide undiluted by oxygen._--Examples of such conditions are
cases of _dilated right heart_ with weakened cardiac musculature.
Such hearts could not be expected to work against a peripheral
resistance suddenly raised, say, from 120 mm. of Hg, to 180 or even
200 mm.--figures well within the possible in deep gas anæsthesia.
Similarly, so great an increase of pressure would be dangerous
to a patient suffering from an _aneurysm_, or from _extensive
arterio-scelorosis_ with high blood pressure.
It will be noted that the above warnings are limited to the use of pure
nitrous oxide, that is, N_{2}O unmixed with oxygen. The extent to which
the dangers referred to can be met by the admixture of oxygen in the
manner to be described in the next chapter is largely a matter of the
skill and experience of the administrator.
Nasal Methods.
The object of using this route is to be able to continue the
administration throughout the period in which the dentist is doing his
work. The essentials of a suitable apparatus are:--
1. A malleable nosepiece which can be closely adapted to the
nose.
2. Two supply pipes from bag to nosepiece.
3. Some means of admitting air to the stream of gas.
4. A two-gallon bag.
5. A supply of gas.
6. A mouth cover with an expiratory valve only.
The patient is instructed to breathe in through the nose, but to
expire through the mouth. The gas is supplied under some pressure and
the mouth cover ensures that no air is inspired by that route. Some
patients find it easier to conduct both inspiration and expiration
through the nose, and for their benefit an expiratory valve is also
provided in the nosepiece. After unconsciousness has supervened, nearly
all patients begin to-and-fro nasal breathing. The mouth cover may then
be removed, and if it be desired to economise gas, the expiratory valve
in the nosepiece may be thrown out of action.
[Illustration: FIG. 17.--Ash’s Modification of
Paterson’s Nasal Gas
DESCRIPTION:--A--Nose-Cap Attachment with Stopcock for
Air and Gas, and with Inspiratory and Expiratory Valves
and Shutter. B--Nose-Cap. C--Sliding Clip on India-rubber
Tubings. D--Bifurcated Mount. E--Bag Mount. F--Gas Bag
Compressor. G--Gas Bag. H--India-rubber Tubing. MC--Mouth
Cover. This is fitted with an expiratory valve, and should
be used at the same time as the Nose-Cap. By using the two
together patients are more quickly anæsthetised than they
would be if only the Nose-Cap were used, and the Nitrous
Oxide is economised.]
Air must be admitted in limited quantity through the tap after
the first thirty seconds or so; by a judicious regulation of this
mechanism, anæsthesia may be prolonged for five or ten minutes.
Facility with nasal gas comes only after some considerable practice.
Fig. 17 shows Ash’s No. 3 Patented Nasal Inhaler, which admits of
Air or Gas being administered either by the Naso-Oral Method, or by
to-and-fro nasal breathing. _Air_ only is admitted when the Shutter A
is open and turned to the right as far as it will go. _Nitrous Oxide_
is admitted when the Shutter A is open and turned to the left as far
as it will go, and the patient will breathe it in-and-out through the
nose. _For the Nasal Oral Method_, close Shutter A and turn it to
the left as far as it will go. This will cause the patient to inhale
through the nose and exhale through the mouth.
Nasal attachments are provided with most of the gas-oxygen apparatuses
mentioned in the next chapter. With them good results can be obtained
with much greater ease.
For a more detailed account of nasal methods, the student is referred
to works devoted entirely to Dental Anæsthesia.