Chapter VIII: Nitrous Oxide and Oxygen

The account given in the previous chapter of anæsthesia by nitrous oxide and air will have convinced the student that it is a somewhat inelegant method with a limited sphere of usefulness. The reason is obvious. In atmospheric air, oxygen exists only in the proportion of about one to four of nitrogen. To sustain life it is therefore necessary to admit to the anæsthetic mixture an amount of air which leaves too little room for the anæsthetic factor--nitrous oxide. If, however, pure oxygen be used, the nitrous oxide is diluted to a much less degree, and far better results are obtained.

The exact scope for gas-oxygen anæsthesia cannot at present be defined with certainty. The work of Crile, and the experience of the war have done much to enlarge it. We may say that the following are definite indications for its use:--

(1) Minor operations lasting 5–15 minutes, particularly if
performed on out-patients.

(2) Operations of any variety upon the subjects of severe shock.

(3) Operations upon patients suffering from acute sepsis.

(4) Operations repeated upon the same subject at short intervals.

As regards (3) and (4), the lack of toxic properties in nitrous oxide gas, and the rapidity with which it is eliminated, give it a tremendous advantage over ether or chloroform. To men with shattered bones and extensive damage to soft tissues, badly infected with sepsis, who required repeated opening up of pockets, changing of gauze packs, etc., the advantage of gas-oxygen over ether was evident, and was easily appreciated by the patients themselves during the late war.

There are, however, certain _drawbacks_ to the method which must be appreciated--

(1) The necessary plant is heavy, bulky, and costly; it cannot
be easily transported.

(2) The running cost is high as compared with ether or
chloroform.

(3) It has been said by some that gas-oxygen can only be given
by an expert. That is a statement too extreme, in the author’s
opinion. Certainly, of all anæsthetics it is the most difficult
to give successfully. Adequate study and proper teaching by an
expert are required, but given these two helps, any one can soon
learn to administer gas-oxygen for minor surgery. Considerably
more experience is, however, necessary before the beginner
should give it for an abdominal section.

Apparatus.

A good gas-oxygen apparatus is necessarily rather complicated. The machines in the market are numerous, and of the most diverse external appearance. Certain broad principles, however, underlie all the machines, and it is to be hoped that some one of them will before long become practically the standard. Once that is effected, hospitals and nursing homes could be expected to provide them. So long as every anæsthetist asks for a different machine, they certainly never will do. A good machine must provide means for the following:--

(1) _An even flow_ of both gases under perfect control.

(2) _A percentage of Oxygen_ in the mixture rising at the
will of the administrator from 2 to 15 or 20.[2] To meet this
requirement it is not necessary that any indicator should be
provided which shows with mathematical precision what percentage
of oxygen is being given. The colour of the patient tells us at
once if too much or too little oxygen is being supplied, and all
we need in the apparatus is some mechanism whereby we can tell
approximately to what extent we are increasing or decreasing the
percentage.

(3) _Positive Pressure._--If the pressure at which the gases are
supplied to the patient can be raised a little above that of
the ordinary atmosphere, absorption is increased and a deeper
anæsthesia produced. In the author’s view, this is an essential
point in a good instrument.

(4) _Re-breathing._--To supply the whole volume of gases
required for inspiration during a long operation is costly and
quite unnecessary. Yet that is what is being done if the whole
administration is conducted upon the “valvular” principle.
Moreover, a prolonged inhalation upon the valves tends to remove
a great deal of CO_{2} from the patient’s blood and tissues
(_see_ Chap. IV.). Periods of partial or complete re-breathing
do much to deepen respiration, and reduce the cost of the
anæsthetic.

(5) _Warming the Gases._--While not essential, this is certainly
an advantage.

(6) _Addition of Ether Vapour to the mixture._--Gas-oxygen
even well given is hardly capable of reducing to quiescence
very robust people, unless the oxygen percentage is kept to an
undesirably low level. The merest trace of ether vapour as an
adjunct is a great assistance during the stages of the operation
where very sensitive structures such as the parietal peritoneum
are being handled. The more experienced the anæsthetist, the
less will he require such assistance.

Hewitt’s Apparatus.

This, with the exception of one designed by Dr Guy and the author, and described on page 130, is the only machine with any pretence to portability by hand. It does not satisfy all the requirements above referred to, but the fact that it was the first practicable means introduced in this country to give gas-oxygen entitles it to full description. (_See_ Fig. 18.)

Essentially it consists of the following:--

(1) A supply of nitrous oxide and oxygen in separate cylinders.
Hewitt’s own stand held two of nitrous oxide and one of
oxygen.[3]

(2) Rubber pipes of supply for each of the two gases. For
convenience, it is sometimes arranged that one of these shall
run inside the other.

(3) Two 2-gallon bags. Nitrous oxide is led into the one, oxygen
into the other. The mouth of _each_ bag is guarded by an
inspiratory valve.

(4) The mixing chamber. Upon the surface of this are marked
successively: “Air,” “N_{2}O,” “O_{2}, 1 2 3 4 5 6 7 8 9 10.”
As the indicator is pushed from “air” to “N_{2}O,” the patient
begins to inhale nitrous oxide only, but as it travels into the
numerals 1, 2, etc., a proportion of oxygen is added.

Immediately below the mixing chamber is the _expiratory valve_.

(5) Lastly, there is the _facepiece_, identical with that used
for pure nitrous oxide.

It would be fallacious to suppose that the numerals 2, etc., on the dial represent accurately the percentage of oxygen yielded by the instrument when the indicator points to one of these figures, nor did Hewitt ever make such a claim. What the figures do represent is a number of holes in the wall of the mixing chamber, opposite to the aperture from the oxygen bag, which are uncovered one by one as the indicator moves over. The amount of oxygen which enters into the mixing chamber is regulated by the number of these holes uncovered, and also by the tension of the oxygen bag. If the figure on the dial is to be even a rough index of the actual percentage of oxygen present in the mixture, it is necessary to keep the tension reasonably constant, _i.e._ to regulate the flow of oxygen from the cylinder by manipulation of the foot key. In a brief administration for, say, a dental case, this is not necessary. It is sufficient to fill the oxygen bag once, and then turn off the supply. If, however, a long administration is required, a constant flow of oxygen of just the requisite amount must be secured.

Administration.

Put the lever at “air,” and fill up each bag to an equal and moderate degree of distention. Adapt the facepiece accurately to the patient’s face, and then push the lever to “N_{2}O”. After a few inhalations, move to 2 of oxygen; regulate the flow of nitrous oxide from the cylinder so that the N_{2}O bag remains slightly distended. Gradually move the indicator along the numerals until the figure 6 or 8 is reached at the end of about a minute or a minute and a half. Women and children require more oxygen than men. The former are easily cyanosed; if the latter are fed too generously with oxygen, they are apt to become excited. Take as your guide to the amount of oxygen required the colour of the patient, the type of respiration, and the size of the pupil.

The _colour_ aimed at can only be learnt by experience, but is best described as a dull pink.

_The Type of Respiration._--Too little oxygen leads to stertor and even stridor; too much oxygen, to a light almost noiseless respiration, which to the experienced ear is the certain precursor of a stage of excitement. Such a stage is clear evidence of too much oxygen having been given.

_The pupil_ should not be dilated to anything like the degree seen with undiluted nitrous oxide. A moderate distention only is to be desired.

_Full Anæsthesia_ should be reached in 100–120 seconds. It is marked by:--

(1) Dull pink complexion; (2) full respiratory movements with
a stertor not exceeding that of gentle snoring; (3) eyeballs
rotated downwards; (4) moderately dilated pupils; (5) loss of
conjunctival reflex; (6) corneal reflex present but not very
active.

If the object be the removal of a tooth, the mask may now be removed, and the dentist may rely upon a period of anæsthesia somewhat longer than that furnished by pure nitrous oxide. He ought to secure approximately one minute in which to do his work.

Anæsthesia by this apparatus may however be prolonged for an indefinite time if desired. In order to maintain the patient in the condition described above, it will be necessary gradually to increase the supply of oxygen. For this purpose Hewitt added to his mixing chamber a supplementary oxygen supply giving 10 or 20 volumes of oxygen. As a matter of fact, all the necessary supply can be got through the original ten holes _if the tension in the oxygen bag be increased_. The regulation of all this requires, of course, considerable practice and experience.

The signs upon which we rely for warning that the supply of oxygen is insufficient to keep the patient safe, are chiefly the colour of the face, which must not pass from dull pink to blue, and the size of the pupil.

Deficiencies of Hewitt’s Apparatus.

These are chiefly two:--

(1) There is no means of producing _positive pressure_. Any
attempt to distend the nitrous oxide bag beyond a certain point
simply leads to escape of the gas through the mixing chamber and
out of the expiratory valve even during inspiration. To this
defect especially must we attribute the fact that an anæsthesia
deep enough for abdominal section is difficult to secure with
the Hewitt instrument.

(2) There is no means of securing _re-breathing_. The whole
administration must of necessity be conducted “upon the valves.”
This latter fault is remedied by the modification introduced by
_Burns_, who took away the inspiratory valve from the mouth of
the N_{2}O bag, and fitted a cap over the expiratory valve which
could be rotated so as to throw the valve out of action. The
author first met this modification at a Base Hospital in France,
and found it a great improvement upon the original instrument.
It is, however, only an imperfect attempt to adopt Teter’s chief
principle.

The Teter and Allied Machines.

The rapid spread of nitrous oxide and oxygen anæsthesia in the U.S.A. brought forward a number of machines of which Teter’s was the forerunner; the other well-known machine of the group is the Clarke. They differ in principle from the Hewitt apparatus in that they permit re-breathing and the use of positive pressure. Diagrammatically, the two are contrasted in Fig. 19.

The key to Teter’s advance is his removal of the inspiratory valve from the mouth of the nitrous oxide bag, and his substitution for Hewitt’s rubber expiratory valve, of a _rigid_ valve, the lift of which can be diminished or entirely abolished, at will (_see_ Fig. 20). By damming, as it were, the flow from the expiratory valve, the administrator can oblige the patient to practise a certain amount of re-breathing, and, if he keeps up a free flow of the gases, he can develop a pressure in the nitrous oxide bag definitely exceeding that of the atmosphere.

Teter also introduced into his apparatus a means to warm the gases, and to add a little ether vapour to the mixture when required.

The Clarke machine is similar in principle to the Teter, but makes a strong point of the intimate mixture of the two gases produced in the mixing chamber which occupies the centre of the apparatus (Fig. 21).

In both these machines, it will be observed, the two bags for N_{2}O and O_{2} respectively are attached to the stand, and the mixed gases are led to the patient by a pipe of wide bore. When re-breathing occurs, it must therefore be up and down this pipe, but the width of the bore seems to obviate any disadvantage which theoretically might be expected from this form of respiration.

In the experience of the author and of many other anæsthetists, very good results can be obtained from either of these machines.

Sight Feed Machine.

In the author’s opinion, machines based upon this principle are likely to take a prominent place in the future of gas-oxygen. Fig. 22 explains the simple mechanism. Each gas is led through a tube dipping into water contained in the sight feed mixing chamber. The ends of the pipes are open, and on the sides of each pipe also are a number of holes. If the pressure at which either gas is delivered is small, bubbles will be seen ascending towards the surface of the water from the upper holes only. The greater the pressure, the further down the pipe does the gas carry before all of it escapes through a hole, and one can therefore get an accurate estimate of the pressure from the number of holes through which the bubbles are seen escaping.

Upon the surface of the water, the two gases meet and enter into mixture and are conveyed away by the third pipe which, of course, does not dip into the water.

Once the eye of the anæsthetist is trained to its use, this is a very simple means of gauging the relative proportions of oxygen and nitrous oxide which are being delivered, and manipulation of the cylinder heads combined with visual inspection of the sight feed enable one to strike the right proportions very easily. The nitrous oxide is usually kept at a constant pressure sufficient to ensure bubbles, not only from all the side holes, but also a few from the open end of the tube. The oxygen pressure is begun at the point where there is a little bubbling from the top hole only, and is gradually increased until there is a full supply from two holes, occasionally a little even from a third.

Messrs Coxeter have recently brought out two sight feed machines designed by Mr Leonard Boyle and Dr Geoffrey Marshall, of which the latter is shown in Fig. 22. This apparatus may be put up in either portable form or a larger type for use in hospitals. An ether chamber is provided for use when necessary in either type.

As originally introduced, the remainder of the apparatus consisted simply of an ordinary two gallon bag, Barth 3-way tap, and rubber facepiece. With such an appliance, it is not possible to secure “positive pressure, a point which the author brought to the notice of the makers. Messrs Coxeter are willing to supply a facepiece and expiratory valve which obviate this defect, being supplied with a mica expiratory valve the lift of which can be controlled. There should be no inspiratory valve.

Administration of Gas-oxygen for the purposes of Major Surgery.

The patient is prepared with the same scrupulous care as if ether or chloroform is to be administered. Half an hour before operation, morphia gr. ⅙ and atropine gr. ¹⁄₁₀₀ are given hypodermically. The anæsthetist before beginning administration, must look over the apparatus most carefully and satisfy himself that every part of it is in perfect order, and that a sufficient supply of both gases is at hand.

The inhalation is begun by the use of nitrous oxide alone, given “on the valves,” and at no great pressure. After a few breaths, oxygen is added very guardedly, the proportion being steadily raised during the first two minutes: after that point, a further increase will not be necessary until several more minutes have elapsed. The pressure at which the mixture is being given is also steadily increased and should reach the maximum permissible within a few minutes. A useful plan is to allow the flow of gases to remain constant, but to close the expiratory valve at frequent intervals for about forty to sixty seconds at a time. During this period of complete re-breathing the tension in the supplying bag will of course rise, falling again slightly when the expiratory valve is allowed once more to come into action. As soon as the tension falls appreciably, the valve is again closed down.

It is wise, particularly in one’s early days, to give a trace of ether vapour during the latter part of the induction stage, and to maintain it until the operation is well under way. Once the anæsthetist is satisfied that the narcosis is proving deep enough for the purposes of the operation, the ether may be shut off and will probably not be required again.

Remember that _depth_ of anæsthesia can be secured in three ways--(1) cutting down the oxygen percentage; (2) increasing the tension of the mixed gases; (3) adding a little ether. Of these, No. 1 is most undesirable, and if carried to the least excess over a period of more than a minute or two may lead to an accident. No. 3 is the means for the beginner to rely upon, until he learns the judicious and skilful use of No 2. The anæsthetist who is learning the method of anæsthesing must resolve that nothing shall tempt him to overstep the stage of dull pink colour, and moderate pupils. If with gas-oxygen alone, he cannot get a satisfactory anæsthesia without resorting to oxygen starvation, let him not be ashamed to turn on his ether.

Abdominal relaxation sufficiently complete to permit the surgeon to explore the abdominal cavity with ease, is not readily secured by gas-oxygen in a patient of robust type. Fortunately, it is the weakly or the severely shocked who really _need_ this form of anæsthesia, and in them abdominal relaxation is fairly easily obtained.

Professor Crile, as has already been explained, does not rely upon the inhalational anæsthetic alone. He infiltrates each layer of the parietes with novocain, thus producing a local anæsthesia. If this method be faithfully carried out by the surgeon, a most complete relaxation of the muscles can be secured.