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 CO2 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.)
Fig. 18.—Hewitt’s gas oxygen apparatus.
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,” “N2O,” “O2, 1 2 3 4 5 6 7 8 9 10.” As the indicator is pushed from “air” to “N2O,” 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 “N2O”. After a few inhalations, move to 2 of oxygen; regulate the flow of nitrous oxide from the cylinder so that the N2O 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.
Fig. 19.—Diagrams to illustrate action of (A) Hewitt (B) Teter gas oxygen apparatus. Note that in A the mouths of both bags are guarded by valves of inspiration, while in B the oxygen bag only possesses it.
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 N2O 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.
Fig. 20.—Details of the Clarke Expiratory Valve. In the position of the lever marked “open” the valve lifts easily and widely, and the breathing will be purely valvular: in the position marked “half open,” the valve lift is diminished, and the breathing is partly valvular, partly to-and-fro. In the position “closed” re-breathing only is possible.
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.
Fig. 21.—The Clark gas oxygen machine.
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 N2O and O2 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.
Fig. 22.—Marshall’s sight-feed gas-oxygen apparatus. Of the two glass bottles the one to the left is the sight-feed, that to the right the ether chamber.
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.