Handbook of anæsthetics

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 N₂O: 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).

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.

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.

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:—

The signs of overdose are:—

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 N₂O. 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 CO₂ 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 N₂O 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, N₂O 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:—

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.

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.