Bible Encyclopedias
Anaesthetics

Nitrous Oxide

One of the surgical lessons of the World War was that persons suffering from severe shock and loss of blood from wounds did not progress favourably following operation under chloroform or ether, but that the prospects of recovery were distinctly improved when performed under the continuous inhalation of nitrous oxide gas. The reasons for this cannot be stated precisely, but it may be said in general terms that nitrous oxide is less depressing, and further that owing to its exceedingly rapid excre-. tion, consciousness and normal bodily conditions are quickly restored after completion of the operation.

Nitrous oxide, or " laughing gas " as it was formerly termed, is familiar as an agent for producing brief periods of narcosis, as for the extraction of teeth. When administered thus in a pure state it excludes the admission of air to the lungs, and if continued would cause complete asphyxia; the problem of continuous administration is therefore the admission of sufficient oxygen to the lungs to satisfy the needs of the body. Air contains about one-fifth of its volume of oxygen, but if nitrous oxide were diluted to this extent its partial pressure would be reduced to about 80%, which is too weak for the convenient production of its full anaesthetic effect, at least in the early stages of its administration. It is possible, however, to reduce the amount of oxygen inhaled below the normal quantity without reducing the oxygen in the blood to the same degree; this is due to the fact that the absorption of oxygen by the blood is a process of loose chemical combination with the haemoglobin, which is not governed by the laws of the simple solution of gases. Oxygen may in fact be reduced to a proportion of one-tenth of an atmosphere without causing discomfort to the patient or even under ordinary circumstances causing the discolouration of the face known as cyanosis. It may even be reduced lower than one-tenth and yet be capable of sustaining life. The continuous administration of nitrous oxide mixed with oxygen is thus made possible by the provision of a sufficiently delicate mechanism to regulate and indicate the relative proportions of the gases. One form of indicator which has been generally adopted consists of pressure dials connected with the supply tubes from the cylinders of compressed gases; these register the pressures at which the gases are supplied, and the proportions are in the same relation as the pressure of flow. Another form of indicator is that known as a " sight-feed," in which the gases bubble through a glass vessel containing water, the flow being regulated so that one bubble of oxygen passes for a given number of nitrous oxide bubbles according to desire.

The continuous administration of nitrous oxide and oxygen is not, however, a method which is adapted for all classes of cases; the relaxation of the body muscles is not sufficient for the convenient performance of certain operations; the narcosis is not always sufficiently deep, and it may have to be supplemented by an admixture of ether vapour; nor is it a method absolutely free from danger. Its advantages in the cases of profound shock referred to appear to be undoubted, but how far it can be adapted for general purposes is as yet undecided.

Ether

The use of ether as an anaesthetic has received considerable stimulus from the introduction of the " open " method of administration. In order to induce anaesthesia in a muscular person, or to " get him under " in ordinary phraseology, a strong vapour may be required, as strong as 25% to 30% in some cases, and it was formerly supposed to be impossible to attain sufficient concentration from ether sprinkled on a piece of fabric stretched on a frame or " mask." In order to attain this end a " close " method has been in general use, in which the patient breathes to and from a rubber bag over a surface of ether. In this way the vapour becomes concentrated in the bag, but at the expense of the oxygen of the contained air, which becomes rapidly used up, so that the inhaler must be removed periodically to allow of an inspiration of pure air in order to obviate total asphyxia. This method is effective, but far from ideal; the patient is generally more or less " blue " from partial asphyxia throughout the administration, there is a profuse secretion of slimy mucus which must be continually wiped away, the respirations are greatly exaggerated from " re-breathing " the carbon dioxide which accumulates in the bag, and they are often at the same time partially obstructed from the pressure of the closely fitting face-piece. The after-effects are generally unpleasant and not infrequently distressing.

In the " open " ether method the breathing is noiseless, effortless, and only slightly exaggerated, so that delicate abdominal operations can be performed with comfort. The flow of saliva is considerably less than in the closed method (probably from the absence of asphyxia) and this can be entirely abolished by the subcutaneous injection of a minute dose of atropine previous to the administration. There is no sign of cyanosis, and the patient's face remains a healthy colour throughout; the only restriction of oxygen is by reason of the displacement of air by ether vapour which at a maximum will be less than one-third its volume, and as in the later stages of an administration much less vapour is required the restriction becomes entirely negligible. The after-effects of ether, such as vomiting and malaise, are considerably less pronounced than following a " close " administration.

The application of the " open " method to ether inhalation has been brought about by an exceedingly simple adaptation. The liquid ether is applied to a pad of open-wove fabric, such as " stock inette " or a number of layers of absorbent gauze, stretched over a framework mask of which the margin is roughly adapted to the contours of the face; the mask rests lightly upon the face, a soft pad being interposed between its edges and the skin to prevent the entrance of air in this direction. In this way the inhaled air is made to pass through the meshes of the fabric, and in doing so every portion of it comes in close contact with the ether, and takes up a greater proportion of vapour than it would if it merely passed over the surface of the fabric, as in the ordinary way of procedure.

The induction of anaesthesia by the open method is liable to be somewhat prolonged, an undoubted disadvantage, but once full anaesthesia has been produced it is maintained without difficulty, and the results attained are in general more satisfactory than those of any other form of inhalation anaesthesia.

The " intratracheal " method of etherization has in recent years been in considerable requisition for special purposes. It is conducted by passing a narrow tube through the larynx into the trachea almost to the level of its bifurcation. Through this tube a continuous current of air and ether vapour is forced into the lungs at a pressure which keeps the lungs moderately distended, but not so much so as to abolish the natural respiratory movements. The air returns through the chink of the vocal cords by the side of the tube, and this continuous return blast blows away any solid or fluid particles, blood or pieces of tissue, in the neighbourhood, and prevents their entering the trachea, an accident which may possibly occur in ordinary inhalation methods. The advantage of intratracheal ether in operations involving the respiratory passages is therefore obvious; it is likewise a convenient arrangement for operations upon the face, which is left entirely uncovered; and in operations upon the interior of the thorax a proper aeration of the lungs can be thus insured.

Chloroform

The form of sudden death which is occasionally encountered under chloroform anaesthesia has acted as a deterrent to its more extended employment in spite of its manifest conveniences. An earnest endeavour was made by an influential committee appointed by the British Medical Association to find a method of preventing these chloroform deaths, by enquiring into the conditions of overdosage and devising apparatus for the precise limitation of chloroform vapour to essential requirements. The final report of this committee was issued in 1910, but the number of deaths from chloroform has remained practically undiminished since that time. An attempt has further been made to reduce the risk of overdosage by diluting the chloroform with ether in varying proportions, but this has proved to be futile as a prophylactic against death, for although the number of deaths under pure chloroform has fallen, the number under mixtures of chloroform has risen ten times in a period of ten years.

It is now becoming realized that the typical sudden chloroform fatality is not conditioned by an overdose at all. It has long been known that the majority of deaths occur in the very early stages of anaesthesia before the patient is fully narcotized, and further enquiry into reports of fatalities shows that there is generally some evidence of light anaesthesia preceding death, or else that overdosage can be ruled out of question.

There is a further point brought out by these reports, which was in fact fully appreciated by John Snow in the middle of the last century: whereas in overdose the respiration is paralyzed before the circulation, in the typical chloroform death the outstanding feature is an absolutely sudden failure of the circulation, and the failure of the respiration is a secondary result.

In 1890, Dr. Robert Kirk, boldly and with strong conviction, advanced the theory that chloroform deaths occurred from underdosage, and although his thesis was supported by important experiments, he failed to formulate an acceptable theoretical basis for it. Dr. A. G. Levy, in 1911, reported certain cases of sudden cardiac failure that he had observed in animals obviously in a light stage of chloroform anaesthesia, and he succeeded in reproducing this death by the intravenous injection of small doses of adrenalin in lightly chloroformed animals, but the experiment failed under full chloroform narcosis. This at once accounted for those cases of syncope and death, a number of which had been recorded, following the injection of adrenalin into the mucous membrane of the nose for the purposes of certain nasal operations which were always conducted under light anaesthesia, the form of this syncope being the same as in an ordinary chloroform fatality.

By following up this line of research it was shown that sudden cardiac failure could be induced likewise by various procedures - excitation of the cardiac accelerator nerves either directly or through a reflex mechanism, stimulating the excretion of the adrenal glands, by intermitting the administration of chloroform, or by withholding the chloroform during excitement and struggling; the event never occurred during deep narcosis. The underlying condition of the cardiac syncope was shown to be that of fibrillation of the ventricles, in which the ventricles are entirely deprived of their power of propelling the blood through the arteries. The seeming paradox of too small a quantity of a drug being dangerous is susceptible of explanation although the theoretical points have not been fully worked out: a relatively small proportion of chloroform renders the heart " irritable " and liable to assume a sequence of irregular beats which may pass into fibrillation, whereas a larger proportion of chloroform, by reason of its depressing effect, makes the heart less irritable, and entirely annuls the tendency to fibrillation.

Many years ago J. A. McWilliam expressed the opinion that ventricular fibrillation would be found to account for otherwise unexplained sudden death met with in various conditions, and this demonstration of its occurrence under chloroform is the first confirmation of his views.

On this theory the prevention of death under chloroform can be compassed by simple precautions, by making the induction of anaesthesia continuous and expeditious and thereafter continuously maintaining a full degree of narcosis. Chloroform should never be employed if the conditions of the operation forbid the observance of these rules, and especially in those special cases in which a light degree of anaesthesia is required. These rules are practically a reversion to the injunctions of Simpson, who introduced chloroform as an anaesthetic, and his colleague Syme, in whose experience only one case of death occurred in 10,000 administrations.

Ventricular fibrillation is not always fatal; probably in more than half the cases the heart spontaneously recovers its normal beat, but this happy result can only occur in the first minute or two following the onset of fibrillation. After that time the only prospect of recovery is through the performance of cardiac " massage." This so-called " massage " is a rhythmic manual compression of the heart, producing an artificial circulation; it is combined with an artificial ventilation of the lungs, and so oxygenated blood is supplied to the heart muscle keeping it alive and active, and giving to it a prolonged chance of recovery. In cats this experiment is uniformly successful in bringing about recovery, but in man there have hitherto been only relatively few successes. It appears to be the case that failures have arisen from an imperfect appreciation and application of the principles of cardiac massage, and it is believed that with better knowledge the majority of cases of ventricular fibrillation should prove amenable to this form of treatment.

(A. G. L.)