In March, 1868, Dr Evans, residing in Paris, after a visit from Dr Cotton, directed the attention of medical men in England to the value of nitrous oxide as an anæsthetic in dental surgery, and shortly afterwards it was first employed to produce anæsthesia at the Dental Hospital. Nitrous oxide is obtained from nitrate of ammonia, and the particulars of its preparation may be found by referring to the article Nitrous oxide.
Immense quantities of the gas are used in dental operations. It has been computed that in 1870 Messrs Coxeter and Barth could not have prepared much less than 60,000 gallons in London alone. To fit it for transit it is reduced by compression. Fifteen gallons may thus be diminished in volume until it fills an iron bottle holding a quart. Five or six gallons of the gas are, on an average, required for each patient. In the preparation of nitrous oxide for surgical purposes Dr Evans advises it to be made at least 24 hours before it is used, and further recommends its being thoroughly washed. An apparatus for the preparation of the gas was devised by Mr Porter, a description of which will be found in the ‘Transactions of the Odontological Society of Great Britain’ for 1868, in which also mention is made of a face-piece for its administration, the invention of Mr Clover. By means of this latter instrument the desiderata that the nitrous oxide should be inhaled without admixture with atmospheric air, and contamination arising from the expired air given off by the patient, are accomplished, for it has been found that when excitement and talking attend the inhalation of the gas, these effects are due to the presence of the carbonic acid thrown off by the lungs.
When inhaled in the ordinary way, nitrous oxide gas induces exhilaration and narcotism, without asphyxia. When, however, the atmospheric air is carefully excluded, it produces, as we have just seen, anæsthesia without exhilaration. The time required to produce anæsthesia varies from 25 to 120 seconds, by from 10 to 60 inhalations. A patient has been subjected for 10 minutes to its action without experiencing any unpleasant symptoms or after effects. Mr Randle says it is perfectly safe in all short operations, and possibly in long ones also, provided there is due admission of air at proper intervals. It seems tolerably certain that nitrous oxide is largely absorbed by the blood-corpuscles, and it is probable that its presence in them may temporarily act to the exclusion of oxygen, and thus prevent for a time that combination of oxygen with hæmoglobin upon which the red colour of the corpuscles depends. Chemistry, however, has failed to show that nitrous oxide is decomposed in the blood, or that it exerts any of the chemical properties of oxygen on the constituent elements of the blood. Whenever the slightest anæsthetic effect is communicated to the nervous system, a simultaneous effect is produced upon the medulla oblongata, the spinal chord, as well as upon the cerebrum and cerebellum.
The whole available force in the body is undoubtedly due to oxidation. This oxidation is accomplished by means of the blood, and it is therefore evident that a continuous flow of oxygenated blood to the nerve centres is necessary as a source of power and of sensibility, as well as for the reintegration of nerve tissue. Any deficiency of oxygen in the blood is followed by a decreased arterialisation of the whole volume of the blood. Under these conditions the exhalation of carbonic acid is relatively less rapid than its formation, and life cannot continue if the blood in the arteries becomes thoroughly venous, as well in colour as in character. That nitrous oxide, when inhaled, changes the colour of the blood-corpuscles is evidenced by the livid appearance of the face and mucous surfaces; the latter, indeed, is a characteristic accompaniment of its administration, and the darkened colour of the blood may be observed as it flows from the severed vessels. This colour of the blood is probably in part due to uneliminated carbonic acid; but that nitrous oxide possesses in a high degree the property of darkening the blood-corpuscles may be easily demonstrated by directing a jet of the gas for a few seconds upon a little arterial blood in a test tube. Yet, from what has previously been advanced on this point, this latter result may more strictly be due to physical than to chemical causes. An interruption of the circulation in any part of the organism is soon followed by local insensibility in the tissues from which the blood supply may have been withdrawn; and it is beyond dispute that, during the anæsthetic state, the circulation of the blood through the capillary system becomes diminished in velocity. A tendency to stasis begins to appear, accompanied at the same time by a considerable reduction in the supply of arterial blood. These are facts that admit of experimental demonstration, as does also another fact, viz. that during the period of insensibility produced by the inhalation of nitrous oxide the
brain itself is in a state of comparative anæmia. In short, it appears most probable that an arrest of the capillary circulation through the brain, to which several writers have attributed a potential influence as the cause of anæsthesia, is simply, so far as it may exist, a result of it.
The anæsthesia produced by the inhalation of nitrous oxide would, therefore, appear to be referable to an altered condition of the blood, whereby the molecular dynamic changes are interfered with, this interruption being probably due either to the retention of carbonic acid, or to the presence of nitrous oxide; or, as the result of both conditions, to the exclusion of oxygen.
For minor operations nitrous oxide possesses many advantages over other anæsthetics. The principal of these is its safety. In America, in 200,000 cases in which it had been administered, there was only one case of death. Furthermore its use is not contra-indicated in patients having any constitutional derangement, nor for women who are either pregnant or suckling.
Nitrogen, coal-gas, and carbonic acid have also been employed as anæsthetics.
The ‘British Medical Journal’ for June 13th, 1868, contains an account of some experiments performed by Dr Burdon Sanderson, at Middlesex Hospital, with nitrogen. It seems to have been longer in producing insensibility than nitrous oxide, but no lividity of countenance accompanied, nor sickness or headache followed, its administration.
ANALEP′TIC. Syn. Analep′ticus, L.; Analeptique, Fr. Restorative; that recruits the strength lost by sickness.