A New Anæsthetic Technique for the Horse: A Preliminary Note on the Use of Cyclopropane-Oxygen Anæsthesia

A New Anæsthetic Technique for the Horse: A Preliminary Note on the Use of Cyclopropane-Oxygen Anæsthesia

C Y C L 0 P R 0 PANE -0 X Y G EN AN 1£ S THE S l A 187 GENERAL ARTICLES A NEW AN~STHETIC TECHNIQUE FOR THE HORSE: A Preliminary Note on the Use ...

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GENERAL ARTICLES A NEW

AN~STHETIC

TECHNIQUE FOR THE HORSE:

A Preliminary Note on the Use of Cyclopropane-Oxygen Anresthesia By K. M. DYCE, B.Sc., M.R.C.V.S.,* 0. G. JONES, M.R.C.V.S.,t and F.

J.

WADSWORTH, B.Sc., M.R.C.V.S.*

IN this note we draw attention to a method for inducing a state of general amesthesia in horses which may have a future in equine practice. In making radiological examinations of the trunk and upper parts of the limbs of ponies it was necessary to be able to place the animal on a PotterBucky pouch and to position it there with the minimum of restraint. For this purpose we required an anresthetic which would comply with the following conditions :(r) It must be safe-a risk perhaps ac8eptable in the performance of a major surgical operation is not justified in the course of a clinical examination. (2) It must be suited to the confined space of an X-ray room. There must be no large casting bed required and induction must be smooth and free from struggling, with risk to the animal and to the apparatus. (3) It must be passible to maintain anresthesia at a level that permits free manipulation of the subject for prolonged periods. Until techniques are standardised repeated exposures, with scrutiny of the plates after each, will be required. (4) It is desirable to have provision for the suspension of the respiratory movements for the several seconds necessary to make exposures. (5) There must be a minimum of equipment obscuring the animal and hindering free alteration of position. (6) It must be possible to terminate anresthesia immediately the examination is completed. (7) Recovery should be rapid and uneventful. These rather exacting requirements are not met by any of the agents in current use in equine anresthesia, and, searching for an alternative method, we were guided by the fortunate experience some of us have had with cyclopropaneoxygen anresthesia in other species to investigate the possibility of adapting this method to the horse. Our first results when using cyclopropane in connection with radiological examinations were so satisfactory that we continued its use

* Royal

Veterinary College.

t Zoological Society of London.

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for this purpose, and also tried the method on the few surgical cases which came our way. In this preliminary account we base our observations on a short series of J 4 cases, all ponies or donkeys. Of these animals, I o were anresthetised for radiological and four for surgical purposes (three castrations, one minor interference); all were in good or moderate condition and were free from respiratory or circulatory disease. They ranged in age from eight months to I 5 years, and in weight from 150-700 lb. Cyclopropane-oxygen ana:sthesia has been used in human anresthetic practice for some time, and in recent years had quite extensive employment in small animal surgery. The properties of the gas and the general technique of its use are therefore familiar and need not be described in detail in the present connection. Apparatus

Because of the comparatively high cost and explosive nature of cyclopropane, this gas must be administered, mixed with oxygen, in a closed-circuit anzsthetic apparatus that permits total rebreathing. The apparatus available to us was a Heidbrink ana:sthetic machine, previously modified for the administration of cyclopropane-oxygen ana:sthesia, and which only required the substitution of a larger (1 gallon) rebreathing bag to be suitable for equine anresthesia. The features of this apparatus are well known and are described in the standard textbooks of anresthesia : the essential points are the completely closed circuit of the circle type, a rebreathing bag on the inspiratory side of the circuit, a sodalime canister providing variable CO, absorption, and provision for the direct inflation of the lungs with oxygen. The apparatus is connected by a Y-piece to a cuffed endotracheal tube (Hewer's modification of Magill's). These tubes should be available in several sizes so that one may be selected that provides the maximum airway possible; if the oral rather than the nasal route is used the size of the trachea is the limiting factor. Convenient sizes for the animals in our series were: 70 em. in length and 2 em., 2.6 em., and 3 em. in external diameter. Administration and Course of Anzsthesia

The animal is prepared by fasting for I 8 hours; no premedication IS necessary. In the place where it is proposed to induce anresthesia, the animal is backed into a padded comer. A site for jugular puncture is prepared in the usual way and anresthesia is induced by the rapid intravenous injection of a minimal calculated* dose of "Sagatal." After an interval of a few seconds the animal sinks *The dose, surprisingly small, is calculated according to weight-14 c.c. is sufficient, m our experience, for a 240 lb. pony, but more investigations on this poi!tt and Ql1 the inflY~:nc;~ gf a~e and condition are required.

FIG. 1 General view of the apparatu s in use.

FIG. 2 Gag and tube in position

(Article by Dyce, paye 187) .

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to its haunches and almo8t immediately rolls on its side; there has been no struggling or excitement in any animal in our series. This injection provides 5-Io minutes of gradually lightening amesthesia-ample time for passage of the endotracheal tube and connection to the anresthetic apparatus. The pony is now secured, and, if desired, a small animal may be transferred to a Potter-Bucky couch or to an operating table. The endotracheal tube is now inserted : the head and neck are extended and the tube is passed through the mouth and larynx for some distance into the trachea before the cuff is inflated. A probang gag may be used to protect the tube from the teeth and to assist in holding it in position. The rebreathing bag is filled with oxygen and the apparatus is connected to the tracheal tube. At this stage the animal is generally showing some signs of recoverying from the injection, and the first quantity of cyclopropane should lx added to the circuit; during the next few minutes further quantities of cyclopropane are added until the desired level of an~thesia i.!l obtained. Theoretically, as there is complete rebreathing, an~thesia could be maintained at this level indefinitely without administering additional an~thetic : in practice further quantities must be given to compensate for loss by leakage from the system and elimination from the animal by portals other than the lungs. It is a simple matter to vary the depth of anresthesia : deeper an~thesia is obtained by increasing the proportion of cyclopropane in the gas-oxygen mixture; lighter anresthesia by releasing some of the mixture from the circuit and restoring the volume with oxygen. Oxygen must, of course, be added either continuously or in frequent bursts to keep pace with utilisation and to maintain the volume of gas in the circuit. We have not as yet recorded detailed observations on the development and course of a~thesia. A pony of 240 lb. requires I!- I i litres of cyclopropane within the first few minutes after transference to the closed circuit, and uses a similar quantity hourly to maintain an~thesia at this level. The initial dose was increased by about 50 per cent for the production of the deeper anresthesia required for the castration operations. So far as we can at present judge, cyclopropane has no undesirable effect on the respiration and is without any marked effect on the circulation. The reflexes, notoriously unreliable in the horse, are affected in the conventional order. A valuable feature of the method is the means of controlling the rate and depth of respiration by varying the amount of expired gas that passes through the soda-lime canister, and therefore the percentage of carbon dioxide in the inspired gas. With some experience the machine can be set so that little adjustment is necessary. A further advantage is the ease with which the abdomen and thorax can be immobilised for periods sufficiently long to enable radiographs ·Of these parts to be made. This is achieved by exerting manual pressure on the inflated rebrea.thing bag sufficient to (}vercome the expiratory movement.

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There is, however, a certain risk that incautious "nflation may develop an excessive pressure within the lungs with the p08Sibility of emphysema occurringthis complication has occurred in man, and the predisposition of the horse to this condition suggests a similar danger. On oompletion of the operation or examination the supply of cyclopropane is cut off and the circuit gradually opened to the air before the apparatus is disconnected. Any hobbles may be removed at this stage, and if on a table the animal should be transferred ·to the floor. The tube is retained in position until consciousness returns, as so long as it occupies the trachea there is no possibility of obstruction of the upper respiratory passages. After a few minutes consciousness will have returned sufficiently for the tube and gag to be removed with safety : recovery is now rapid. Within I o minutes of ancesthesia being discontinued the animal will be supporting itself on its brisket, and another I o- I 5 minutes is sufficient to see it on its feet. Muscular co-ordination returns quickly and the pony may be safely returned to its box within 20-30 minutes from the termination of ancesthesia. Provided the animal is permitted to recover in its own time and is not forced to its feet too soon, there is no sruggling or excitement. We know of no ill-effects resulting from the ancesthesia, and this despite prolonged ancesthesia on a number of occasions. One pony has been lightly anresthetised for · upwards of two hours on three occasions within a fortnight without any apparent ill-effects.

Discussion

Any observations we make on the basis of our present experience are subject to the reservations entailed by the small number of cases, the size of the animals and the depth of amesthesia employed. None the less, our experience within these limitations has been so satisfactory that we have no hesitation in stating that the results we obtained could not have been achieved with any other ancesthetic technique, and in particular the way now lies open for radiological investigations of parts previously considered inaccessible to the radiologist. The advantages of cyclopropane will be apparent since it fulfills the requirements laid down in the introduction to this paper. The more general interest will no doubt be in the application of cyclopropane ancesthesia to surgery for the horse. Here we would be more cautious, as our experience is so slight and as we have never attempted deep surgical ancesthesia. We see no reason to doubt that deeper ancesthesia than that we have required could be obtained with this method, but if this should be unsatisfactory, provision exists for the inclusion in the circuit of an ether or chloroform bottle that could be brought into use when it was necessary to augment the level of anresthesia. These, however, are questions that must await more extensive

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investigations by others more experienced in equine amesthesia before they can be profitably answered. It is to be hoped that the results will fulfill the early promise. Acknowledgments

We are grateful for this opportunity of expressing our thanks to those who have helped us; to Professor McCunn for permlSSton to conduct this work in his department and for his advice and encouragement; to R. H. A. Merlen, M.R.C.V.S., for much generous assistance; to Messrs. S. J. Bryant, T. Crosby, C. A. Fowler and C. P. Levey for technical assistance, and in particular to Mr. Bryant for the design and construction of the gag; to Messrs. W. T. Price and F. Roberts for the care of the animals ; and to Mr. R. Shepherd for the photographs.

THE OCCURRENCE OF BLOOD FILAMENTS OR "PSEUDO-SPIROCHA!TES" IN CERTAIN NEOPLASTIC By

J.

CONDITIONS

G. CAMPBELL, F.R.C.V.S., Ph.D., F.R.M.S.

British Empire Cancer Campaign Unit at the Poultry Research Centre, King's Buildings, West Mains Road, Edinburgh

WHEN fresh blood is examined by dark-ground illumination, a number of filamentous objects may be seen which have, in the past and even quite recently, led to errors in diagnosis through misinterpretation. It is surprising that no reference is made to these structures in any of the current textbooks of hcematology so far consulted. The first mention of such filaments appears to be by Nuttall and GrahamSmith (I 907) in one of their classical papers on piroplasmosis. They found filaments, extruded vesicles, and beaded structures in the blood of dogs, and at first they considered them to be parasites. Later, filaments were also found in the blood of various animals including the fowl. Nuttall and Graham-Smith concluded that these objects are degeneration products of red blood corpuscles, supporting this contention by showing that the production of filaments could be enhanced by warming the blood above its physiological temperature, and they appended some good drawings of typical structures. Kuhn and Steiner (I 9 I7) studied these filaments, claiming that they were spirochcetes and were the cause of multiple sClerosis. They described them as often more delicate than W eil's organism, some showing a loop, and others a highly Tefractile ball at each end. They claimed to have transmitted the