Veterinary Anaesthesia (Tenth Edition)

W. B. Saunders An imprint of Harcourt Publishers Limited © Harcourt Publishers Limited 2001 is a registered trademark of Harcourt Publishers Limited The right of L.W. Hall, K.W. Clarke and C.M. Trim to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act, 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior permission of the publishers (Harcourt Publishers Limited, Harcourt Place, 32 Jamestown Road, London NW1 7BY), or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Limited, 90 Tottenham Court Road, London W1P 0LP. First edition published in 1941 by J.G. Wright Second edition 1947 (J.G. Wright) Third edition 1948 (J.G. Wright) Fourth edition 1957 (J.G. Wright) Fifth edition 1961 (J.G. Wright and L.W. Hall) Sixth edition 1966 (L.W. Hall) Seventh edition 1971 (L.W. Hall), reprinted 1974 and 1976 Eighth edition 1983 (L.W. Hall and K.W. Clarke), reprinted 1985 and 1989 Ninth edition 1991 (L.W. Hall and K.W. Clarke) ISBN 0 7020 2035 4 Cataloguing in Publication Data: Catalogue records for this book are available from the British Library and the US Library of Congress. Note: Medical knowledge is constantly changing. As new information becomes available, changes in treatment, procedures, equipment and the use of drugs become necessary. The authors and the publishers have taken care to ensure that the information given in this text is accurate and up to date. However, readers are strongly advised to confirm that the information, especially with regard to drug usage, complies with the latest legislation and standards of practice. Printed in England Prelims 8/28/00 11:50 AM Page ix Preface This, the 10th edition of Veterinary Anaesthesia, is a direct descendant of Anaesthesia & Narcosis of Animals & Birds by Sir Frederick Hobday published in 1915 by Baillière, Tindall and Cox. The 1st edition of Veterinary Anaesthesia, which had marked similarities to Hobday’s book, was written by Professor J.G. Wright and published in 1942. Wright’s intention was to ‘incorporate in a small volume the present status of knowledge on anaesthesia in the domestic animals’. This he accomplished in 198 small pages with fewer than 100 references to publications – including many to his own work. Since then tremendous advances have been made and now almost every week that passes sees new publications relating to veterinary anaesthesia and its related subjects.Thus, we have had to acknowledge the impossibility of including a comprehen- sive bibliography in this edition, but we have selected references which should provide a useful introduction to the literature. In addition, we have indicated where attention should be given to material published before computerized databases became available because, despite the increasing complexity of anaesthesia, simple methods also work. Although extensively revised, the general concepts remain as in previous editions. The aim is still to provide a text for undergraduate veterinary students, a reference work for veterinarians in general practice and laboratory scientists, and a stimulating introduction to the subject for those wishing to specialize in veterinary anaesthesia through the examinations of the Royal College of Veterinary Surgeons, the American College of Veterinary Anesthesiologists and the European College of Veterinary Anaesthesia. Because we believe that clinical anaesthesia encompasses much more than an exercise in applied pharma- cology, emphasis is still on the effects of clinically useful doses of drugs and of practical techniques in animal patients, rather than on pharmacological effects demonstrated in healthy experimental animals in laboratories. We wish to express our appreciation of the invaluable help given by Mrs Lorraine Leonard, librarian of the University of Cambridge Veterinary School, and the librarians at the Royal College of Veterinary Surgeons in tracing older references. Mr Stephen Freane of the Royal Veterinary College gave much assis- tance with computer-generated figures. Finally, our warmest thanks are due to the publishers for their patience, and especially to Deborah Russell whose encouragement ensured completion of the manuscript. L.W. Hall K.W. Clarke C.M. Trim 2000 chap-1 8/28/00 10:44 AM Page 1 General considerations 1 INTRODUCTION ‘pain is the conscious perception of a noxious stimu- lus’ two conditions may be envisaged: general The clinical discipline concerned with the anaesthesia where the animal is unconscious and reversible production of insensibility to pain is apparently unaware of its surroundings, and anal- known as ‘anaesthesia’, a term coined by Oliver gesia or local anaesthesia where the animal, although Wendell Holmes in 1846 to describe a new phe- seemingly aware of its surroundings, shows nomenon in a single word. It is essentially a prac- diminished or no perception of pain. tical subject and although becoming increasingly General anaesthesia is a reversible, controlled based on science it still retains some of the attrib- drug-induced intoxication of the central nervous utes of an art. In veterinary practice anaesthesia system in which the patient neither perceives nor has to satisfy two requirements: (i) the humane recalls noxious or painful stimuli. Analgesia may handling of animals and (ii) technical efficiency. be produced by centrally acting drugs such as Humanitarian considerations dictate that gentle morphine given in doses insufficient to produce handling and restraint should always be em- unconsciousness or by substances having a local, ployed; these minimize apprehension and protect transient, selective paralytic action on sensory the struggling animal from possible injury. nerves and nerve endings (local anaesthetics). The Technical efficiency is not restricted to facilitation analgesia produced by these latter substances may of the procedure to be carried out on the animal, it be classified as local or regional, applied as they must also take into account the protection of per- are by topical application, subdermal or submu- sonnel from bites, scratches or kicks as well as the cous infiltration and by peripheral, paravertebral risks of accidental or deliberate self-injection with or spinal perineural injection. dangerous or addictive drugs. Moreover, today it The anaesthetist aims to prevent awareness of is considered that personnel need protection from pain, provide immobility and, whenever this is the possible harmful effects of breathing low con- needed, relaxation of the skeletal muscles. These centrations of inhalation anaesthetic agents. objectives must be achieved in such a way that the While anaesthesia has precisely the same mean- safety of the patient is not jeopardized during the ing as when it was first coined, i.e. the state in perianaesthetic period. Many animals fear and which an animal is insensible to pain resulting resist the restraint necessary for the administration from the trauma of surgery, it is now used much of anaesthetics thereby increasing not only the more widely and can be compared to terms such as technical difficulties of administration but also ‘illness’ and ‘shock’ which are too non-specific to the dangers inseparable from their use. A fully be of real value. Starting with the premise that conscious animal forced to breathe a strange and 1 chap-1 8/28/00 10:44 AM Page 2 2 PRINCIPLES AND PROCEDURES possibly pungent vapour struggles to escape and ‘nociceptors’ but he realized that impulses in them sympatho-adrenal stimulation greatly increases would not necessarily lead to pain sensation the risks associated with the induction of anaes- unless the central nervous system accepted them thesia. For this reason, veterinary anaesthetists and conveyed them to a pain-perceiving region of often employ sedative drugs to facilitate the com- the brain. The nociceptor fibres were found to end pletion of general anaesthesia as well as to over- preferentially on cells in laminae 1, 2 and 5 of the come the natural fear of restraint inherent in dorsal horn and from there impulses were be- animals and to control any tendency to move sud- lieved to reach the thalamus by way of spinothala- denly during operations under local analgesia. mic fibres which ran in the ventrolateral white In addition, the veterinary anaesthetist must matter of the spinal cord. From the thalamus recognize that not only does the response of each impulses were said to reach a site of pure pain sen- species of animal to the various anaesthetics differ sation (pain centre) in the association areas of the due to anatomical and physiological differences, cerebral cortex. but that there is often a marked variation in response between breeds within each particular Afferent mechanisms species. Another factor which must be considered is that in many parts of the world veterinarians In recent years pain studies have resulted in an must perform tasks without highly skilled assist- almost explosive expansion of knowledge relating ance and when employing general anaesthesia, to pain mechanisms in the body and have shown after inducing it themselves, have to depute its that these earlier ideas must be abandoned in maintenance to a nurse or even to a lay assistant. favour of an interlocking, dynamic series of bio- (It must be pointed out that in many countries, logical processes which need still further invest- including the UK, the delegation of tasks related to igation. It is apparent that in the periphery the old anaesthesia to an untrained person may in law be picture of fixed property nociceptor cells in dam- considered negligent if a mishap occurs). Thus, the aged tissue passively detecting the products of cell continued development in recent years of safe, breakdown does not accurately represent what is simple, easily applied techniques of general anaes- actually happening.

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