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Regional analgesia in domestic animals
Current Anaesthesia and Critical Care (1998) 9, 25-32
© 1998 Harcourt Brace & Co Ltd
Medicine
Regional analgesia in domestic animals
R. F. McGregor and R. S. Jones Regional analgesia is commonly practised in veterinary medicine, either alone, with sedation, or in conjunction with general anaesthesia. Many major surgical procedures can be satisfactorily performed under local analgesia with or without sedation depending on the species, temperament and health of the animal. In adult cattle, many procedures are performed under regional analgesia on standing animals and heavy sedation is best avoided to prevent the animal becoming recumbent. This is particularly useful as the dangers of prolonged recumbency can be avoided, and the surgeon can provide local analgesia, and operate without the assistance of an anaesthetist. In the other species however, deep sedation will greatly facilitate surgery by reducing the risk of sudden movement. A number of compounds are used to produce local analgesia in animals. The most universally used compound is lignocaine hydrochloride, but more recently bupivacaine has become available for use in veterinary practice. Bupivacaine has about four times the potency of lignocaine, and lasts at least twice as long. Other local anaesthetic agents used in veterinary practice include mepivacaine, prilocaine, proparacaine and benzocaine, and trials have recently begun using ropivacaine. The toxicity of local analgesic solutions is often disregarded in large animal practice. However, in foals, calves, lambs, goats and small animals it must be considered. Accurate figures are not available for the toxic dose of lignocaine, but it is generally considered unwise to exceed a dose of 5 mg/kg with any of the regional analgesic techniques. As well as the local anaesthetic agents, the ~2 agonists are utilized in epidural regional analgesia as they have significant local analgesic effects thought to be due to activation of spinal cord ~2 adrenoreceptors. Finally, the opiates can also be utilized to produce spinal and intra- articular analgesia due to the presence of opiate receptors within the spinal cord and on the synovium.
Regional analgesia of the head
division of the Vth (Trigeminal) cranial nerve. The horn corium and the skin around its base derives its sensory component from this nerve, thus analgesing it enables the horn buds of young calves to be effectively cauterized without pain ( Fig. 1). In adult cattle with well developed horns, two further injections are required to block the posterior division of the nerve and the cutaneous branch of the 2nd cervical nerve, and it is often easier in these cases to perform a
The most commonly analgesed nerve in cattle practice is probably the cornual nerve, a branch of the ophthalmic R. F. McGregor BVSE, MRCVS, Professor R. S. Jones, Department of Anaesthesia, Royal Liverpool University Hospital, Daulby Street, Liverpool L69 3GA, UK. Correspondence to: R. F. MeG_
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26 CURRENT ANAESTHESIA AND CRITICAL CARE
Fig. 1 Needle placement for infiltration of the eyelids (A) and nerve blocks on the head in cattle: auriculopalpebral (B); infraorbital (C); and cornual nerve (D).
ring block around the base of the horn, to ensure analgesia. Goats horns are supplied by the infratrochlear nerve as well as the cornual branch, and both these nerves require analgesia before dehorning should be attempted (Fig. 2). The auriculopalpebral nerve supplies motor fibres to the orbicularis oculi muscle and blocking this nerve prevents eyelid closure during examination of the eyeball. No analgesia is produced by this method but in conjunction with topical analgesia it is useful for the removal of foreign bodies from the cornea and conjunctival sac in cattle and horses (Fig. 1). The eyelids can also be effectively analgesed by a linear subcutaneous infiltra-
Fig. 2 Needle placement for desensitizing the infratrochlear nerve (B) as well as the cornual nerve (A) in the goat.
tion of lignocaine about 0.5 cm from the margin of the dorsal and ventral eyelids (Fig. 1). Analgesia for enucleation of the eye in cattle is achieved by 1 of 2 methods: retrobulbar nerve block or the method described by Peterson. 1'2 In other species enucleation is usually conducted under general anaesthesia; however, in cattle regional anaesthesia in conjunction with sedation is a successful method. The retrobulbar block is hazardous to perform as it involves pushing the needle along the floor of the orbit until it penetrates the tough periorbita, 20-30 ml of 2% lignocaine solution being injected in small increments as the needle is advanced (Fig. 3). If correctly executed the local analgesic solution produces corneal analgesia, mydriasis and proptosis of the eyeball. Paralysis of the eyeball should also occur. Adequate sedation and restraint is vital to decrease the risk of orbital haemorrhage, penetration of the globe, or damage to the optic nerve or its meninges. The Peterson technique requires more skill than the retrobulbar block but it is less hazardous. The nerves which are analgesed by this technique include the oculomotor, trochlear, abducens and three branches of the trigeminal nerve (Fig. 4). The needle can also be redirected to block the auriculopalpebral nerve. Both these techniques will prevent blinking for several hours so if the purpose of the procedure is other than enucleation it is important to keep the cornea moist. Another common regional analgesic technique used in all domestic species is the infraorbital nerve block (Fig. 1). This involves administering local anaesthetic into the infraorbital foramen to block the infraorbital nerve which is a continuation of the maxillary division of the Vth cranial nerve and is entirely sensory to the upper lip, cheek, nostrils and lower parts of the face. In horses this block enables minor surgery to be successfully carried out under sedation, whilst in small animals it is usually used in conjunction with general anaesthesia for increased analgesia during painful procedures such as rhinotomies. In cattle it also enables the repair of nasal lacerations and the insertion of nose rings. Analgesia of the mental nerve as it emerges from the
Fig. 3 Retrobulbar needle placement t h r o u g h the medial canthus (A) to the orbital apex (B) in cattle.
REGIONALANALGESIAIN DOMESTICANIMALS 27
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Fig. 4 Needle placement for 'Peterson' eye block (a: coronoid process; b: pterygoid crest; c: foramen orbitorotundum). L
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Fig. 6 Needle placement for nerves of the distal forelimb of the horse, lateral (k), palmar (P), and medial (M) views: lateral and medial palmar digital nerves (A); dorsal branches (B); lateral and medial palmar digital nerves (abaxial sesamoid) (C); lateral and medial palmar nerves (D and G); lateral and medial palmar metacarpal nerves (E); communicating branch (F).
Fig. 5 Sites for needle placement to desensitize the supraorbital (A), auriculopalpebral (E and F), infratrochlear (G and H), mental (I) and alveolar mandibular nerves (K).
mental foramen on the lateral aspect of the ramus produces regional anaesthesia of the lower lip enabling suturing of lower lip wounds to be conveniently carried out. Other less frequently practised facial blocks include the mandibular nerve block which will desensitize the whole of the lower jaw, all the teeth and alveoli on the affected side (useful in many dental procedures), and the supraorbital nerve block which desensitizes the upper eyelid (Fig. 5).
Regional analgesia of the limbs As well as allowing minor surgical procedures to be performed without general anaesthesia or to provide added analgesia in conjunction with general anaesthesia, regional limb anaesthesia in horses is used extensively to provide diagnosis of the site of pain producing lameness. Once the characteristics of a lameness have been established regional analgesia is commenced at the most distal point of the limb and various blocks are employed progressively up the leg until lameness is abolished (Fig. 6).
For diagnostic procedures mepivacaine is the drug of choice as it is less likely to cause a tissue reaction than lignocaine which can commonly cause moderate inflammation and oedema. It is also advisable to avoid preparations containing adrenaline as this can produce vascular spasm and the production of white hairs at the site of injection_ Most horses will tolerate nerve blocks without chemical sedation and this is advisable to prevent confusion of results. The Palmar Digital Nerve Block analgeses the medial and lateral palmar digital nerves and will anaesthetize the caudal third of the hoof. Abolition of sensation around the bulbs of the heels will indicate the block has worked. This usually takes about 10 min. The Abaxial Sesamoid Nerve Block blocks the medial and lateral palmer nerves higher up the leg at the level of the proximal sesamoid bones and this blocks the whole of the foot below this point. Confirmation that the block has worked is by abolition of skin sensation around the coronary band. Seventy per cent of the causes of equine lameness are located within the foot but if lameness has not been abolished then the next block to perform is the Low Four-point Block which blocks the medial and lateral palmar and palmar metacarpal nerves just above the metacarpophalangeal joint and anaesthetizes every structure below this point causing abolition of skin sensation over the fetlock and dorsoproximal pastern region. Continuing up the leg, the Subcarpal Block analgeses the palmer and palmar metacarpal nerves immediately distal to the carpus to induce regional anaesthesia of everything below this point. There is no reliable test that this block has worked.
28
CURRENT ANAESTHESIA AND CRITICAL CARE
Regional analgesia above the carpus is not usually required but if necessary then a Median and Ulnar Nerve Block can be performed to analgese the whole of the carpus and distal structures. Lameness in the hindlimb is usually investigated in a similar manner and the regional analgesic techniques for the foot are the same as for the foreleg. However, to analgese the metatarsophalangeal joint in the hindleg, a low six-point block is required to include the medial and lateral dorsal metatarsal nerves. More proximally, a Subtarsal Block is similar to the subcarpal block in the foreleg, and further proximally still, the tibial and fibular nerves can also be analgesed but this is a difficult procedure as the fibular nerve lies between muscle bellies about 3-5 cm deep. All the joints of equine limbs can also be analgesed using strict aseptic techniques with varying degrees of difficulty either to enable diagnosis or to aid analgesia. In small animal practice limb analgesia is not commonly practised although infiltration of the digital nerves is occasionally attempted by infiltration lateral and medial to the first phalanx of the digit to be analgesed (Fig. 7). Another block occasionally used in the dog is a Brachial Plexus Block. 3 This is a complicated procedure involving insertion of a 7.5 cm needle lateral to the chest wall but medial to the subscapularis muscle (Fig. 8). Many complications can occur such as puncture of a major blood vessel producing a large haematoma, damage to the brachial plexus, puncture of the thorax admitting air to the pleural cavity or the introduction of infection into the axilla. However, if adequate care is taken it is a relatively safe procedure of particular value where general anaesthesia is contraindicated. In cattle, the nerve supply to the digits is much more complex than in the horse and so regional analgesia is more difficult to produce. Also, the skin below the tarsus and carpus is tense with fibrous subcutaneous tissue so that precise location of the nerves is not easy. Blocking the individual nerves is possible but a simple ring block
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Fig. 8 Needle placement for a brachial plexus block. Inset: ventral branches of (a) sixth; (b) seventh; (c) eighth cervical and (d) first thoracic spinal nerve; (e) tuberosity of humerus; (f) first rib.
is a more reliable way of producing regional analgesia of the digit, particularly in the presence of pathology in the area. However, the most commonly used means of providing regional analgesia of cattle limbs at present is by the use of intravenous regional analgesia (IVRA). This technique involves the placing of a tourniquet proximal to the lesion acquiring attention and local analgesic solution is then injected intravenously distal to the tourniquet (approximately 25 ml is required in the adult cow) (Fig. 9). Although the mechanism of production of analgesia is unclear, the whole area distal to the tourniquet is effectively analgesed within 5-10 min, and if the tourniquet has been effectively applied the area remains analgesed until removal of the tourniquet. This technique is simple as it requires only one injection and is much less time consuming than other methods. It is also used in sheep, goats, pigs and dogs_
Regional analgesia for laparotomy
Fig. 7 Needle placement for infiltration of the digital nerves lateral and medial to the digits.
The abdominal wall can be analgesed by a number of different techniques, the simplest being an infiltration technique or line block, to produce analgesia of the incision site. An Inverted L Block may also be used and has the advantage that as the lignocaine is away from the surgical site it will not interfere with wound healing. However, in cattle the classical and most commonly used regional analgesia for laparotomy is the Paravertebral Nerve Block9 This technique involves injecting local analgesic solution about the spinal nerves as they emerge from the vertebral canal through the intervertebral foramina and is an improvement on field infiltration because the abdominal wall, including the peritoneum, is completely and uniformly desensitized; excellent muscle relaxation is produced and intraabdominal pressure is decreased. The method is safe, simple and quick and also uses less local anaesthetic than
REGIONALANALGESIAIN DOMESTICANIMALS 29
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Fig. 10 Needle placement for paravertebral nerve block in cattle. (A)left lateral aspect of thoracolumbar vertebrae T13 to L4 with the needle tip placed at spinal nerves T13, L1 and L2. (B) cranial view of a transection of the first lumbar vertebra at the location of the intervertebral foramen (a: dorsal branch; b: ventral branch of L1 vertebral nerve).
Fig. 9 Tourniquet and needle placement for intravenous regional analgesia of the bovine hindleg, (a) cranial branch of the lateral saphenous vein, (b) lateral plantar digital vein.
cranial or caudal epidural anaesthesia. This procedure is commonly practised in all species. In cattle, caudal epidural analgesia is a simple procedure regularly carried out, and it is well tolerated without additional sedation usually being required. The injection is performed through the space between the arches of the first and second coccygeal vertebrae (Fig. 11). This site is selected in preference to the sacrococcygeal space because it is large and thus more easily penetrated. The site is located by raising and lowering the tail and palpating the first obvious articulation behind the sacrum.
field infiltration. The nerves anaesthetized are the thirteenth thoracic nerve and the first and second lumbar nerves as these innervate the area of flank bounded cranially by the last rib, caudally by the angle of the ilium and dorsally by the lumbar transverse processes. However, each of these nerves has a dorsal and ventral branch and unless both are adequately analgesed the block will be incomplete (Fig. 10). Successful innervation of the nerves is indicated first by the development of a belt of hyperaemia which causes an appreciable rise in skin temperature. Full analgesia develops in about 10 rain and persists for about 90 min. When a unilateral block is fully developed it produces a curvature of the spine, the convexity of which is towards the analgesed side.
Regional analgesia of the pelvic viscera and genitalia Regional analgesia of the anus, perineum, rectum, vulva, vagina, urethra and bladder can be achieved using either
Fig. 11 Needle placement for caudal epidural analgesia and catheterization of the sacral epidural space in cattle. Co-1: first coccygeal vertebra; Co-2: second coccygeal vertebra. Desensitized subcutaneous area after caudal block is stippled.
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CURRENT ANAESTHESIA AND CRITICAL CARE
Only the coccygeal nerves, the thin phylum terminale, blood vessels, epidural fat and connective tissue remain in the spinal cord at this site so the risks of damage are minimal if an aseptic technique is used. Indications for epidural analgesia in cattle include obstetrical procedures such as to overcome straining for the manipulative correction of malpresentations, operative correction of parturient injuries and reduction of a prolapsed uterus, or for general surgical procedures such as operations on the tail, suturing wounds to the perineum and vulva or to enable vaginal and rectal examination in a fractious animal. A dose of 1 ml of 2% lignocaine per 100 kg body weight is usually administered slowly and maximal analgesia takes 10-20 min to produce full desensitization of the pelvic viscera and genitalia, which will last for 30-150 rain. Abdominal contractions usually cease and successful block is recognized by flaccidity of the tail. Locomotor function of the hindlegs and uterine contractions should remain unaffected. Cranial epidural block in cattle is achieved by injecting 100-150 ml of 2% lignocaine into the first intercoccygeal space so that the analgesic solution passes cranially. This technique involves interference with the motor function of the hindlegs and is not as commonly practised as it can result in prolonged recumbency, but is useful for more extensive udder surgery or hind limb operations such as amputation of a digit. A significantly longer duration of action is observed if c~2 agonists are used to produce the analgesia. However, in cattle this technique commonly has detrimental side effects such as sedation, mild ataxia, bradycardia, hypotension, hypoxaemia, respiratory acidosis and ruminal amotility so lignocaine is the drug of choice for bovine epidurals. To increase the length of analgesia, a continuous caudal epidural technique has been used in which a 16 or 17 gauge thin-walled catheter is placed within the epidural space. A dose of 5 ml of 2% lignocaine can then be administered every 4-6 h to prevent straining. In horses, the caudal epidural technique is also frequently practised and the site is the first intercoccygeal space as in cattle. This space is easily detected in horses when the tail is moved up and down as the sacrococcygeal joint is usually fused. Horses need to be sedated prior to attempting to inject the epidural space, but once they are adequately sedated and restrained it is a relatively simple procedure. Indications for this technique in the horse are similar to those in cattle although obstetrical complications are more infrequent in horses and more reconstructive vaginal surgery is carried out. The effects of a lignocaine block take longer to manifest themselves in horses but tend to have the same duration (30-150 min). a2 agonists are commonly used in equine epidurals to prolong the duration of effect and this does not appear to have the side-effects that it has in cattle. 5 A dose of 0.17 mg/kg of xylazine combined with lignocaine to produce a total dose of 10 ml produces effective analgesia for up to 4 h. Opioids can also be used effec-
tively in the epidural space of horses for analgesia and are most commonly used in conjunction with general anaesthesia in foals undergoing hindlimb orthopaedic surgery. The most commonly used opiate is morphine and this has also been used effectively in calves during general anaesthesia. Cranial epidural block is not used in horses as the loss of hind leg function would cause panic and severe risks of injury. In dogs, caudal epidurals are occasionally carried out to enable minor surgical procedures on the tail, but the more commonly utilized technique is lumbar epidural analgesia. 6'7 This technique is relatively simple and provides analgesia and muscle relaxation of the perineal, caudal abdominal and hindlimb regions. The spinal cord ends at the junction of the 6th and 7th lumbar vertebrae in the dog so the meninges are unlikely to be penetrated at this point. If successful, the technique can be used to enable surgery to be performed on high risk cases but more commonly it is used in conjunction with general anaesthesia to increase the level of analgesia following surgery of the caudal abdomen, anus or hindlegs. Two per cent lignocaine with adrenaline will produce 1.5-2 h of analgesia but 0.75% bupivicaine will last for 5-6 h. If an opiate such as morphine is used, up to 24 h of postoperative analgesia is obtained without may interference to motor function so this is usually the drug of choice. Possible complications with this technique are hypotension due to peripheral vasodilation, hypothermia due to loss of shivering and respiratory paralysis in cases of overdose. This procedure is contraindicated in cases of skin sepsis or blood clotting disorders. Lumbar epidurals are also occasionally used in cats and the technique is identical to that used in dogs. However, the risk of hypothermia developing is high and as the spinal cord extends further in cats the procedure is more hazardous_
Regional analgesia for castration Castration is commonly performed as a management technique in all veterinary species (horses, cattle, sheep, goats and pigs) and the testicles can be analgesed by a variety of different techniques to avoid the use of general anaesthesia. 1. A large amount of lignocaine can be injected into the depth of each testicle until they feel firm and along the median raphe of the scrotum. Castration can then be carried out painlessly after 10 min. 2. The spermatic cord can be grasped just above the testicle and lignocaine can then be injected into the subcutaneous tissues of the region. Care must be taken to keep the needle still to avoid blood vessels as lignocaine is injected around each spermatic cord. The scrotal skin must also be injected along the line of the proposed incisions. This method does not appear to be as effective. 3. A long needle can be inserted through the testicle and directed into the spermatic cord whilst lignocaine
REGIONALANALGESIAIN DOMESTICANIMALS 31 is injected. After treatment of both spermatic cords, the scrotal skin is infiltrated. The injected local analgesic agent passes out from the testicle along the lymph vessels and diffuses to block the nerve fibres present in the spermatic cord. The bulk of the drug is carried on in the lymph to enter the blood stream and for this reason excessive dosage must be avoided or intoxication will occur.
with local analgesic solution once all the milk has been withdrawn. A tourniquet placed as high as possible on the teat will confine the local analgesic solution. Perineal nerve block can be utilized to desensitize the caudal part of the udder for the removal of supeluumary teats and the repair of wounds. This is a useful technique as the analgesic solution is distant to the surgical site and will not interfere with wound healing.
Other veterinary regional analgesic techniques Regional analgesia of the udder and teats The nerve supply to the bovine udder is multiple and complex as it receives innervation from four of the lumbar nerves and from three of the sacral nerves (Fig. 12). The vast majority of surgical procedures on the teats are carried out under local analgesia with adequate restraint in the standing animal. However, if major surgery is required then either general anaesthesia or an anterior epidural block is employed. One of the simplest and most effective means of analgesing the teats is a ring block technique which is well tolerated in the standing cow. Care must be taken to avoid using lignocaine preparations containing adrenaline as this may produce vasoconstriction with associated tissue damage and even necrosis of the teat. An alternative to the ring block is the inverted V block. For lesions within the teat the cistern can be infused
Fig. 12 Schematic illustration of the nerve supply to the cow's udder. (A) inguinal nerve, (B) internal anterior, (C) posterior inguinal nerves, (D) external inguinal nerve, (E) perineal inguinal nerve. (L1 to L4) ventral branches of the first to fourth lumbar vertebral nerves. ($2, $3, $4) ventral branches of the second, third and fourth sacral vertebral nerves.
P u d e n d a l nerve block
The pudendal nerve block is a useful technique to enable surgery of the penis and preputial membrane of the bull. The block produces analgesia of the integumentary surfaces and relaxation of the retractor penis muscle. Both these objectives may be achieved by general anaesthesia, but this has greater risk of complications such as regurgitation and inhalation of lumen contents. Analgesia can also be obtained using an epidural block but it is difficult to achieve satisfactory analgesia and relaxation of the penis without also affecting the bull's ability to stand. Pudendal nerve block avoids these disadvantages, the bull may be kept standing or be cast if required. Several nerves are blocked during this technique, they are' 1. The caudal rectal nerves which provide motor function to the proximal segment of the retractor penis muscle. 2. The pudendal nerve which splits into: a) the dorsal nerve of the penis which supplies motor function to the distal segments of the retractor penis muscle and sensation to the free end of the penis b) the superficial perineal nerve which splits into: i) the preputial branch which supplies sensation to the preputial membrane ii) The scrotal branch which in part supplies cutaneous sensation to the scrotum. 3. The pudendal branch of the ischiatic nerve which joins the pudendal nerve proper.
Fig. 13 Needle placement to the internal pudendal nerve in cattle. The position of the hand palpating the internal pudendal nerve (A) on the right side is shown. (B) caudal rectal nerve, (C) internal pudendal artery, (D) sacrosciatic ligament, (E) sciatic nerve.
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CURRENT ANAESTHESIA AND CRITICAL CARE
The procedure for analgesing these nerves is relatively complex and involves maintaining a hand within the rectum to guide the needle to the pudendal nerve (Fig_ 13). Intercostal nerve block
Intercostal nerve block is frequently used in small animals to relieve pain during and after thoracotomy, pleural drainage and rib fractures. A minimum of two adjacent intercostal spaces both cranial and caudal to the incision site are selectively blocked because of overlap of nerve supply. The analgesic solution is injected caudally to each rib near the intervertebral foramen and post-thoracotomy pain is generally controlled for 3-6 h following successful block with bupivacaine (Fig. 14).
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References 1. Peterson D R. Nerve block of the eye and associated structures. J A m Vet Med Assoc 1951; 118: 145-148. 2. Hare W C D. A regional method for the complete anaesthetization and immobilization of the bovine eye and its associated structures. Can J Comp Med 1957; 21: 228-234. 3. Nutt P. Brachial plexus analgesia in the dog. Vet Rec 1962; 74: 874-876. 4. Farquharson J. Paravertebral lumbar anesthesia in the bovine species. J Am Vet Med Assoc 1940; 97: 54-57. 5. Leblanc P H, Caron J P, Patterson. S, Brown M, Matta M A. Epidural injection of xylazine for perineal analgesia in horses. J Am Vet Med Assoc 1988; 193: 1405. 6. Klide A M, Soma L R_ Epldural analgesia in the dog and cat. J A m Vet Med Assoc 1968; 153: 165-173. 7. Bone J K, Beck J G. Epidural anesthesia in dogs. J A m Vet Assoc 1956; 128: 236-238.
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Fig. 14 Needle placement for inducing intercostal n e r v e blocks. Inset: (a) skin, (b) subcutaneous tissue, (c) intercostal muscles, (d) rib, (e) subcostal space, (f) pleura costalis and fascia, (g) interpleural space, (h) pleura pulmonalis, (i) intercostal artery, vein and nerve, (j) lung.
Further reading Thurmon J C, Tranquilli W J, Benson G J. Lumb & Jones' Veterinary anaesthesia, 3rd edn. London: Williams and Wilkins, 1996. Hall L W, Clarke K W. Veterinary anaesthesia, 9th edn. London: WB Saunders, 1991. Short C E. Principles and practice of veterinary anaesthesia_ Baltimore: Williams and Wilkins, 1987.
© 1998 Harcourt Brace & Co Ltd
Medicine
Regional analgesia in domestic animals
R. F. McGregor and R. S. Jones Regional analgesia is commonly practised in veterinary medicine, either alone, with sedation, or in conjunction with general anaesthesia. Many major surgical procedures can be satisfactorily performed under local analgesia with or without sedation depending on the species, temperament and health of the animal. In adult cattle, many procedures are performed under regional analgesia on standing animals and heavy sedation is best avoided to prevent the animal becoming recumbent. This is particularly useful as the dangers of prolonged recumbency can be avoided, and the surgeon can provide local analgesia, and operate without the assistance of an anaesthetist. In the other species however, deep sedation will greatly facilitate surgery by reducing the risk of sudden movement. A number of compounds are used to produce local analgesia in animals. The most universally used compound is lignocaine hydrochloride, but more recently bupivacaine has become available for use in veterinary practice. Bupivacaine has about four times the potency of lignocaine, and lasts at least twice as long. Other local anaesthetic agents used in veterinary practice include mepivacaine, prilocaine, proparacaine and benzocaine, and trials have recently begun using ropivacaine. The toxicity of local analgesic solutions is often disregarded in large animal practice. However, in foals, calves, lambs, goats and small animals it must be considered. Accurate figures are not available for the toxic dose of lignocaine, but it is generally considered unwise to exceed a dose of 5 mg/kg with any of the regional analgesic techniques. As well as the local anaesthetic agents, the ~2 agonists are utilized in epidural regional analgesia as they have significant local analgesic effects thought to be due to activation of spinal cord ~2 adrenoreceptors. Finally, the opiates can also be utilized to produce spinal and intra- articular analgesia due to the presence of opiate receptors within the spinal cord and on the synovium.
Regional analgesia of the head
division of the Vth (Trigeminal) cranial nerve. The horn corium and the skin around its base derives its sensory component from this nerve, thus analgesing it enables the horn buds of young calves to be effectively cauterized without pain ( Fig. 1). In adult cattle with well developed horns, two further injections are required to block the posterior division of the nerve and the cutaneous branch of the 2nd cervical nerve, and it is often easier in these cases to perform a
The most commonly analgesed nerve in cattle practice is probably the cornual nerve, a branch of the ophthalmic R. F. McGregor BVSE, MRCVS, Professor R. S. Jones, Department of Anaesthesia, Royal Liverpool University Hospital, Daulby Street, Liverpool L69 3GA, UK. Correspondence to: R. F. MeG_
25
26 CURRENT ANAESTHESIA AND CRITICAL CARE
Fig. 1 Needle placement for infiltration of the eyelids (A) and nerve blocks on the head in cattle: auriculopalpebral (B); infraorbital (C); and cornual nerve (D).
ring block around the base of the horn, to ensure analgesia. Goats horns are supplied by the infratrochlear nerve as well as the cornual branch, and both these nerves require analgesia before dehorning should be attempted (Fig. 2). The auriculopalpebral nerve supplies motor fibres to the orbicularis oculi muscle and blocking this nerve prevents eyelid closure during examination of the eyeball. No analgesia is produced by this method but in conjunction with topical analgesia it is useful for the removal of foreign bodies from the cornea and conjunctival sac in cattle and horses (Fig. 1). The eyelids can also be effectively analgesed by a linear subcutaneous infiltra-
Fig. 2 Needle placement for desensitizing the infratrochlear nerve (B) as well as the cornual nerve (A) in the goat.
tion of lignocaine about 0.5 cm from the margin of the dorsal and ventral eyelids (Fig. 1). Analgesia for enucleation of the eye in cattle is achieved by 1 of 2 methods: retrobulbar nerve block or the method described by Peterson. 1'2 In other species enucleation is usually conducted under general anaesthesia; however, in cattle regional anaesthesia in conjunction with sedation is a successful method. The retrobulbar block is hazardous to perform as it involves pushing the needle along the floor of the orbit until it penetrates the tough periorbita, 20-30 ml of 2% lignocaine solution being injected in small increments as the needle is advanced (Fig. 3). If correctly executed the local analgesic solution produces corneal analgesia, mydriasis and proptosis of the eyeball. Paralysis of the eyeball should also occur. Adequate sedation and restraint is vital to decrease the risk of orbital haemorrhage, penetration of the globe, or damage to the optic nerve or its meninges. The Peterson technique requires more skill than the retrobulbar block but it is less hazardous. The nerves which are analgesed by this technique include the oculomotor, trochlear, abducens and three branches of the trigeminal nerve (Fig. 4). The needle can also be redirected to block the auriculopalpebral nerve. Both these techniques will prevent blinking for several hours so if the purpose of the procedure is other than enucleation it is important to keep the cornea moist. Another common regional analgesic technique used in all domestic species is the infraorbital nerve block (Fig. 1). This involves administering local anaesthetic into the infraorbital foramen to block the infraorbital nerve which is a continuation of the maxillary division of the Vth cranial nerve and is entirely sensory to the upper lip, cheek, nostrils and lower parts of the face. In horses this block enables minor surgery to be successfully carried out under sedation, whilst in small animals it is usually used in conjunction with general anaesthesia for increased analgesia during painful procedures such as rhinotomies. In cattle it also enables the repair of nasal lacerations and the insertion of nose rings. Analgesia of the mental nerve as it emerges from the
Fig. 3 Retrobulbar needle placement t h r o u g h the medial canthus (A) to the orbital apex (B) in cattle.
REGIONALANALGESIAIN DOMESTICANIMALS 27
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Fig. 4 Needle placement for 'Peterson' eye block (a: coronoid process; b: pterygoid crest; c: foramen orbitorotundum). L
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Fig. 6 Needle placement for nerves of the distal forelimb of the horse, lateral (k), palmar (P), and medial (M) views: lateral and medial palmar digital nerves (A); dorsal branches (B); lateral and medial palmar digital nerves (abaxial sesamoid) (C); lateral and medial palmar nerves (D and G); lateral and medial palmar metacarpal nerves (E); communicating branch (F).
Fig. 5 Sites for needle placement to desensitize the supraorbital (A), auriculopalpebral (E and F), infratrochlear (G and H), mental (I) and alveolar mandibular nerves (K).
mental foramen on the lateral aspect of the ramus produces regional anaesthesia of the lower lip enabling suturing of lower lip wounds to be conveniently carried out. Other less frequently practised facial blocks include the mandibular nerve block which will desensitize the whole of the lower jaw, all the teeth and alveoli on the affected side (useful in many dental procedures), and the supraorbital nerve block which desensitizes the upper eyelid (Fig. 5).
Regional analgesia of the limbs As well as allowing minor surgical procedures to be performed without general anaesthesia or to provide added analgesia in conjunction with general anaesthesia, regional limb anaesthesia in horses is used extensively to provide diagnosis of the site of pain producing lameness. Once the characteristics of a lameness have been established regional analgesia is commenced at the most distal point of the limb and various blocks are employed progressively up the leg until lameness is abolished (Fig. 6).
For diagnostic procedures mepivacaine is the drug of choice as it is less likely to cause a tissue reaction than lignocaine which can commonly cause moderate inflammation and oedema. It is also advisable to avoid preparations containing adrenaline as this can produce vascular spasm and the production of white hairs at the site of injection_ Most horses will tolerate nerve blocks without chemical sedation and this is advisable to prevent confusion of results. The Palmar Digital Nerve Block analgeses the medial and lateral palmar digital nerves and will anaesthetize the caudal third of the hoof. Abolition of sensation around the bulbs of the heels will indicate the block has worked. This usually takes about 10 min. The Abaxial Sesamoid Nerve Block blocks the medial and lateral palmer nerves higher up the leg at the level of the proximal sesamoid bones and this blocks the whole of the foot below this point. Confirmation that the block has worked is by abolition of skin sensation around the coronary band. Seventy per cent of the causes of equine lameness are located within the foot but if lameness has not been abolished then the next block to perform is the Low Four-point Block which blocks the medial and lateral palmar and palmar metacarpal nerves just above the metacarpophalangeal joint and anaesthetizes every structure below this point causing abolition of skin sensation over the fetlock and dorsoproximal pastern region. Continuing up the leg, the Subcarpal Block analgeses the palmer and palmar metacarpal nerves immediately distal to the carpus to induce regional anaesthesia of everything below this point. There is no reliable test that this block has worked.
28
CURRENT ANAESTHESIA AND CRITICAL CARE
Regional analgesia above the carpus is not usually required but if necessary then a Median and Ulnar Nerve Block can be performed to analgese the whole of the carpus and distal structures. Lameness in the hindlimb is usually investigated in a similar manner and the regional analgesic techniques for the foot are the same as for the foreleg. However, to analgese the metatarsophalangeal joint in the hindleg, a low six-point block is required to include the medial and lateral dorsal metatarsal nerves. More proximally, a Subtarsal Block is similar to the subcarpal block in the foreleg, and further proximally still, the tibial and fibular nerves can also be analgesed but this is a difficult procedure as the fibular nerve lies between muscle bellies about 3-5 cm deep. All the joints of equine limbs can also be analgesed using strict aseptic techniques with varying degrees of difficulty either to enable diagnosis or to aid analgesia. In small animal practice limb analgesia is not commonly practised although infiltration of the digital nerves is occasionally attempted by infiltration lateral and medial to the first phalanx of the digit to be analgesed (Fig. 7). Another block occasionally used in the dog is a Brachial Plexus Block. 3 This is a complicated procedure involving insertion of a 7.5 cm needle lateral to the chest wall but medial to the subscapularis muscle (Fig. 8). Many complications can occur such as puncture of a major blood vessel producing a large haematoma, damage to the brachial plexus, puncture of the thorax admitting air to the pleural cavity or the introduction of infection into the axilla. However, if adequate care is taken it is a relatively safe procedure of particular value where general anaesthesia is contraindicated. In cattle, the nerve supply to the digits is much more complex than in the horse and so regional analgesia is more difficult to produce. Also, the skin below the tarsus and carpus is tense with fibrous subcutaneous tissue so that precise location of the nerves is not easy. Blocking the individual nerves is possible but a simple ring block
, ,,a.< / "l~
Fig. 8 Needle placement for a brachial plexus block. Inset: ventral branches of (a) sixth; (b) seventh; (c) eighth cervical and (d) first thoracic spinal nerve; (e) tuberosity of humerus; (f) first rib.
is a more reliable way of producing regional analgesia of the digit, particularly in the presence of pathology in the area. However, the most commonly used means of providing regional analgesia of cattle limbs at present is by the use of intravenous regional analgesia (IVRA). This technique involves the placing of a tourniquet proximal to the lesion acquiring attention and local analgesic solution is then injected intravenously distal to the tourniquet (approximately 25 ml is required in the adult cow) (Fig. 9). Although the mechanism of production of analgesia is unclear, the whole area distal to the tourniquet is effectively analgesed within 5-10 min, and if the tourniquet has been effectively applied the area remains analgesed until removal of the tourniquet. This technique is simple as it requires only one injection and is much less time consuming than other methods. It is also used in sheep, goats, pigs and dogs_
Regional analgesia for laparotomy
Fig. 7 Needle placement for infiltration of the digital nerves lateral and medial to the digits.
The abdominal wall can be analgesed by a number of different techniques, the simplest being an infiltration technique or line block, to produce analgesia of the incision site. An Inverted L Block may also be used and has the advantage that as the lignocaine is away from the surgical site it will not interfere with wound healing. However, in cattle the classical and most commonly used regional analgesia for laparotomy is the Paravertebral Nerve Block9 This technique involves injecting local analgesic solution about the spinal nerves as they emerge from the vertebral canal through the intervertebral foramina and is an improvement on field infiltration because the abdominal wall, including the peritoneum, is completely and uniformly desensitized; excellent muscle relaxation is produced and intraabdominal pressure is decreased. The method is safe, simple and quick and also uses less local anaesthetic than
REGIONALANALGESIAIN DOMESTICANIMALS 29
L-1
Fig. 10 Needle placement for paravertebral nerve block in cattle. (A)left lateral aspect of thoracolumbar vertebrae T13 to L4 with the needle tip placed at spinal nerves T13, L1 and L2. (B) cranial view of a transection of the first lumbar vertebra at the location of the intervertebral foramen (a: dorsal branch; b: ventral branch of L1 vertebral nerve).
Fig. 9 Tourniquet and needle placement for intravenous regional analgesia of the bovine hindleg, (a) cranial branch of the lateral saphenous vein, (b) lateral plantar digital vein.
cranial or caudal epidural anaesthesia. This procedure is commonly practised in all species. In cattle, caudal epidural analgesia is a simple procedure regularly carried out, and it is well tolerated without additional sedation usually being required. The injection is performed through the space between the arches of the first and second coccygeal vertebrae (Fig. 11). This site is selected in preference to the sacrococcygeal space because it is large and thus more easily penetrated. The site is located by raising and lowering the tail and palpating the first obvious articulation behind the sacrum.
field infiltration. The nerves anaesthetized are the thirteenth thoracic nerve and the first and second lumbar nerves as these innervate the area of flank bounded cranially by the last rib, caudally by the angle of the ilium and dorsally by the lumbar transverse processes. However, each of these nerves has a dorsal and ventral branch and unless both are adequately analgesed the block will be incomplete (Fig. 10). Successful innervation of the nerves is indicated first by the development of a belt of hyperaemia which causes an appreciable rise in skin temperature. Full analgesia develops in about 10 rain and persists for about 90 min. When a unilateral block is fully developed it produces a curvature of the spine, the convexity of which is towards the analgesed side.
Regional analgesia of the pelvic viscera and genitalia Regional analgesia of the anus, perineum, rectum, vulva, vagina, urethra and bladder can be achieved using either
Fig. 11 Needle placement for caudal epidural analgesia and catheterization of the sacral epidural space in cattle. Co-1: first coccygeal vertebra; Co-2: second coccygeal vertebra. Desensitized subcutaneous area after caudal block is stippled.
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CURRENT ANAESTHESIA AND CRITICAL CARE
Only the coccygeal nerves, the thin phylum terminale, blood vessels, epidural fat and connective tissue remain in the spinal cord at this site so the risks of damage are minimal if an aseptic technique is used. Indications for epidural analgesia in cattle include obstetrical procedures such as to overcome straining for the manipulative correction of malpresentations, operative correction of parturient injuries and reduction of a prolapsed uterus, or for general surgical procedures such as operations on the tail, suturing wounds to the perineum and vulva or to enable vaginal and rectal examination in a fractious animal. A dose of 1 ml of 2% lignocaine per 100 kg body weight is usually administered slowly and maximal analgesia takes 10-20 min to produce full desensitization of the pelvic viscera and genitalia, which will last for 30-150 rain. Abdominal contractions usually cease and successful block is recognized by flaccidity of the tail. Locomotor function of the hindlegs and uterine contractions should remain unaffected. Cranial epidural block in cattle is achieved by injecting 100-150 ml of 2% lignocaine into the first intercoccygeal space so that the analgesic solution passes cranially. This technique involves interference with the motor function of the hindlegs and is not as commonly practised as it can result in prolonged recumbency, but is useful for more extensive udder surgery or hind limb operations such as amputation of a digit. A significantly longer duration of action is observed if c~2 agonists are used to produce the analgesia. However, in cattle this technique commonly has detrimental side effects such as sedation, mild ataxia, bradycardia, hypotension, hypoxaemia, respiratory acidosis and ruminal amotility so lignocaine is the drug of choice for bovine epidurals. To increase the length of analgesia, a continuous caudal epidural technique has been used in which a 16 or 17 gauge thin-walled catheter is placed within the epidural space. A dose of 5 ml of 2% lignocaine can then be administered every 4-6 h to prevent straining. In horses, the caudal epidural technique is also frequently practised and the site is the first intercoccygeal space as in cattle. This space is easily detected in horses when the tail is moved up and down as the sacrococcygeal joint is usually fused. Horses need to be sedated prior to attempting to inject the epidural space, but once they are adequately sedated and restrained it is a relatively simple procedure. Indications for this technique in the horse are similar to those in cattle although obstetrical complications are more infrequent in horses and more reconstructive vaginal surgery is carried out. The effects of a lignocaine block take longer to manifest themselves in horses but tend to have the same duration (30-150 min). a2 agonists are commonly used in equine epidurals to prolong the duration of effect and this does not appear to have the side-effects that it has in cattle. 5 A dose of 0.17 mg/kg of xylazine combined with lignocaine to produce a total dose of 10 ml produces effective analgesia for up to 4 h. Opioids can also be used effec-
tively in the epidural space of horses for analgesia and are most commonly used in conjunction with general anaesthesia in foals undergoing hindlimb orthopaedic surgery. The most commonly used opiate is morphine and this has also been used effectively in calves during general anaesthesia. Cranial epidural block is not used in horses as the loss of hind leg function would cause panic and severe risks of injury. In dogs, caudal epidurals are occasionally carried out to enable minor surgical procedures on the tail, but the more commonly utilized technique is lumbar epidural analgesia. 6'7 This technique is relatively simple and provides analgesia and muscle relaxation of the perineal, caudal abdominal and hindlimb regions. The spinal cord ends at the junction of the 6th and 7th lumbar vertebrae in the dog so the meninges are unlikely to be penetrated at this point. If successful, the technique can be used to enable surgery to be performed on high risk cases but more commonly it is used in conjunction with general anaesthesia to increase the level of analgesia following surgery of the caudal abdomen, anus or hindlegs. Two per cent lignocaine with adrenaline will produce 1.5-2 h of analgesia but 0.75% bupivicaine will last for 5-6 h. If an opiate such as morphine is used, up to 24 h of postoperative analgesia is obtained without may interference to motor function so this is usually the drug of choice. Possible complications with this technique are hypotension due to peripheral vasodilation, hypothermia due to loss of shivering and respiratory paralysis in cases of overdose. This procedure is contraindicated in cases of skin sepsis or blood clotting disorders. Lumbar epidurals are also occasionally used in cats and the technique is identical to that used in dogs. However, the risk of hypothermia developing is high and as the spinal cord extends further in cats the procedure is more hazardous_
Regional analgesia for castration Castration is commonly performed as a management technique in all veterinary species (horses, cattle, sheep, goats and pigs) and the testicles can be analgesed by a variety of different techniques to avoid the use of general anaesthesia. 1. A large amount of lignocaine can be injected into the depth of each testicle until they feel firm and along the median raphe of the scrotum. Castration can then be carried out painlessly after 10 min. 2. The spermatic cord can be grasped just above the testicle and lignocaine can then be injected into the subcutaneous tissues of the region. Care must be taken to keep the needle still to avoid blood vessels as lignocaine is injected around each spermatic cord. The scrotal skin must also be injected along the line of the proposed incisions. This method does not appear to be as effective. 3. A long needle can be inserted through the testicle and directed into the spermatic cord whilst lignocaine
REGIONALANALGESIAIN DOMESTICANIMALS 31 is injected. After treatment of both spermatic cords, the scrotal skin is infiltrated. The injected local analgesic agent passes out from the testicle along the lymph vessels and diffuses to block the nerve fibres present in the spermatic cord. The bulk of the drug is carried on in the lymph to enter the blood stream and for this reason excessive dosage must be avoided or intoxication will occur.
with local analgesic solution once all the milk has been withdrawn. A tourniquet placed as high as possible on the teat will confine the local analgesic solution. Perineal nerve block can be utilized to desensitize the caudal part of the udder for the removal of supeluumary teats and the repair of wounds. This is a useful technique as the analgesic solution is distant to the surgical site and will not interfere with wound healing.
Other veterinary regional analgesic techniques Regional analgesia of the udder and teats The nerve supply to the bovine udder is multiple and complex as it receives innervation from four of the lumbar nerves and from three of the sacral nerves (Fig. 12). The vast majority of surgical procedures on the teats are carried out under local analgesia with adequate restraint in the standing animal. However, if major surgery is required then either general anaesthesia or an anterior epidural block is employed. One of the simplest and most effective means of analgesing the teats is a ring block technique which is well tolerated in the standing cow. Care must be taken to avoid using lignocaine preparations containing adrenaline as this may produce vasoconstriction with associated tissue damage and even necrosis of the teat. An alternative to the ring block is the inverted V block. For lesions within the teat the cistern can be infused
Fig. 12 Schematic illustration of the nerve supply to the cow's udder. (A) inguinal nerve, (B) internal anterior, (C) posterior inguinal nerves, (D) external inguinal nerve, (E) perineal inguinal nerve. (L1 to L4) ventral branches of the first to fourth lumbar vertebral nerves. ($2, $3, $4) ventral branches of the second, third and fourth sacral vertebral nerves.
P u d e n d a l nerve block
The pudendal nerve block is a useful technique to enable surgery of the penis and preputial membrane of the bull. The block produces analgesia of the integumentary surfaces and relaxation of the retractor penis muscle. Both these objectives may be achieved by general anaesthesia, but this has greater risk of complications such as regurgitation and inhalation of lumen contents. Analgesia can also be obtained using an epidural block but it is difficult to achieve satisfactory analgesia and relaxation of the penis without also affecting the bull's ability to stand. Pudendal nerve block avoids these disadvantages, the bull may be kept standing or be cast if required. Several nerves are blocked during this technique, they are' 1. The caudal rectal nerves which provide motor function to the proximal segment of the retractor penis muscle. 2. The pudendal nerve which splits into: a) the dorsal nerve of the penis which supplies motor function to the distal segments of the retractor penis muscle and sensation to the free end of the penis b) the superficial perineal nerve which splits into: i) the preputial branch which supplies sensation to the preputial membrane ii) The scrotal branch which in part supplies cutaneous sensation to the scrotum. 3. The pudendal branch of the ischiatic nerve which joins the pudendal nerve proper.
Fig. 13 Needle placement to the internal pudendal nerve in cattle. The position of the hand palpating the internal pudendal nerve (A) on the right side is shown. (B) caudal rectal nerve, (C) internal pudendal artery, (D) sacrosciatic ligament, (E) sciatic nerve.
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CURRENT ANAESTHESIA AND CRITICAL CARE
The procedure for analgesing these nerves is relatively complex and involves maintaining a hand within the rectum to guide the needle to the pudendal nerve (Fig_ 13). Intercostal nerve block
Intercostal nerve block is frequently used in small animals to relieve pain during and after thoracotomy, pleural drainage and rib fractures. A minimum of two adjacent intercostal spaces both cranial and caudal to the incision site are selectively blocked because of overlap of nerve supply. The analgesic solution is injected caudally to each rib near the intervertebral foramen and post-thoracotomy pain is generally controlled for 3-6 h following successful block with bupivacaine (Fig. 14).
~d
--f i
References 1. Peterson D R. Nerve block of the eye and associated structures. J A m Vet Med Assoc 1951; 118: 145-148. 2. Hare W C D. A regional method for the complete anaesthetization and immobilization of the bovine eye and its associated structures. Can J Comp Med 1957; 21: 228-234. 3. Nutt P. Brachial plexus analgesia in the dog. Vet Rec 1962; 74: 874-876. 4. Farquharson J. Paravertebral lumbar anesthesia in the bovine species. J Am Vet Med Assoc 1940; 97: 54-57. 5. Leblanc P H, Caron J P, Patterson. S, Brown M, Matta M A. Epidural injection of xylazine for perineal analgesia in horses. J Am Vet Med Assoc 1988; 193: 1405. 6. Klide A M, Soma L R_ Epldural analgesia in the dog and cat. J A m Vet Med Assoc 1968; 153: 165-173. 7. Bone J K, Beck J G. Epidural anesthesia in dogs. J A m Vet Assoc 1956; 128: 236-238.
h
g
Fig. 14 Needle placement for inducing intercostal n e r v e blocks. Inset: (a) skin, (b) subcutaneous tissue, (c) intercostal muscles, (d) rib, (e) subcostal space, (f) pleura costalis and fascia, (g) interpleural space, (h) pleura pulmonalis, (i) intercostal artery, vein and nerve, (j) lung.
Further reading Thurmon J C, Tranquilli W J, Benson G J. Lumb & Jones' Veterinary anaesthesia, 3rd edn. London: Williams and Wilkins, 1996. Hall L W, Clarke K W. Veterinary anaesthesia, 9th edn. London: WB Saunders, 1991. Short C E. Principles and practice of veterinary anaesthesia_ Baltimore: Williams and Wilkins, 1987.