C. M. Trim Atbens, Georgia, U.S.A.

Transcription

1 127. MUSCLE RELAXANTS IN LARGE AN1IJIP.L ANAESTHESIA C. M. Trim Atbens, Georgia, U.S.A. Muscle relaxants contribute to the quality of znaesthesia by providing skeletal muscle relaxation, preventing patient movement, and allowing reduction in the dose of anaesthetic drugs. The emphasis on use of muscle relaxants is different in large and small animal anaesthesia. Indiiction of anaesthesia in the adult horse or bovine is accompanied by the risk of injury as the animal fslb to the ground. Good mus-le relaxation reduces this risk and, therefore, muscle relaxants are often incorporated into the anaesthetlc induction protocol. Surgical procedures which re&re a high degree of muscle relaxation, such a3 thoracotomy, laparotomy, and major fracture repairs, are less frequently performed iri large animals than in small animals. Furthermore, a greater proportion of large animal patients undergoing surgery are systemically healthy, and the need for balanced anaesthesia is small. Drugs commonly used 1. Centrally acting muscle relaxants Although not strictly regzrded as 'muscle relaxants' by some anaesthesiologists, guaifenesin, methocarbamol, and diazqam, are classified as centrally acting muscle relaxants in the literature (3ianchine, 1980). Giiaifenesin is commonly used in the United States comtined with thiobarbiturate (thiopentone or thiamylal) for induction of anaesthesia in the horse and adult bovine (Short, 1981; Trim, 1981). Metho-

2 128. carbamol is mainly used in the treatment of horses with post-anaesthetic myositis (Bobaxic, 1980). Diazepam is included in the premetiication to improve muscle relaxation when using injectatile drugs, especially ketamine (Butera et al., 1978). It is also injected before nyelography to reduce the incidence of seizures. 2. Neuromusculax blocking drugs Clinical use generally involves either succinylcholine or pancuronium. D-tubocurarine, methyltubocurarine, and ga.llamine are used cccasionally. Succinylcholine is utilized in the induction process, whereas succinylcholine or pancuronium are used for balanced anaesthesia. 3. Direct action on skeletal muscle Dantrolene produces some CNS depressant effects, but does not affect neurommcular transmission. Dantrolene reduces contra-tion of skeletal muscle Sjr a direct action on excitationcontraction coupling (Bianchine, 1980). It is mainly used to treat malignant hyperthermia (Gronert et al., 1976). Clinical use and pharmacology Guaifenesin GecolateR (Summit; Hill) Biotec) Glyceryl GuaiacolateR (Professional Vet Labs) Guaifenesin is.marketed as a powder for reconstitution with water or 5% dextrose. A 5% solution of guaifenesin is used for both horses and cattle. The volume of solution needed is large and many veterinarians use a 10% solution for horses to increase speed of induction. Potential problems with guaifenesin are: (a) Haemolysis. This is related to the concentration of drug used and not to the total dose. In horses a 10% solu,tion produces minimal haemolysis and, in fact,

3 129. solutions of 10% guaiferesin in water have been shown to have an osmolality of 242 rnosm/kg, which is closer tb the osmolality of equine plasma than the recommended 5% solution or those in dextrose (Grandy and McDonell, 1980). (b) Tissue damage. A 7.5 or 10% sol.ution will cause swelling, abscesses, and necrosis if injected perivascularly. Subcutaneous infiltration of saline (300 ml) will reduce the damage but may not completely prevent it. Ferivascular Injectfon is less likely if a catheter (I@G, 12G, 14G) is used. (c) Erecipitation. k 10% solution precipitates readily in the bottle when left at room temperature. Erecipitation often occurs in less than an hour. Guaifenesiri is administered in 3 basic ways for induction of anaesthesia in horses. a & b. Prernedication with acepromazine, 0.05 mg/kg intravenously (IV), or xyiaziie, 0.6 mg/kg IV, is followed by: (a) 4. mixed sol-.itio:i of guaifenesin arid thi.oharbiturate, 2G bezbiturate to each 50G guaifenesin, given 'to effect'. Approximate dosage of 110 r.g/kg guaifenesir, and 4.4 mg/kg barbiturate. (b) Guaifenesin is infused to produce relaxation without recumbency, about 50 mg/kg IV, and followed immediately by a bolus of thiobarbiturate, 4-4 mg/kg IV. Additional guaifenesin is infused when the horse is recumbent, if needed. Duration of immobilization is about 20 minutes in both techniques. (c) Premedication with xylazine, 1.1 mg/kg IV, and ketamine, 1.7 mg/kg IV. The dosages in a, b, c are modified accmding to patient status and the anaesthesiologist's drug preferences.

4 Cardiovascular depression from guaifenesin has been shown to be dose dependent. In healthy horses, guaifenesin and thiobarbiturate at the doses described above caused a small increase in heart rate and a decrease in arterial blood pressure (Funk, 1970; Heath and Gabel, 1970; Wright et al., 1979; Schatzman, 1980/1981). The change in heart rate was less than described for thiopentme alone, but the decrease in blood pressure was similar (Brouwer et al., 1980). The frequency of respiration increased but the tidal volume and Pa02 were decreased (Schatzman, 1980/1981). The reduction in Pa02 was less than with thiopentone alone. Horses receiving guaifenesin and ketamine had the same decrease in heart rate and cardiac output as horses receiving ketamine 2.2 mg/kg IV, but the decrease in arterial blood pressure was greater. There was no difference in arterial blood gases (Muir et al., 1977, 1978). The duration of guaifenesin in male horses was found to be 1.5 times that in mares. However, sex differences were not observed in dosage (Davis and Wolff, 1970). The author's clinical impression is that horses recover with less depression and better limb co-ordination from guaifenesin/thiopentone anaesthesia than from thiopentone alone. Precautions: (a) Perivascular injection must be avoided. (b) Use of guaifenesin alone is not advised in any except the very depressed horse. Without barbiturate or ketamine, the horse appears to retain consciousness and resist casting, with the result that excessive amounts of guaifenesin are administered in an attempt to induce muscle relaxation. With overdose of guaifenesin, the horse becomes stiff, and often the swallow-

5 131. ing reflex is retained which interferes with endotracheal intubation. (c) Additional guaifenesin can be safely administered during anaesthesia. However, because the onset of full action of the drug is delayed several minutes, the infusion must be stopped before the desired relaxation is achieved. Failure to do this will result in unacceptable cardiovascular depression (Fig. 1 ). Guaifenesin, as a 576 solution, is often used with thiobarbiturate for anaesthesia in ruminants (Funk, 1970; Trim, 1981). Up to a body weight of 500 kg and without premedication the dosage is the sane as for horses. Less drug is needed for mature bulls. 1 litre of 5% guaifenesin with 2G thiobarbiturate is usually sufficient. Premedication severely decreases the anaesthetic drug requirement in ruminants. Succinylcholine SucostrinR (Squibb) QuelicinR (Abbott) AnectineR (Burroughs- Wellcome). Succinylcholine has been widely used in horses (Hall, 1971) but is less so now with the increasing popularity of guaifenesin and ketamine. A bolus injection of thiopentone followed by succinylcholine, 0.1 mg/kg, IV results in a smooth controlled induction with less than a minute of apnoea. This used to be the author's preferred technique for induction of horses requiring abdominal surgery for colic (Trim, 1977/1978) but this has been replaced by guaifenesin. Succinylcholine has also been used as an infusicn to provide continuous muscle relaxation during surgery (Hall, 1971). Recently, a comparison between succinylcholine infusion and intermittent injection has been published (Benson et al.,

6 ). The heart rate and arterial blood pressure remained stable in the infusion group (loading dose of 0-33 mg/kg and 2.2 mg/kg/hr) in contrast to the marked increases seen in the horses receiving intermittent injections of 0.33 mg/kg. Serum potassium levels increased in both groups, with a greater change produced in the infusion group from 3-76 to 5-38 meq/l after 90 minutes of anaesthesia. No dysrhythmias were observed in either group. Pancuronium FavulonR ( Organon Currently pancuronium is the non-depolarizing muscle relaxant of choice in horses, but its use in clinical patients is small, In the halothane-anaesthetized horse a dose of 0.06 mg/kg IV will provide about 40 minutes paralysis. This can be prolonged byo.03 mg/kg. Although adequate spontaneous breathing often returns, it is necessary to reverse the relaxant completely with atropine, 0.2 mg/kg, and neostigmine, up to 0.4 mg/kg, IV. A report that use of neostigmine increases the incidence of leakage from intestinal anastomoses in humans (Bell and Lewis, 1968) is of concern considering that the most likely indication for balanced anaesthesia in the horse is for gastrointestinal surgery. Further details on monitoring neuromuscular blocking drugs during anaesthesia are available (Klein, 1981).

7 133. REFERENCES Bell, C.M.A. & Lewis, C.B. (1968). Brit. Med. J. &, 587. Benson, G.J., Hartsfield, S.M., Smetzer, D.L. & Thurmon, J.C. (1979). Am. J. Vet. Res. 40, Bianchine, J.R. (1980). Drugs for Parkinson's disease: Centrally acting muscle relaxants. In The Pharmacological Basis of Therapeutics. Eds A. Goodman Gilman, L. S. Goodman & A. Gilman. Macmillan Publishing Company, Inc., New York. Brouwer, G.J., Hall, L.W. & Kuchel, T.R. (1980). Vet. Rec , Butera, T.S., Moore, J.N., Garner, H.E., Amend, J.F., Clarke, L.L. & Hatfield, D.G. (1978). Vet. Med./Sm. An. Clin Davis, L.E. & Wolff, W.A. (1970). Am. J. Vet. Res. 2, Funk, K.A. (1970). Equine Vet. J. 2, Grandy, J.L. & McDonell, W.N. (1980). J. Am. Vet. Med. ASSOC. 176, Gronert, G.A., Milde, J.H. ology, 44, & Theye, R.A. (1976). Anesthesi- Hall, L.W. (1971). Equine anesthesia. In Textbook of Veterinary Anesthesia. Ed. L. R. Soma. Williams & Wilkins Company, Baltimore. Heath, R.B. & Gabel, A.A. (1970). J. Am. Vet. Med. Assoc , Klein, L.V. (1981). Vet. Clin. N. Amer. 3, (1) Muir, W.W., Skarda, R.T. & Milne, D.W. (1977). Am. J. Vet. Res. 38, Muir, W.W., Skarda, R.T. & Sheehan, W. (1978). Am. J. Vet. Res. 39, RobaxinR -V (1980). AH Robins Mfg. Company. Schatzman, U. (1980/1981). Proc. Assoc. Vet. Anaes. Gt. Br. & Ir. 54, Short, C.E. (1981). Vet. Clin. N. her. 3, (1) Trim, C.M. (1977/1978). Proc. Assoc. Vet. Anaes. Gt. Br. & Ir. 7, Trim, C.M. (1 981 ). California Vet. 35, Wright, M., McGrath, C.J. & Baffe, M. (1979). Vet. Anesth. VI,

8 134. SB 6m M Stifle 120 I G A R E OOO.O...O ~ Figure 1 A 6 month Standardbred colt was anaesthetized with acepromazine, guaifenesin, thiamylal, halothane for surgery. At G, the horse began to move. The vaporizer $ was increased and guaifenesin 5% infused until movement ceased. Note that arterial BP decreased from 120/90 to 70/55. The PaC02 at A was 57 mmhg. At R, the colt was rolled over. The vaporizer was increased from 2% to 3%, then 4%, then 5% for 5 minutes each to maintain anaesthesia. The horse did not move. BP decreased from 115/95 to 95/80. 8 = heart rate, o = spontaneous resp. rate.