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EQUINE VETERINARY EDUCATION / AE / MARCH 2015


Donkeys and mules Donkeys and mules are different in their response to anaesthetic drugs in that they tend to metabolise anaesthetic drugs faster than horses resulting in a shorter duration of anaesthesia (Matthews and Taylor 2000; Matthews and VanDijk 2004). For example, mules tend to require higher doses of xylazine (1.6 mg/kgbwt i.v.) or detomidine (0.03 mg/kgbwt i.v.) to produce desirable sedation. In donkeys, especially unhandled and miniature donkeys, satisfactory sedation occurs when combining butorphanol (0.04 mg/kg bwt i.v.) or diazepam (0.03 mg/kg bwt i.v.) with higher doses of xylazine or detomidine (Matthews and Taylor 1992, 2000). Subsequent administration of ketamine(2.0–3.0 mg/kgbwti.v.)canbeused to induce short-term anaesthesia. However, rough recoveries tend to occur when the ketamine dose is greater than 3.3 mg/kg bwt (Trawford 2000). A significantly shorter duration of anaesthesia (14.7 ± 9.4 min) with xylazine, butorphanol and ketamine in combinationwasobserved in miniature donkeys as compared to that of standard donkeys. Although this combination induced recumbency, poor muscle relaxation andaninadequate anaesthetic effect were observed resulting in vocalisation in 67% of the miniature donkeys in response to surgery (Matthews et al. 2002). On occasion, standard doses of xylazine, butorphanol and ketamine do not produce satisfactory anaesthesia in donkeys and mules. Diazepam (0.03–0.06 mg/kg bwt i.v.) or midazolam (0.06 mg/kg bwt i.v.) can be added to improve muscle relaxation and enhance the anaesthetic effect of xylazine, butorphanol and ketamine combination. Detomidine (0.04 mg/kg bwt i.v.) followed by ketamine (2.2 mg/kg bwt i.v.) provided approximately 10 additional minutes of anaesthesia than xylazine and ketamine combination (Matthews et al. 1992). ‘Equine Triple Drip’ can be used to induce and maintain


longer durations of anaesthesia in these animals. However, donkeys are more sensitive to guaifenesin and only require 60% of the dose used in horses to induce recumbency (Matthews and Taylor 2000; Matthews and VanDijk 2004). Nevertheless, this combination is often given ‘to effect’ with close monitoring and frequent adjustment of infusion rate of the mixture to maintain adequate depth of anaesthesia. Because of the sensitivity to guaifenesin5 of donkeys, a modified ‘Equine Triple Drip’ containing 25 mg/ml (2.5%) guaifenesin, 1 mg/ml ketamine and 0.5 mg/ml oxylazine has been used to prevent accumulation of guaifenesin and prolonged recovery (Bidwell 2010). Alternatively, donkeys can be sedated with detomidine (0.005–0.02 mg/kg bwt i.v. or i.m.) and methadone (0.1 mg/kg bwt i.v.) and anaesthesia can be induced with midazolam (0.06 mg/kg bwt i.v.) and ketamine (2.0–3.0 mg/kg bwt i.v.). A mixture of detomidine (10 mg) and ketamine (1000 mg) in 500 ml of 10% guaifenesin can then be used to maintain anaesthesia at 0.6–1 ml/kg bwt/h (Matthews and VanDijk 2004). Recovery in donkeys is usually gradual and smooth and they tend to remain in sternal recumbency until they are ready to stand unassisted. Mules have a temperament similar to horses, thus, sedation and assistance is often required for smooth and safe recovery (Matthews and Taylor 2000, 2002). In donkeys, anaesthesia induced with romifidine


(0.1 mg/kg bwt i.v.), midazolam (0.1 mg/kg bwt i.v.) and ketamine (2.2 mg/kg bwt i.v.) combination and maintained with a continuous infusion of midazolam (0.065 mg/kg bwt/h) and ketamine (6.6 mg/kg bwt/h) caused a mild but transient (10–15 min) increase in respiratory rate and decrease in heart rate (Amin et al. 2012a,b).


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Draught horses Similar anaesthetics and anaesthetic combinations used in regular horses, donkeys and mules can be used in draught horses. However, because of their slower metabolic rate, stoic temperament and large body mass, drugs used for chemical restraint and general anaesthesia tend to have enhanced depressing effects and prolonged elimination in draught horses. A 10–25% reduction in dosage is preferred when sedating or anaesthetising these animals (H.C. Lin, personal communication). Lower doses of xylazine (0.75–1.1 mg/kg bwt i.v.) and ketamine (1.65–2.2 mg/kg bwt i.v.) have been used to successfully produce a short duration of anaesthesia in draught horses. Prolongation of anaesthesia can be achieved by administering a half dose of each drug, which can be repeated more than once if necessary. ‘Equine Triple Drip’ is a better anaesthetic combination when good muscle relaxation, excellent analgesia and longer duration of anaesthesia are required. A combination of acepromazine (0.33 mg/kg bwt i.v.), followed in 15 min by gravity-flow of 5–10% guaifenesin until ataxia is evident and then 2.2 mg/kg bwt of ketamine administered i.v. has been used to induce anaesthesia. However, the longer duration of ataxia associated with this technique may increase the chance of injury during induction of anaesthesia. Detomidine (0.02–0.04 mg/kg bwt i.v.) followed in 15 min by ketamine (2.2 mg/kg bwt i.v.) has also been used in draught horses. Severe ataxia is a concern when these horses are recovering from anaesthesia (Geiser 1992).


Authors’ declaration of interests No conflicts of interest have been declared.


Manufacturers’ addresses


1Pfizer Inc., New York, USA. 2West-Ward, Eatontown, New Jersey, USA. 3Lloyd Laboratories, Shenandoah, Iowa, USA. 4Spectrum Chemical MFG. Corp. Gardena, California; New Brunswick,


New Jersey, USA. 5Abbott Laboratories, North Chicago, Illinois, USA. 6Mylan Institutional LLC, Rockford, Illinois, USA. 7Boehringer Ingelheim, Vetmedica Inc., St. Joseph, Missouri, USA. 8Hospira Inc., Lake Forest, Illinois, USA. 9Akorn Inc., Lake Forest, Illinois, USA. 10Mallinekrodt Inc., St. Louis, Missouri, USA. 11Gruppo Lepetit Srl, Localita Valcanello, Anagni, Italy. 12Med-Pharmex Inc., Pomona, California, USA. 13Amneal Pharmaceuticals, Glasgow, Kentucky, USA.


References


Abrahamsen, E. (2009) Managing severe pain in ruminants. In: Current Veterinary Therapy: Food Animal Medicine, 5th edn., Eds: D.E. Anderson and D.M. Rings, Saunders/Elsevier, St. Louis, Missouri. pp 570-574.


Alam, S., Saito, Y. and Kosaka, Y. (1996) Antinociceptive effects of epidural and intravenous ketamine to somatic and visceral stimuli in rats. Can. J. Anaesth. 43, 408-413.


Altura, B., Altura, B. and Carella, A. (1980) Effects of ketamine on vascular smooth muscle function. Br. J. Pharmacol. 70, 257-267.


Amin, A., Ali, A. and Al-Mutheffer, E. (2012a) Cardiopulmonary effects of the anesthesia by romifidine as a premedication, midazolam and ketamine induction and infusion in donkeys. J. Vet. Sci. 5, 125-129.


Amin, A., Ali, A. and Al-Mutheffer, E. (2012b) Clinical evaluation of TIVA by romifidine as a premedication, midazolam and ketamine in donkeys. Vet. Sci. Conf. pp 203-208.


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