Anaesthetics - Donkey

Current best practice aspects of anaesthesia in donkeys are described in detail in Anaesthesia and Sedation. A brief outline of the pharmacology of some of the agents used commonly in donkeys for anaesthesia and analgesia is given below. These agents should never be administered without appropriate preanaesthetic evaluation and preparation and adequate monitoring is essential (Matthews and Taylor, 2000; Matthews and Van Dijk, 2004).

Ketamine and tiletamine

Ketamine and tiletamine are dissociative anaesthetic agents that produce a dose-dependent loss of consciousness, usually associated with an increase in skeletal muscle tone, which progresses to a trance-like state (catalepsy). Ocular and pharyngeal reflexes remain intact and nystagmus is common. Ketamine and tiletamine are effective analgesics, indirectly stimulate cardiac function, do not significantly depress respiration and produce bronchodilation.

The total anaesthetic time with ketamine is even shorter in donkeys than in horses (12 to 20 minutes). Some donkeys, particularly those not used to being handled, and miniature donkeys, may require higher doses of ketamine (and xylazine). Tiletamine is only available in a fixed combination with the benzodiazepine zolazepam.

Thiopentone (thiopental)

Barbiturates depress central neuronal activity resulting in anaesthesia, anti-convulsant activity, an increase in the threshold of spinal reflexes and excellent muscle relaxation. They also decrease intracranial and intraocular pressures. Barbiturates cause respiratory and cardiovascular depression and do NOT produce analgesia. Redistribution of the thiopentone from the brain to other body tissues terminates the anaesthetic effect. However clearance is slow, meaning that repeated dosing can significantly prolong recovery.

Systemic acidosis can increase the proportion of un-ionised thiopental resulting in increased delivery of active drug to the brain. The extreme alkalinity of thiopentone sodium solution (pH >10) can cause significant local tissue damage if administered accidentally by the perivascular or intraarterial routes. Thiopentone should only be administered after adequate sedation.

Guaifenesin

Guaifenesin, a centrally acting skeletal muscle relaxant with only mild sedative and analgesic effects, can be administered intravenously to donkeys in combination with xylazine and ketamine for extended periods of general anaesthesia (Matthews and Taylor, 2000). Careful monitoring of the depth of anaesthesia is essential, particularly because donkeys are more sensitive (40% less required to produce recumbency) to guaifenesin than horses (Matthews et al, 1997; Matthews and Taylor, 2000). Overdosage is associated with abnormal respiration nystagmus, hypotension, and contradictory muscle rigidity.

Suggested combination regimens for sedative/tranquillisers, opioids and anaesthetic agents in donkeys

• Sedation: xylazine or detomidine
• To improve the quality of xylazine or detomidine sedation: diazepam or butorphanol
• Tranquillisation for minor procedures: ACP – analgesia must be provided!
• Field anaesthesia: xylazine or detomidine (with or without butorphanol) i.v. followed in three to five minutes by ketamine (with or without diazepam) i.v.
• Prolongation of xylazine/ketamine anaesthesia: i.v. bolus doses of thiopentone (5% solution), up to a maximum of three repeated doses

Local anaesthetic agents

Local anaesthetics (e.g. lignocaine (lidocaine), mepivacaine, bupivacaine) are used to interrupt the nociceptive process between the peripheral, high threshold nociceptor and the cerebral cortex, either by blocking transduction, by infiltration at the site of injury or incision, or by preventing transmission in afferent myelinated Aδ and unmyelinated C fibres, by blockade of peripheral nerves, nerve plexuses or by epidural injection. In fact, local anaesthetics depress all excitable cells, in part explaining some of their undesired effects, e.g. CNS stimulation, bradycardia and hypotension, which often follow inadvertent intravascular injection or overdosage and systemic absorption. The onset of action (pKa), potency (lipid solubility) and duration of action (protein-binding) of each agent depends on its physicochemical properties:

• lignocaine has a rapid onset and medium duration of action
• mepivacaine has a rapid onset and a slightly longer duration of action
• bupivacaine has an even longer duration of action

Local anaesthesia is used commonly in donkeys. Adequate restraint and/or sedation are important and the site must be properly and aseptically prepared. For intra-articular administration, the volume injected depends on the joint size, but in practice the smallest volume possible should be administered. The activity of local anaesthetics (pKa 7.7-9.0) can be enhanced by increased extraneuronal pH (addition of bicarbonate) and by co-administration of a vasoconstrictor such as adrenaline (epinephrine) for lignocaine or of hyaluronidase.

Local tissue reactions, including inflammation and necrosis, can occur, particularly if the formulation used contains adrenaline. Local anaesthetics are less effective in conditions associated with acid pH, e.g. inflammation. Due to the longer duration of action of bupivacaine, nerve blocks should not be repeated within four to six hours to avoid accumulation and toxicity.

Literature Search

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Anaesthesia in donkeys publications

References

• Horspool, L. (2008) Clinical pharmacology In Svendsen, E.D., Duncan, J. and Hadrill, D. (2008) The Professional Handbook of the Donkey, 4th edition, Whittet Books, Chapter 12
• Matthews, N.S., Peck, K.E., Mealey, K.L., Taylor, T.S., and Ray, A.C. (1997). ‘Pharmacokinetics and cardiopulmonary effects of guaifenesin in donkeys’. Journal of Veterinary Pharmacology and Therapeutics 20. pp 442-226.
• Matthews, N.S., Taylor, T.S. (2000). ‘Anesthetic management of donkeys and mules’. Recent Advances in Anesthetic Management of Large Domestic Animals. (ed). E.P. Steffey. International Veterinary Information Service [www.ivis.org], New York . A0607.0700.
• Matthews, N.S., Van Dijk, P. (2004). Anesthesia and analgesia for donkeys In Veterinary Care of Donkeys. N.S. Matthews, T.S. Taylor (eds). International Veterinary Information Service [www.ivis.org], New York. A2902.0904.

PHARMACOLOGY - DONKEY

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