Anaesthesia and Sedation - Donkey
Pre-anaesthetic evaluation and preparation
Donkeys can be more stoical than horses and, therefore, it may be more difficult to recognize the subtle signs that they are not well. The donkey presented for illness or a painful condition is likely to be sicker or in more pain than anticipated. A thorough pre-operative physical examination will help assess the patient’s condition, but caution must be used in interpreting blood values. Donkeys do not hemoconcentrate until they are seriously dehydrated, so an increase in hematocrit (packed cell volume, or PCV) won’t be seen with mild to moderate dehydration as would occur in horses.
If the donkey is in pain, it is important to stabilize the animal with pre-operative analgesics before anaesthesia is induced. Failure to do so may be associated with a donkey which ‘crashes’ shortly after induction. N. Matthews has seen severe cardiovascular collapse which responded to treatment, but might have been prevented by better pre-operative care.
Pre-operative evaluation should also include an estimate of weight derived by use of a weight-tape or use of the heart-girth nomogram. Body condition should also be evaluated. Debilitated animals usually have markedly reduced anaesthetic requirements.
This pre-operative evaluation is also an opportunity to evaluate the donkey’s temperament and training (or lack thereof) which may also affect drug choice, dosage and route of administration (i.v. or i.m.).
Donkeys seem to respond well to firm and secure restraint, and they do not seem to be claustrophobic like horses. We prefer to tie the donkey on a short lead with his head slightly elevated (which protects the neck from injury) when restraint is needed (as for catheterisation). A nose twitch is not very effective on most donkeys. Patience seems to be the most effective characteristic in working with donkeys. They do become very stressed in new environments, but cannot be ‘bullied’ into compliance as horses can. It pays to take your time and let them evaluate a new environment step by step.
Intramuscular injections may be better tolerated if the needle is slowly introduced through the skin and muscle, rather than with the ‘slapping’ motion commonly used in horses. Since a donkey’s skin is thicker than a horse’s, intravenous injections require that the needle be angled slightly differently - closer to 60º to the skin rather than the 45º angle which would be used for horses. Again, placing the needle on the skin and slowly increasing the pressure may cause the donkey to ‘lean into’ the needle rather than bounce away from it. For intravenous catheter placement, the site should always be blocked with a small bleb of local anaesthetic placed in the skin and subcutaneously over the vein.
Sedation for standing procedures
Many minor surgical procedures may be accomplished with a combination of sedation/analgesia and a local anaesthetic. A variety of drugs at differing dosages have been used to accomplish this (Matthews et al, 1997). Generally the best combination for sedation and analgesia seems to be an alpha-two agonist (such as xylazine, detomidine or romifidine) with an opioid. Butorphanol (El-Maghraby and Atta, 1997) and buprenorphine are the most commonly used, but other opioids can be used depending on availability.
We most commonly use xylazine with butorphanol for a short procedure (20 to 30 minutes).
Alternatively, detomidine can be used with butorphanol or buprenorphine. The duration of sedation and analgesia is longer with this combination (40 to 60 minutes).
All of the above mentioned drugs can also be administered i.m. if the animal is fractious, and generally it is necessary to give twice the dose to achieve the desired effect. Another option is to administer i.m. xylazine or detomidine, wait 20 to 30 minutes for sedation to occur, then add additional drugs i.v. as indicated by the patient.
In addition to the analgesic effects of the above-mentioned drugs, use of a local anaesthetic ( lidocaine, or the longer-acting bupivicaine) is always recommended. This may be locally infiltrated, or a line-block or anatomic nerve block may also be used.
Acepromazine may be used for tranquillization, but one must remember that it has no analgesic effects, so appropriate analgesia must be provided with other drugs.
Injectable (field) anaesthesia
Many drug combinations and techniques have been successfully used for injectable anaesthesia in donkeys. General guidelines which may be helpful are:
- different sizes of donkeys appear to metabolize drugs at differing rates. Miniature donkeys seem to require more drug/kg body weight than larger donkeys (Matthews et al, 2001; Matthews et al, 2002)
- donkeys metabolize ketamine faster than horses, but are more sensitive to guaifenesin. Doses of these drugs must be adjusted appropriately (Matthews et al, 1997).
Since donkeys are more stoical than horses, the anaesthetist must be vigilant about monitoring the depth of anaesthesia to make sure that they are maintained at a suitable plane. Routine use of local anaesthetics in conjunction with injectable anaesthesia is encouraged, since it facilitates intra-operative analgesia as well as post-operative analgesia.
For short procedures, pre-medication with xylazine or detomidine followed in 3 to 5 minutes by induction with ketamine is generally effective. Addition of butorphanol to the xylazine will increase the degree of sedation achieved. In some instances the donkey will lie down with these premeds and anaesthesia can be induced with the ketamine with the donkey already sternally recumbent.
Diazepam (can be mixed with the ketamine) can also be used to ‘smooth’ the effects of the ketamine . Since diazepam produces muscle relaxation but little cardiovascular depression, it is a useful addition for sick or debilitated animals, where it is important to minimize the dose of xylazine or detomidine used. These combinations should provide approximately 15 to 30 minutes of anaesthesia in standard and Mammoth donkeys.
Miniature donkeys appear to be much more ‘resistant’ to xylazine and ketamine anaesthesia (Matthews et al, 2002).
Acceptable injectable anaesthesia can be accomplished if xylazine and butorphanol are used to pre-med, with diazepam and ketamine used for induction and a local block is added. Alternatively, good anaesthesia is provided by pre-medicating with xylazine (or detomidine) and butorphanol, then the combination of tiletamine-zolazepam.
For longer periods of anaesthesia (40 minutes to 2 hours), we have found that the combination of guaifenesin-xylazine-ketamine works well (Taylor et al, 2002). Following premedication with xylazine, anaesthesia is induced by rapid administration of the combination. Once the donkey becomes recumbent the infusion rate is slowed. This rate should be adjusted as is appropriate for the individual based on monitoring.
Thiopental has been used for induction and maintenance of anaesthesia in donkeys. It is usually used following sedation and can be combined with guaifenesin for administration as a slow drip. Lower doses of thiopental can be used as an adjunct to xylazine/ketamine anaesthesia, to increase muscle relaxation and the length of anaesthesia (Crane, 1997). Recoveries are likely to be longer when thiopental is used, especially if multiple doses are given. Use of an intravenous catheter is strongly advised, since perivascular administration of thiopental causes tissue sloughing.
Inhalant anaesthesia and monitoring
Induction of anaesthesia may be accomplished by any of the above techniques, when it will be maintained with an inhalant. Following induction, donkeys can be orotracheally intubated in the same manner as horses. Occasionally, it may be necessary to use a smaller endotracheal tube than anticipated for a donkey, or one can place a sterile stomach tube into the trachea which serves as a guide tube, over which the endotracheal tube is passed. A difficult intubation may also be assisted by making sure that the donkey is at a deep enough plane of anaesthesia (i.e. not vigorously swallowing).
Maintenance of anaesthesia with halothane, isoflurane or sevoflurane in donkeys is similar to maintenance of horses with these agents. Minimal alveolar concentrations (MAC-value) of halothane and isoflurane in donkeys are similar to values reported in ponies (Matthews et al, 1997). Although the MAC-value for sevoflurane in donkeys has not been measured, our clinical experience with this agent in donkeys indicates that vaporizer settings are very similar to those used in horses. Intermittent small boluses of an opioid (e.g. butorphanol) may be used to provide additional analgesia, and for some surgical procedures, the use of a long-acting local anaesthetic (e.g. bupivicaine; not to exceed 2 mg/kg) will provide intra- and post-operative analgesia (i.e. distal limb procedures where the peripheral nerves can be blocked). Since less inhalant is required when additional analgesics are used, cardiovascular stability is improved. MAC-values for all inhalants are generally reduced in older or debilitated animals so vaporizer settings must be adjusted, based on monitoring.
Intra-operative fluid administration is important when the inhalants are used, since they produce a dose-dependent vasodilation. A balanced electrolyte solution given at a rate of 10 ml/kg/hr (or greater if pre-operative dehydration has not been corrected) will help maintain blood pressure. Aggressive fluid therapy (up to 40 mls/kg/hr) or use of hypertonic saline (4 ml/kg) or hetastarch (4 ml/kg) may be needed to maintain blood pressure in hypovolemic animals.
Monitoring the depth of anaesthesia
Monitoring the depth of anaesthesia in donkeys is similar, yet somewhat different from monitoring horses. Palpebral and corneal reflexes, rotation of the eye and presence or absence of lacrimation, muscle relaxation, anal tone, jaw tone, heart rate and respiratory rate and character should all be used to maintain anaesthesia at the appropriate level. Unlike horses, N. Matthews has observed donkeys who hold their breaths when the plane of anaesthesia is too light, so the character of respiration should be closely observed. Resting respiratory rates are higher in donkeys (20-30 bpm) than in horses (6-12 bpm), and the amount of chest movement is less than in horses; these differences must be recognized.
Monitoring blood pressure seems to be a more sensitive indicator of anaesthetic depth. An increasing blood pressure indicates that anaesthesia is decreasing. Blood pressure can be measured indirectly (with cuff placed around the tail or leg) or directly (with arterial catheter placed in the dorsal metatarsal, facial or auricular arteries).
Hypotension (defined as a mean blood pressure < 70 mm of Hg, or a systolic blood pressure < 90 mm of Hg) should be treated by:
- decreasing the anaesthetic plane
- giving intravenous fluids
- using an inotrope such as dobutamine
Complications of anaesthesia
The commonest complication seen in donkeys seems to be movement in response to surgical stimulus. This probably occurs because they metabolize injectable anaesthetics more rapidly than horses, so the dosing interval may need to be adjusted. Additionally, since they can be a little more difficult to monitor (since their tendency is to lie quietly), the anaesthetist may be fooled into thinking the donkey is at a surgical plane of anaesthesia when it is not. The addition of blood pressure monitoring is very helpful here, since it is more sensitive to changes in anaesthetic depth than other reflexes.
A less common (but very serious) complication is cardiovascular collapse. This can occur in a donkey with an extremely painful pre-operative condition (e.g. osteomyelitis or laminitis), where the degree of pain has not been alleviated through pre-anaesthetic analgesics. Cardiovascular collapse can also occur in a very debilitated animal when pre-operative fluid therapy has not been initiated. If bradycardia is the first indication, cardiovascular collapse should be aggressively treated with atropine and/or epinephrine. Cause of the cardiovascular collapse should be determined and treated.
Although donkeys do not have the large muscle mass seen in horses, it is still suggested that padding be used to protect them from the possible complications of neuropathy and myositis. Foam mattresses, airbags, inner tubes, and thick bedding can all be used as padding depending on availability.
Recovery and analgesics
Recovery from anaesthesia is usually calm and smooth in donkeys, although it is wise to insufflate oxygen nasally when possible, provide good padding for recovery and a quiet environment when possible. Causes of rough recovery are pain and difficulty in breathing.
Analgesia may be provided in a number of ways including local aesthetics, opioids, which can be given i.v., i.m. or administered epidurally (Naeni et al, 1999), and non-steroidal anti-inflammatories (NSAIDs). Xylazine and detomidine are also good analgesics and can be administered i.v., i.m. or used epidurally.
Phenylbutazone and flunixin meglumine are more rapidly metabolized in donkeys than in horses; doses should be larger or given more frequently than for a horse. Carprofen lasts longer in donkeys than in horses, so administration once a day should be sufficient (Matthews et al, 2003).
Oedema of the nasal passage may occur during recumbency and cause difficult breathing following extubation. Intranasal neosynephrine spray usually quickly treats this problem. Laryngeal obstruction due to soft palate displacement can also cause difficulty breathing and have serious consequences (e.g. pulmonary oedema) or require an emergency tracheotomy. It can usually be relieved by passing a small tube into the nasopharynx or nasotracheally, or by leaving the endotracheal tube in place until the donkey stands up.
In short, although equine practitioners tend to treat donkeys like little horses, they are significantly different and these differences can make anaesthesia frustrating if not acknowledged. However, because of their calm nature, donkeys can also be very good patients when properly handled.
The donkey should be starved by being fitted with a muzzle for four hours prior to surgery, or for a longer period for abdominal surgery. Starvation may not be possible in an emergency.
Clip over both jugular veins and, if possible, the surgical site, and groom and pick out feet. Separate from friends when the patient is moved to the induction area.
Feed the companion, if there is one, and ensure it settles.
A 14 g catheter is placed in the jugular vein and glued or sutured in position; the intravenous anaesthetic is administered through this. Standard aseptic technique is recommended. Flush the catheter and keep it patent for emergency use and administration of intravenous fluids.
If gaseous anaesthesia is being used, extension of the head facilitates insertion of an endo-tracheal tube. A 16 mm tube is suitable for most donkeys, with a 14 mm tube on stand-by. Small donkeys and foals may only accommodate a 14 mm or 12 mm tube.
The depth of anaesthesia should be constantly monitored and the pulse and respiration rates recorded. Palpebral and corneal reflexes are assessed and the eye will rotate medially. Anal reflex is not always reliable.
In lateral recumbancy, pull the lower forelimb forwards and support the uppermost front and hind limbs to help avoid myopathy.
To reduce contamination, cover feet and limbs with rectal gloves before preparing the surgical site.
On recovery, the gag and endo-tracheal tube are left in position until the swallow reflex returns. Donkeys generally recover quietly and do not require further sedation. When the donkey is standing and stable, it can be returned to its friend and given food.
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Anaesthesia and sedation in donkeys related publications
Donkey anesthesia and analgesia - not just small horses. Matthews, N.; The North American Veterinary Conference, Gainesville, USA, Large animal. Proceedings of the North American Veterinary Conference, Orlando, Florida, USA, 16-20 January 2010, 2010, pp 208-210 - Full Text Article
Comparison of caudal epidural administration of lidocaine and xylazine to xylazine/ketamine combination in donkey (Equus asinus). Sarrafzadeh-Rezaei, F.; Rezazadeh, F.; Behfar, M.; Iranian Veterinary Surgery Association, Kermen, Iran, Iranian Journal of Veterinary Surgery, 2007, 2, 5, pp 7-15, 47 ref. - Full Text Article
Influence of atropine premedication on cardiac rate in donkeys injected with xylazine and xylazine-ketamine combination. Mogoa, E. G. M.; Kenya Veterinary Association, Nairobi, Kenya, The Kenya Veterinarian, 2001, 21, pp 26-28, 17 ref. - Full Text Article
- Matthews, N. (2008) Anaesthesia and sedation In Svendsen, E.D., Duncan, J. and Hadrill, D. (2008) The Professional Handbook of the Donkey, 4th edition, Whittet Books, Chapter 15
- Dabinett, S. (2008) Nursing care In Svendsen, E.D., Duncan, J. and Hadrill, D. (2008) The Professional Handbook of the Donkey, 4th edition, Whittet Books, Chapter 18
- Crane, M., (1997). The Professional Handbook of the Donkey, Third Edition. E.D Svendsen (ed). pp. 43-51. Whittet Books, London.
- El-Maghraby, H.M., Atta, A.H. (1997). ‘Sedative and analgesic effects of detomidine with and without butorphanol in donkeys’. Assiut veterinary medical journal 37. pp 201-211.
- Matthews, N.S., Taylor, T.S., and Hartsfield, S.M. (1997). ‘Anaesthesia of donkeys and mules’. Equine Veterinary Education 9. pp 198-202.
- Matthews, N.S., Peck, K.E., Taylor, T.S., and Mealey, K.L. (2001). ‘Pharmacokinetics of phenylbutazone and its metabolite oxyphenbutazone in miniature donkeys’. American Journal of Veterinary Research 62. pp 673-675.
- Matthews, N.S., Taylor, T.S., and Sullivan, J.A. (2002). ‘A comparison of three combinations of injectable anaesthetics in miniature donkeys’. Veterinary Anaesthesia and Analgesia 29. pp 36-42.
- Matthews, N.S., Mealey, K.L., Peck, K.E., Sinclair, M., and Taylor, T.S. (2003). ‘Pharmacokinetics of carprofen in donkeys: comparison to horses’. Proceedings 8th World Congress of Veterinary Anaesthesia, Knoxville, TN, USA. pp 133.
- Naeni, A.T., Rezakhani, A., and Ahmmadian, M. (1999). ‘Comparison of morphine, fentanyl, methadone, lidocaine and lidocaine/epinephrine as caudal epidural analgesics in donkeys’. Journal of Applied Animal Research 15. pp 181-184.
- Taylor, E.V., Matthews, N.S., Taylor, T.S., and Barling, K.S. (2002). ‘Comparison of injectable anaesthetics in donkeys. British Equine Veterinary Association 41st Congress’. Handbook of presentations and free communications. pp 198-199.