Sedation, Anaesthesia and Analgesia

SEDATION

Sedation may be required to facilitate certain aspects of donkey care, such as management practices, veterinary treatment or to enable standing surgical procedures. Sedation is also a vital aspect of premedication prior to general anaesthesia.

Sedation in donkeys for stressful and painful procedures, or prior to anaesthesia, is broadly similar to horses and ponies. The dose of sedative agent used will depend on the temperament, age and health status of the donkey, as well as the anticipated duration and potential pain level of the procedure.

• All of the commercially available alpha-2 agonists can be successfully used in the donkey using standard equine doses. Precise selection is according to availability, personal experience and the procedure being undertaken.
• Sedation top-ups may need to be administered more frequently in donkeys than in horses and ponies, due to differing rates of drug metabolism.
• Typically alpha-2 agonists are combined with an opioid and data sheets must be checked to calculate suitable dosing regimes.
• Local anaesthetic nerve blocks should always be considered when appropriate.

Constant-rate infusions may be advisable for lengthier procedures. Combinations can be used that replicate those used in horses and ponies; standard equine texts and data sheets should be consulted.

Note that adjustment of the ‘normal’ recommended equine drip rate for constant rate infusions may be required to maintain adequate sedation, particularly in fractious individuals. Adjust the drip rate to effect.

Preparation is important before proceeding with sedation and anaesthesia.

• A thorough clinical examination, with consideration of the history, should precede any procedure. If there is suspicion of underlying disease before an elective procedure then biochemical/haematological screening may provide useful information.

• Carry out regular temperature, pulse and respiratory rate checks, routine monitoring for ataxia, monitoring of blood glucose and triglycerides, fluid and nutritional support as necessary, especially where the duration of the procedure is likely to be long. It is preferable to have a second suitably trained person available to monitor the patient during standing surgical procedures.
• Brief the owner regarding after-care and monitoring in the postoperative period, in particular to monitor the donkey closely for any changes in appetite or demeanour over the following few days.
• Arrange adequate analgesia for the post-operative period.
• Check the patient’s protection against tetanus and use prophylaxis where appropriate.

Intravenous administration

Sedation in donkeys follows similar protocols to those used in horses and ponies. Intravenous injection (I.V.) is the common route for administering sedatives in donkeys, although the donkey’s anatomy makes intravenous access more challenging.

An initial ‘off the needle’ administration of sedative may assist catheter placement in less cooperative patients.

Other routes of administration

There are occasions when it may be preferable to administer sedatives by the intramuscular or oral route. Examples include sedation of mules, or where there is prior knowledge of a problem in handling a donkey. It is preferable to sedate the donkey before they become stressed or experience fear, improving the level of sedation and also reducing the chance of long-term issues with needle sensitisation.

Options for sedating a donkey are similar to those for nervous or fractious horses and ponies. Clinicians may choose to supply an owner with oral sedative for use prior to their arrival. Oral detomidine gel can be used at standard equine doses.

Alternatives include the intramuscular (IM) administration of an alpha 2 agonist/opioid combination, typically at double the IV dose, or IM administration of a combination of detomidine, acepromazine and butorphanol (‘Magic Mix’).

GENERAL ANAESTHESIA

Before planning a general anaesthetic consider whether a standing procedure is a realistic alternative, especially if the donkey in question is in poor health or geriatric.

Pre-anaesthesia preparation

Preparation for anaesthesia will be similar to that for horses and ponies, but it is important to be aware of additional considerations, including the different behaviours and temperaments of the donkey in question.

Handling. Donkeys may be less used to regular handling than horses and ponies. It is essential to have plenty of patience and maintain a calm demeanour. A well-fitting head collar, supplemented with an arm around the neck and/or a hand over the dorsal nose surface, can provide good restraint during sedation or preparation for anaesthesia.

A heightened fight-or-flight response/increased adrenaline resulting from an unfamiliar situation, or a lack of attention to appropriate handling techniques, may increase the volume of sedative required to achieve a smooth induction.

Behaviour and risks from stress. Be aware of the risks of hyperlipaemia perioperatively. Stress due to separation from bonded companions, underlying pain, disease and pre-anaesthetic starvation are particular risk factors.

It is important to consider the strong bond between donkeys when planning anaesthesia or indeed any procedure and to try to keep the bonded companion within view to reduce the stress they experience from separation.

• It is essential that the companion accompanies and remains with the patient for as long as possible before induction.
• If the companion is unsettled or anxious, e.g. during catheter placement of the patient, it can be useful to separate the donkeys so that they can still maintain eye and muzzle contact over a gate or stable door.
• Note that while the patient is undergoing anaesthesia the companion may become very stressed at being seperated; distraction with attention and food can help, but severe cases may require sedation.
• Where severe stress in the companion can reasonably be predicted, an oral sedative such as detomidine gel can be given about 30 minutes before the procedure.
• Where a foal is present there is a high risk of stress to both mare and foal. Donkey foals may require sedation while separated from an anaesthetised mare. Xylazine or detomidine at standard equine foal doses can be used.

Failure to appreciate the significance of companion bonds can induce significant stress and a variable reaction to sedation or anaesthesia.

Pre-operative examination. It is essential that a thorough examination is carried out to check the health status of the animal and to obtain baseline physiological parameters.

Remember that the physiological parameters of the donkey differ from those of horses and ponies.

Pre-anaesthetic blood tests may be a valuable tool and judgement on the use as a pre-anaesthetic check must be made on an individual basis.

Donkey specific reference ranges for biochemistry and haematology must be used for evaluation.

• Pay particular attention to any evidence of underlying respiratory disease or subtle lameness.
• Although donkeys may not display overt signs of pain, perioperative analgesia is essential for smooth anaesthesia and the ongoing wellbeing of the patient.

Weight estimation. Accurate weight estimation is necessary for the calculation of anaesthetic drug dosages; scales may not be available.

The difficulty in calculating a donkey’s weight in the field situation is possibly the biggest cause of poor sedation and anaesthesia.

A weigh tape has been developed specifically for the donkey. This is suitable for estimating the weight of a donkey whilst horse and pony weigh tapes are not.

See online at www.donkeyweightape.com for a donkey-specific weigh tape.

Other considerations. There are important points to plan for that are unique to the species.

• Prolonged periods of starvation before an anaesthetic are not recommended; 12 hours should be sufficient, although many equine anaesthetists argue that pre-operative starvation is unnecessary. At The Donkey Sanctuary we withhold hard feed and hay overnight and remove the donkey from straw or grazing on the morning of the operation for elective procedures. Suckling foals should not be starved.
• Very few drugs are licensed for use in the donkey. Take note of the legal requirements of the country with regard to prescribing.
• In the interests of maintaining hygiene it is advisable to groom donkeys prior to induction.

It is advisable to have a crash kit available for emergencies. Suggested dosages of drugs used for this purpose are currently the same as in horses and ponies.

Induction

The Donkey Sanctuary uses the typical theatre induction protocol outlined below.

• The donkey’s mouth should have been flushed prior to induction
• While donkeys lack the bulk of horses, they can still go down quickly and with force, so induction should be carried out in a well-padded environment. 
• The placement of an intravenous catheter for administration of anaesthetic drugs is strongly recommended.
• The selection of alpha-2 agonist and opioid should be made according to individual clinician preference and experience, and on the procedure being performed and butorphanol, buprenorphine and morphine have all been used.
• Ensuring the donkey is adequately sedated prior to the administration of the induction agent should help ensure a smooth induction.
• The dose of ketamine varies between individual clinicians and the nature of the individual donkey and typically, 2.5 mg/kg bwt I.V. of ketamine post adequate sedation gives a smooth induction.
• Thiopental and propofol have also been used in donkeys but are not currently used at The Donkey Sanctuary.

Maintenance of anaesthesia

Anaesthesia may be maintained with inhalation or injectable agents.

Inhalation anaesthesia

If planning inhalational anaesthesia be aware of the potential difficulties when performing endotracheal intubation:

• the nasopharyngeal airway is short, constricted in the middle and flared dorsally and ventrally
• the nasopharyngeal recess (a diverticulum of the pharyngeal mucosa lying between the guttural pouches) is enlarged into a pouch 2—3cm wide and 4—6cm long (in the horse this is 2.5cm long)
• the epiglottis is short and more pointed, and the laryngeal opening tilted more caudally
• the nasal passages are narrower relative to head size compared with horses and ponies
• the average 180kg donkey requires an endotracheal tube of 16mm internal diameter; if possible, have a range of 14—18mm tubes available
• endoscopy may assist laryngeal visualisation.

Donkeys and foals weighing less than 150kg may need a small animal circuit, because a large animal circuit with excess dead space can result in laboured respiration and hypoxia.

Vaporiser setting can be minimised by appropriate use of local anaesthetics and injectable anaesthetic agents.

No donkey-specific differences in minimum alveolar concentrations of volatile agents have been noted.

Injectable anaesthesia

If injectable agents are used for maintenance of anaesthesia, top-ups may be required more frequently than in horses and ponies due to the shorter half-life of these drugs in donkeys. This is due to the different drug metabolism seen in donkeys.

Miniature donkeys may require even shorter top-up intervals.

At The Donkey Sanctuary ketamine is commonly used for maintenance as in the table below.

The ‘triple drip’ combination of guaiphenesin, ketamine and xylazine has also been used for induction and maintenance of anaesthesia in donkeys.

Note that:

• a smaller volume of guaiphenesin is needed to induce recumbency in donkeys
• higher amounts of ketamine may be needed in these combinations
• guaiphenesin is irritating to perivascular tissues so the catheter should be thoroughly flushed with saline before removal.

As in horses and ponies, there are many recipes for ‘triple drip’. An example used at The Donkey Sanctuary for maintenance of anaesthesia is given below. This may also be used for induction after an alpha-2 pre-medication.

Monitoring Anaesthesia

Monitoring anaesthesia follows the same principles as for horses and ponies. Eye position, pulse, respiratory rate, blood pressure and muscle tension can all be used as indicators of anaesthetic depth.

• Monitor trends in parameters over time so that sudden changes become obvious.
• Respiratory rate tends to be higher than in horses and ponies.
• Spontaneous ventilation is the norm in anaesthetised donkeys. Theatre personnel should be able to provide adequate ventilation manually where assistance is needed. Aim for 8–10 breaths per minute, adjusting as necessary.
• Donkeys may breath-hold if the plane of anaesthesia is too light.
• Trends in arterial blood pressure are particularly helpful; be alert to any sudden increases that may indicate a lightening of depth.
• The auricular artery has been used for direct measurement of arterial blood pressure. The facial artery has also been used, or arterial blood pressure can be measured indirectly via a cuff.
• Standard machines recording ECG traces, pulse oximetry and capnography can be valuable additional monitoring resources.

Be aware that the braying of a patient’s companion may lighten the depth of anaesthesia. Methods to alleviate this include distracting or sedating the companion, as previously described, and placing cotton wool in the patient’s ears.

Although myopathies and neuropathies occur less frequently in donkeys undergoing general anaesthesia, appropriate positioning is recommended with padding of the trunk and limbs, and fluid therapy as necessary to maintain blood pressure is still recommended.

Note that perioperative fluid therapy follows the same guidelines as for horses and ponies.

Anaesthesia in the field

It will often be appropriate, or even necessary, to carry out anaesthesia in the field.

• Induction should be carried out with care and not on a hard surface. For anaesthesia in the field it is advisable to take appropriate sized endotracheal tubes and a portable oxygen supply.
• The theatre induction protocols described above may also be used where induction is to be carried out in the field.
• Fractious or feral donkeys may require higher doses. The data sheets should always be consulted, particularly if using variations on suggested protocols.
• It is useful to take a towel to protect the eye during anaesthesia.

Triple drip recipes can also be used for field surgery but should be approached with caution by the less experienced clinician. Clinicians are advised to familiarise themselves with guaiphenesin anaesthesia using standard equine anaesthesia texts. With triple drip, signs of an appropriate plane of surgical anaesthesia can differ from those seen with inhalational anaesthesia or ketamine top-ups alone. Regular monitoring of the donkey and recording of vital parameters by trained personnel can be a vital aid to ensuring smooth and safe anaesthesia.

Emergency protocols

A crash kit with emergency resuscitation drugs and vasopressor agents for maintenance of blood pressure should always be available. Suggested dosages of such drugs are currently the same for donkeys as for horses and ponies.

Recovery

The donkey is usually calmer in recovery than horses and ponies, and rarely requires sedation. Extubation is performed once the swallowing reflex is observed and voluntary movement to sternal recumbency usually follows almost immediately. The donkey will typically delay standing until they are ready and assistance is rarely required, although occasionally a tail pull may be necessary. There are always exceptions to the rule, so the donkey must be observed until standing. It is important to ensure analgesia is adequate for the whole post-operative period as donkeys will not always show visible signs of pain.

The owner should be briefed to monitor the donkey closely for any changes in appetite or demeanour over the 2—3 days following the procedure.

ANALGESIA

Analgesia is mandatory where any procedure is performed that may be anticipated to cause pain.

• The use of meloxicam is not currently advised in donkeys as it is metabolised too quickly to have any lasting efficacy.
• Firocoxib has been used successfully in the USA but has not been tried at The Donkey Sanctuary, so we do not yet have any data on suitable dosages.
• Other analgesic agents including ketamine, lidocaine and opioids (morphine, buprenorphine) have all been used at The Donkey Sanctuary, using dosages extrapolated from equine formularies.

REGIONAL ANAESTHESIA

Nerve blocks

Nerve blocks may be indicated to assist in lameness evaluation, facilitate surgery of the head or distal limb, or for dental procedures.

Nerve blocks are commonly used for dental procedures that are performed in the standing sedated patient. Using effective local anaesthesia will enable a reduction in the requirement for sedation and analgesia.

Local anaesthesia can also provide an invaluable adjunct when performing procedures under general anaesthesia or when performing standing surgery.

The same standard local anaesthetic and regional nerve-blocking techniques used in horses and ponies can be employed. Doses of local anaesthetic may be reduced to reflect the smaller size of the donkey.

Nerve blocks are performed using aseptic techniques and, if necessary after the donkey has been given sedation, to ensure accurate needle placement. This may not be possible in the case of lameness evaluation as sedation can lead to ataxia.

For the mandibular and maxilliary nerve blocks that affect the deeper structures of the head, careful monitoring post procedure should be employed and feed withheld until the block has sufficiently worn off, in order to prevent inadvertent self-harm. Typically there may be damage to the tongue or buccal mucosa, or feed aspiration.

The ophthalmic, maxillary and mandibular branches of the trigeminal nerve may be anaesthetised at accessible foramen using the four most common dental nerve blocks: mental, infraorbital, mandibular and maxillary.

Epidurals

When carrying out epidurals be aware that the spinal cord terminates at the level of S2, while the dura may extend to the level of C1-2.

Donkeys typically have less musculature over the rump and tail head than horses and ponies, consequently the angle of needle introduction is shallower than in horses and ponies. Direct the needle at an angle of approximately 30° from the horizontal plane.

In donkeys the second intercoccygeal space is wider and most accessible for needle puncture.

Drugs suitable for epidural administration are as in horses and ponies, using volumes at the lower end of the dose range.
 

ANAESTHESIA IN FOALS

Donkey foal anaesthesia follows the same general principles as for other equine foals. Due to the small size of donkey foals, a small animal circuit may be used.

The monitoring and maintenance of body temperature and blood glucose levels are of particular importance in young foals, especially during prolonged periods of anaesthesia.

There is a high risk of stress to both mare and foal when either one is to be anaesthetised or undergo any procedure. Donkey foals often require sedation during separation from an anaesthetised mare.