Sedation, Anaesthesia and Analgesia

SEDATION

Sedation may be required to facilitate a routine dental examination or to enable standing surgical procedures. Sedation is also a vital aspect of pre- medication where general anaesthesia will be taking place.

Preparation is important and the following should be considered:

• For both sedation and general anaesthesia a thorough clinical examination and history taking should precede any procedure. If there is suspicion of underlying disease before an elective procedure then biochemical/haematological screening may provide extra, useful information.

• Check the patient’s history for any warning signs relating to laminitis, which may be a risk factor where prolonged standing surgery is to be undertaken.

• Check the patient’s protection against tetanus and use prophylaxis where appropriate.

• In ideal situations, although not possible for the majority of cases, a second person should be available to monitor the patient during standing surgical procedures. This is particularly important where the duration of the procedure is likely to be long. Carry out:
  • regular TPR checks
  • routine monitoring forataxia
  • monitoring of blood glucose and triglycerides
  • fluid and nutritional support as necessary.

• The owner should be carefully briefed regarding after care and monitoring in the post-operative period, in particular to monitor the donkey closely for any changes in appetite or demeanour over the next few days.

• Arrange adequate analgesia for the post-operative period.

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.

Sedation can be done ‘off the needle’, but where the duration of the planned procedure may be long, catheterisation is advisable because frequent top-ups are often necessary and I.V. injection can be challenging in the donkey. Catheterisation should be considered mandatory in surgical procedures where continuous venous access is needed.

An off the needle initial administration of sedative may assist catheter placement in less cooperative patients. The use of oral or intramuscular sedation in fractious animals is discussed below.

The dose of sedative agent required will depend on temperament, age, health status, anticipated duration and potential pain level of the procedure.

Note:

• 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.

  • The combination of an alpha-2 agonist with an opioid such as butorphanol may assist sedation and reduce tongue movement. Some clinicians feel that the opioid may promote head weaving and other unwanted locomotor and behavioural issues. At The Donkey Sanctuary, we have found this to be most pronounced when buprenorphine is used.

  • If tongue movement is still persistently affecting examination and procedure, then diazepam at 0.005–0.01mg/kg bwt I.V. can be given. Use of diazepam will cause relaxation of the whole body, not just the tongue, and the clinician should be wary of ensuing ataxia. For this reason, the use of diazepam is only recommended when the standing patient is restrained in stocks.

  • Drug combination and dosage should be selected according to the individual situation and with consideration of recommendations noted in the product data sheets.

• Sedation top-ups may need to be administered more frequently in donkeys, due to differing rates of drug metabolism.

• Local anaesthetic nerve blocks should always be considered when appropriate.

Constant-rate infusions are advisable for lengthier procedures and all standing surgical procedures. Suitable combinations are the same as those used in horses and ponies. The clinician is advised to consult standard equine texts and data sheets.

Note: Adjustment of the ‘normal’ recommended equine drip rate for constant rate infusions may be required to maintain adequate sedation, particularly in fractious individuals, drip rate should be titrated to effect.

Different routes of administration

There will be 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 it becomes stressed or experiences fear and thereby reduce the chance of long-term issues with needle sensitisation and improve sedation.

Options for alternative sedation routes for donkeys are similar to those for nervous or fractious horses. Vets may choose to supply an owner with an oral sedative for use before the vet arrives. The Donkey Sanctuary uses oral detomidine gel at standard equine doses. Alternatives include the intramuscular (IM) administration of a combination of detomidine, acepromazine and butorphanol (‘Magic Mix’).

ANAESTHESIA

A general anaesthetic will only rarely be used for dental procedures, even complex procedures are normally carried out using sedation and effective local nerve blocks. However, it may become necessary in the absence of suitable equipment and resources necessary to carry out standing procedures under sedation, or where the animal’s behaviour compromises the safety of the patient or the handler.

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

• We do not recommend prolonged periods of starvation before an anaesthetic; 12 hours should be sufficient, although several 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.

• Always try to keep the bonded companion within view to reduce stress from separation.

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

Horse and pony weigh tapes are not suitable for donkeys. A donkey-specific weigh tape or The Donkey Sanctuary nomogram should be used.

A weigh tape has been developed specifically for the donkey.

Induction

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

Note:

• The selection of alpha-2 agonist and opioid should be made according to individual clinician preference and experience, and on the procedure being performed. Butorphanol, buprenorphine and morphine have all been used.

• The dosage of ketamine varies between individual clinicians and the nature of the individual donkey. 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 inhalational or injectable agents.

Inhalation anaesthesia

If planning inhalational anaesthesia, be aware of the potential difficulties when performing endotracheal intubation. Donkeys have a large pharyngeal recess. The epiglottis is relatively shorter and more pointed than in horses and the laryngeal opening caudally tilted. The trachea is also narrower relative to head size than in horses and ponies.

Note:

• The average 180kg donkey requires an endotracheal tube of 16mm internal diameter; have a range of 14–18mm tubes available if possible.

• Endoscopy may assist laryngeal visualisation.

• Foals and donkeys less than 150kg may need a small animal circuit; a large animal circuit with excess dead space results in a risk of dyspnoea and hypoxia.

• When using inhalational anaesthetics, as with horses, judicious use of injectable agents can help to minimise the vaporiser setting required to maintain an adequate plane of anaesthesia.

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

The use of local anaesthetics around the surgical field as standard dental nerve blocks can help reduce general anaesthetic drug volumes.

Injectable anaesthesia

If using injectable agents for maintenance, usually ketamine, top-ups will be required more frequently than in horses due to their shorter half-life in donkeys. This is due to the different drug metabolism seen in donkeys.

Miniature donkeys may require even shorter top-up intervals.

The ‘triple drip’ combination of guaiphenesin, ketamine and xylazine has also been used for induction and maintenance of anaesthesia in donkeys. However, triple drip anaesthesia is only suitable for shorter procedures and is therefore unlikely to be appropriate for the majority of dental surgeries. It would be appropriate where a short acting general anaesthetic is necessary on behavioural grounds to perform a quick dental procedure.

Note: A smaller volume of guaiphenesin is needed to induce recumbency in donkeys and higher amounts of ketamine may be needed in the combination (Taylor et al, 2008). As in horses and ponies, there are a multitude of different recipes for the maintenance of anaesthesia, an example used at The Donkey Sanctuary is given below. This may also be used for induction after an alpha-2 pre-med.

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

Note:

• Trends in parameters over time should be monitored so that sudden changes become obvious.

• Respiratory rate tends to be higher in donkeys than in horses.

• 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/minute, adjusting as necessary.

• Donkeys may breath-hold if the plane of anaesthesia is too light.

• Although myopathies and neuropathies occur less frequently in donkeys undergoing general anaesthesia, probably due to their reduced body mass, appropriate positioning and padding of trunk and limbs, and fluid therapy as necessary to maintain blood pressure is still recommended.

Note: Perioperative fluid therapy for donkeys follows the same guidelines as for horses and ponies.

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

• Donkeys are usually calmer in recovery than horses and rarely require 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 ready and assistance is rarely required although occasionally a tail pull may be called for.

• There are always exceptions to the rule so ensure that the donkey is observed until standing.

• The owner should be briefed to monitor the donkey closely for any changes in appetite or demeanour over the next few days.

• Dispense adequate analgesia for the post-operative period.

NERVE BLOCKS

Nerve blocks are used commonly 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.

Nerve blocks are placed using aseptic techniques and, if necessary, after the donkey has been given sedation, to ensure accurate needle placement.

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 may be anaesthetised at accessible foramen using the four most common blocks: mental, infraorbital, mandibular and maxillary.

Mental nerve block

Mental nerve blocks are used for desensitising:

• the skin of ipsilateral lip and chin, for superficial placement of local anaesthetic

• the lower incisor arcade, alveoli and gingiva, for central incisors 301/401, which need a bilateral block due to crossing over of nerve fibres rostrally, and some insertion of needle into the foramen

• canine teeth 304 and 404.

Location

The mental nerve exits the mental foramen at the lateral horizontal ramus of mandible, below the commissure of the lips in the inter-alveolar space. The tendon of the depressor labii inferioris muscle covers the nerve and foramen, and must be lifted dorsally to palpate the subtle bony depression that indicates the foramen. The donkey may not tolerate the injection of local anaesthetic into the foramen as well as injection around the nerve, but injecting into the foramen is needed to fully anaesthetise the incisors, canines and the rostral molar.

Approach

2–4mls of local anaesthetic are used depending on the structures that need desensitising, using a 23-gauge needle.

Infraorbital block

Infraorbital blocks are used for desensitising:

• the motor and sensory to ipsilateral side of the face, nostril and upper lip

• upper incisors and canines

• the rostral molar teeth, this requires insertion of a needle into the foramen and injecting local under pressure, which is not well tolerated.

Location

The infraorbital foramen is located 1–3cm caudal to a line drawn between the rostral end of the facial crest and the commissure of the nasal bone. It can be palpated as a slight depression under the levator labii superioris muscle, which may need to be displaced for accurate placement.

Approach

3–5mls of local anaesthetic are used, using a 21/22-gauge needle.

Mandibular nerve block

Mandibular nerve blocks are used for desensitising:

• the medial aspect of the mandible

• the ipsilateral mandibular molars, premolars, canines and incisors.

Location

The mandibular foramen is located on the medial aspect of the mandible and is found by drawing an imaginary line along the buccal edge of the upper molars and extending this back to the mandible, then drawing a second imaginary line from the lateral canthus of the eye which crossed the previous line at right angles; the intersection of the lines is the approximate location of the foramen medially.

Approach:

To anaesthetise this foramen:

• Consider sedating the donkey before administering this nerve block in order to avoid movement during the introduction of the needle.

• Extend the donkey’s head and insert a 10–15cm long, 20/22-gauge needle (spinal needle) from either a ventral or caudal location; it is advisable to locate the site of injection on the mandible laterally and to use this to measure the length of needle required for the deposition of local anaesthetic.

• Keep the needle point directed as close to the medial mandible as possible and not angled inwards to vital structures.

• It is preferable to aim to deposit the local anaesthetic dorsal to the foramen so that the nerve is reached before it enters the foramen.

• Use 10mls of local anaesthetic, and allow at least 10–15 minutes before starting any painful procedure.

Maxillary nerve block

Maxillary nerve blocks are used for desensitising:

• the apices of the molar, premolar, canine and incisor teeth on the ipsilateral upper arcade

• paranasal sinuses (part of)

• the nasal cavity (part of).

Note: Branches of the nerve outside the infraorbital canal supply soft tissue next to the dental arcades, additional infiltration of local anaesthetic is advisable when these tissues are manipulated.

Location

The maxillary foramen is found at the pterygopalatine fossa, just lateral to two other large significant foramen carrying the palatine artery (caudal palatine foramen) and the malar artery (sphenopalatine foramen). The nerve runs close to the ethmoidal and palatine arteries, and if these are punctured by the needle there can be significant haemorrhage in the retro bulbar space. Always draw back before injecting the local anaesthetic to avoid inadvertent intra-arterial injection.

Approach

For this nerve block it can help to place a small bleb of local anaesthetic subcutaneously before introducing the spinal needle.

There are two possible approaches:

1. Insert a 20–22 gauge 10cm spinal needle just below the zygomatic bone at the level of the posterior third of the bony orbit. The needle is situated in the space between the mandible and the zygomatic bone. The needle is directed at 90° to the skin and advanced until the bone is contacted at approximately 4–6cm, depending on the size of the donkey.

2. Palpate the most dorsal aspect of the zygomatic process and insert a 22-gauge 10cm needle, aiming rostroventral towards the rostral end of the contralateral facial crest.

10mls of local anaesthetic is deposited. It is preferable to use bupivacaine if the procedure is estimated to take over 90 minutes.

Complications of this nerve block can include facial swelling and exophthalmos and temporary Horner’s syndrome. The complications should be managed symptomatically with pressure bandaging where possible, lubrication of the eye until swelling has decreased, and anti-inflammatories.

Infection in deeper tissues can occur with poor asepsis and can have serious consequences.

Infiltration of local anaesthetic

Nerve blocks can be augmented by the use of topical and subgingival infiltration of local anaesthetic agents.

Lidocaine gel 5% is useful to desensitise small areas of gingiva before injection.

Typically, small amounts (2-4 mls) of local anaesthetic are used sub-gingivally before wolf tooth removal (which is rarely indicated in the donkey), and where possible to aid in desensitisation of the soft tissue before incisor extraction.

Local anaesthetic may be used directly around the soft tissue for the maxillary and the mandibular cheek teeth, particularly if there is likely to be a lot of periodontal elevation. Inject along the plane of the periodontal ligament in order to avoid the palatine artery.

Choice of local anaesthetic

The choice of local anaesthetic is based on the duration of the procedure, with longer extractions requiring drugs with a longer action.

Table showing local anaesthetics and properties.

Complications

Typical complications include:

• unsuccessful anaesthesia

• puncture of arteries/blood vessels

• inadvertent anaesthesia of the optic nerve/oculomotor musculature

• temporary/extended paralysis of the facial nerve

• neuritis (chronic/permanent).

ANALGESIA

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

Note:

• 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.