As in any animal, anaesthesia in a horse carries a risk, although it is much higher than that of other domestic species. It is therefore important to try and minimise these risks as much as possible when performing any procedure. Problems can be encountered at any stage of the anaesthetic and so each stage shall be considered separately here.
Almost every horse undergoing anaesthesia will be standing at induction. This in itself carries a risk for not only the horse but also the personnel involved. For example, if a horse is presenting for surgical correction of a fractured limb it is at high risk of causing further injury to the affected limb if induction is not carried out in a controlled and safe manner. Other examples may be a colic that despite analgesia and sedation may still be thrashing about, posing a danger for injury to the horse itself or any personnel that may be trying to control the horse.
There are a number of methods to try and reduce these risks as follows.
Sedation and Induction Agents
Sedatives are often used in the premedication protocol in horses along with acepromazine. Depending on the procedure and the nature of the horse the use of either a alpha 2 agonist or opioid may be considered. It is important to also have a quiet environment to allow of full effect of the drugs.
Induction is most commonly intravenous using agents such as thiopentone or ketamine with a muscle relaxant such as diazepam. At this time, the minimum personnel should be present within the knockdown box or near to the horse if being performed in the field. Again, it is essential to have a quiet enviroment at this time to allow the horse to relax fully.
There are many different methods used to aid horses from conciously standing to a state of unconciousness. These include :-
- The horse is induced and allowed to fall completely unsupported.
- Most safely carried out in a padded knockdown/recovery box to prevent injury, or in field techniques when a large open space is available so that the horse has nothing to fall into.
- Freestanding knockdowns should be avoided in horses with fractures or other unstable injuries.
- Supported by personnel
- The horse is induced and then a wall, ideally padded, is used to help slide the horse into lateral recumbancy.
- The horse can either be held parallel to the wall and allowed to slide down, or with their hindquarters in the corner of the room with the handler holding onto a lead rope attached to a head collar applying gentle pressure on the shoulder of the horse so that it can be pushed backwards into a dog-sitting position to allow for better positioning.
- Requires no specialised equipment.
- Personnel should be experienced due to the risk to the handlers if induction does not go smoothly.
- Tilt tables
- The horse is induced with the table in an upright position. Once relaxed swift application of belly bands is performed and the table is tilted.
- Requires complex equipment and experienced personnel.
- Squeeze box/Swing door
- The horse is positioned such that its hindquarters is at the hinged wall with the door closed onto it. Induction agents are given and the door/gate is used to support the horse in an upright position allowing it to sink into sternal recumbancy.
- Requires a purpose built door or gate.
- Useful when minimal/inexperienced personnel available.
- Slings and belly bands
- The horse is strapped into the sling before induction. Once in, the horse is induced and lowered to the floor and into lateral recumbancy once unconciousness has been reached.
- Requires a good natured horse that is used to application of belly bands and straps.
- Requires specialised equipment.
- May be useful in fractured limb cases.
Many problems encountered during anaesthesia in domestic species occur in the horse also. These include hypotension, cardiac dysrrhythmias, hypoxaemia and hypercapnia.
Post Operative and Recovery
Many complications of anaesthesia in horses is not detected until the recovery period.
Post Operative Myopathy
Post Operative Myopathy occurs most commonly in large well muscled horses, evident only when the horse tries to stand upon recovery and is a serious cause of post operative anaesthetic morbidity. It usually affects the dependent muscles during surgery and is thought to be caused by ischaemic damage to underperfused muscles. Clinical signs vary depending on severity but include :-
- Mild lameness to inability to stand
- Hard, swollen and painful muscles
- Extreme distress
- Difficulty breathing
Treatment is symptomatic and includes analgesia, sedation and diuresis. Prevention is the best management for this condition and should include preventing hypotension, correct positioning of the horse during surgery and appropriate and correct padding used.
Neuropathy again is seen after the horse tries to stand upon recovery and displays similarly to that of myopathy. It is thought to be caused by ischaemia-induced hypoxia due to either direct pressure on the nerve or artery supplying the area. Clinical signs vary depending on the nerve affected and are similar to that of a myopathy expect the muscles are non painful. Nerves commonly affected include the radial, facial and femoral nerves. Laryngeal paralysis can occur if the neck is over extended during surgery. Treatment, again, is symptomatic and slings and supports may be beneficial in some cases. Prevention is similar to that of myopathies.
Self Inflicted Injury
Self inflicted injury commonly occurs upon recovery in horses due to their size, temperament and flight instinct meaning they try to stand before they are fully ready. Injuries can vary in severity but can result in euthanasia if serious. Prevention methods include sedation, analgesia, quiet environment, good surface, an empty bladder, respiratory support and if possible manual support/assisted recoveries.
Post Operative Colic
General anaesthesia agents reduce the gut motility. This in turn can lead to mild transient discomfort and decreased faecal output, which can lead to caecal impaction which itself can be fatal if rupture occurs.
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A review of the American College of Veterinary Anesthesiologists guidelines for anesthesia of horses. Hubbell, J. A. E.; Werner, H. W.; American Association of Equine Practitioners (AAEP), Lexington, USA, Proceedings of the 54th Annual Convention of the American Association of Equine Practitioners, San Diego, California, USA, 6-10 December 2008, 2008, pp 48-53, 30 ref. - Full Text Article