General anaesthesia in horses: all you need to know

• This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.

The decision to put a horse under a general anaesthetic should not be taken lightly as one in every 100 healthy horses will not survive the procedure or the seven days that follow surgery

General anaesthesia in horses: Risks | Preparation | Five stages of anaesthesia | Pain control | Post-anaesthesia care

When a horse has to undergo anything beyond minor standing surgery, which can be achieved under heavy sedation, a general anaesthetic is required so that the animal is safely immobilised, unconscious and provided with suitable pain relief throughout the procedure.

While the challenges associated with placing a horse under general anaesthesia should not be under-estimated as the risks associated with it are significant, there has been a notable increase in the knowledge, equipment and medications available to anaesthetise horses safely in recent years. This allows veterinary teams to successfully perform much more complicated procedures, such as colic surgery and repair certain fractures, than was previously possible.

What are the risks?

Research reviewing more than 41,000 cases across six years¹ found that one in 100 (0.9%) horses die during or within seven days of anaesthesia, with the figure higher still (1.6%) for emergency cases. This compares to one in 1,000 for small animals and one in 29,000 for humans. This unsatisfactory figure in horses has not changed appreciably over the past 20 years.

In part this is because as anaesthesia technology has improved, more sophisticated longer surgical procedures are being performed. Equally surprising perhaps is that the figure applies to healthy horses undergoing surgery for routine procedures; the risk is inevitably greater in emergency cases involving sick horses. For these reasons it is important that anaesthesia and surgery are only ever performed when they are genuinely justifiable.

So why are horses at a much higher risk than humans and small animals?

Firstly you have the challenge of getting around half a tonne of horse from standing awake on all four legs to recumbent and anaesthetised, then moved on and off an operating table prior to recovering smoothly to standing up again. Even when using a padded ‘knock down’ room for this purpose, such as the ones that are available at many equine hospitals, there are risks as the horse goes down and also when they regain their feet after surgery. Unfortunately horses may sustain a fatal fracture either during induction at the start of anaesthesia or more likely as they regain their feet during the recovery phase.

The horse’s bulk also complicates anaesthesia. Horses are extremely heavy, and it is possible for muscle damage to occur on their underside when they are lying still under anaesthetic for any length of time – put simply this can be similar to severe muscle cramps, which then impede their recovery.

If a horse needs to be positioned on their back during a surgical procedure, which is often the case, for instance with colic surgery, the bulky guts tend to squash the lungs and reduce their ability to inflate and, or oxygenate blood. While such changes can occur in humans and smaller animals, the large size of the horse, along with anatomic adaptations for athletic performance, makes the problem considerably worse. The use of mechanical lung ventilators help, but do not fully resolve problems with breathing and blood oxygenation.

Cardiac arrest was once thought to be the most common cause of anaesthetic death in horses. However, progressive improvement in monitoring and medicines to support the cardiovascular system has occurred over recent years, so that anaesthesia alone is less likely to be fatal.

Unfortunately – and to some extent unavoidably – horses still succumb during recovery from anaesthesia to catastrophes that would not have occurred had the horse not been anaesthetised, hence the safety of “standing surgery”. Probably the most distressing of these – to both horses and those attending them – are unexpected fractures that occur in recovery when horses stand clumsily, or normally, but when bone is weakened by surgery, undetected injuries or disease.

Preparing a horse for an anaesthetic

If your horse is insured, then you should speak to your insurance company before your horse is booked in for any routine operation to ensure the procedure is included in your cover.

In the case of an emergency, you should notify your insurance company as quickly as possible. If surgery goes ahead without your insurer’s knowledge then it is possible you may find yourself with a very large vet bill to pay. Some companies may request an extra premium to cover the higher risk of a general anaesthetic.

Most equine hospitals will ask for the horse’s shoes to be removed, to reduce the risk of injury, but this is not always the case so check with your vet. You should also ask whether the horse should be starved ahead of the procedure and if so, for how long. Current thinking is that starving a horse prior to surgery is not necessarily beneficial and can be linked to complications such as gastric ulcers or post-operative colic. This may also be linked to the medications administered, so careful monitoring before, during and after surgery is really important.

While minor, brief, simple procedures can be done safely in the field, challenging operations need to be conducted where optimum facilities are present, which really means an equine hospital with full theatre facilities and vets with specialist skills in both anaesthesia and surgery. This reduces the risks involved with anaesthetic and, if problems do occur, more equipment and expertise is readily available.

However, optimum facilities also require dedicated and skilled personnel. Despite the introduction of new drugs, equipment and knowledge, the most important safety factor after the horse itself is the hospital team, comprising anaesthetists, grooms, nurses and surgeons.

The five stages of anaesthesia

The risks of anaesthesia-related tragedies can be reduced by proper planning, training and paying meticulous attention to detail. Most anaesthetics are uncomplicated and uneventful, but in others critical events can occur for (occasionally) no obvious reasons. These demand prompt action to avoid serious consequences. Regrettably, not all cases result in a successful outcome.

• The vital planning phase

Any horse that is due to be anaesthetised will be given a thorough check-up to detect risks of any potential complications. The anaesthetic regime can then be modified to suit that horse. It’s possible that a routine operation will need to be postponed until the horse is fit enough to be anaesthetised. In addition the horse’s temperament can be assessed. Usually, the neck is clipped and a catheter inserted into the jugular vein to provide a pain-free route for giving the anaesthetic and intravenous fluid therapy. The area for surgery may also be clipped to save time later.

• Pre-anaesthetic

This often includes giving the horse a “pre-med” injection as a tranquilliser, which reduces anxiety and provides protection for the horse’s heart once anaesthetised. The horse is then groomed, the feet and mouth washed and the tail bandaged.

• Induction or start

Unlike people, horses do not lie down quietly counting to 10 as anaesthetic is injected and so measures are required to ensure they do not harm themselves (nor those around them) as they collapse at the start of anaesthesia (induction). Modern anaesthetic techniques help, but purpose-built padded induction and recovery boxes fitted with various means to support the horse are becoming increasingly popular, so that the horse lies down on a soft surface. Initially, the horse is heavily sedated and then several minutes later an anaesthetic injection is given to render it unconscious.

• Maintenance of anaesthesia

Once the horse is lying down, an “endotracheal tube” — a long, hollow tube — is passed through the mouth via the larynx and down into the windpipe. A cuff is blown up around the tube so that the horse breathes only the gases supplied via the anaesthetic machine. This is a mixture of oxygen and the anaesthetic gas that keeps the horse unconscious. Frequently, cocktails of intravenous anaesthetic drips are used instead of or as well as breathing in gases.

An overhead hoist may be used to move the horse from the padded box into the theatre — some patients may weigh up to 1,000kg. The horse is then carefully positioned on the operating table on its side or back, depending on the surgery to be performed.

Positioning is very important, especially for operations lasting several hours, to avoid a complication known as post-anaesthetic myopathy, where horses’ muscles become swollen and painful as a result of poor blood supply while being “squashed” during the operation.

During any operation, the horse is constantly monitored so that the depth of the anaesthetic is known. The eye reflexes are noted and the rate and character of breathing are recorded. Sometimes it is necessary to control the breathing via a ventilator. Sophisticated monitoring equipment records the horse’s heart rate and rhythm and other vital signs. A catheter may be placed in an artery to monitor blood pressure and if it drops, specific treatment is given. Low blood pressure during surgery can result in major complications.

• Recovery

After the operation the horse is hoisted back into a padded recovery room. Usually it is left alone to come round in a darkened, quiet environment while being watched from a safe distance. It can be necessary to give further sedatives to make the recovery smoother.

Recovery from anaesthesia in horses is frequently unpredictable and it is not until the horse is safely back on its feet that an operation can be considered successfully completed. Many different techniques have been used to ensure a horse stands up successfully after surgery, from a quiet independent “natural” recovery to interventional systems using ropes² or slings. In some US and Swiss centers, horses are recovered on modified rafts in purpose-designed swimming pools.

Understandably, not all horses are temperamentally suited to these (or other “interventions”) and there is no one fail-safe technique. This is why so many different options are being tried. However, a recent survey showed that 40% of equine anaesthetists provide some form of assistance during recovery, although techniques that suit one horses may not be tolerated by others. The reality is that a limb dislocation or fracture only requires the horse to make one unsteady step in its attempt to stand.

Pain control

Pain has a major effect on the quality of a horse’s recovery from anaesthesia, with animals experiencing the highest levels of pain having the worst recoveries. Fortunately, there is now a considerable amount that can be done to make horses more comfortable at this time. Options range from giving “bute” in minor cases to potent analgesics like morphine, and newer morphine-like drugs, where required.

Different painkillers can also be given in several ways, ranging from sophisticated mixtures given by intravenous “drip”, to epidural catheters: thin tubes passed into the spine via the tail-head. The latter are particularly good for controlling chronic debilitating hind limb lameness.

Previously, concerns with drug side effects or fears that painkillers would “mask” clinical signs were used as a reason not to give them to horses. Fortunately (for horses) these fears are being increasingly recognised as groundless.

Furthermore, it is being increasingly appreciated that pain itself produces major problems, e.g., it delays healing. Consequently, analgesics are being used more aggressively, and in different ways. For example, giving analgesics before surgery (and pain) begins – in an approach known as pre-emptive analgesia – improves the quality of pain relief produced, so is becoming standard practice.

The potential side-effects of potent drugs are also being minimized by multi-modal treatment, which involves giving a combination of different pain-killers, together, at reduced doses, to produce a comfortable animal with lower risk of adverse drug effects.

Post-anaesthetic after care

The horse’s care after coming around from an anaesthetic will depend on the length of surgery and the procedure that was performed. Most horses will need to spend days in the equine hospital before being discharged to return home to reduce the risk of post-surgery complications. The aftercare required at home will also vary, but your vet will be able to advise on your horse’s needs. All aftercare instructions should be closely adhered to in order to avoid the likelihood of complications.

References

• The confidential enquiry into perioperative equine fatalities (CEPEF): mortality results of Phases 1 and 2 September 2002
• A retrospective report (2003–2013) of the complications associated with the use of a one-man (head and tail) rope recovery system in horses following general anaesthesia February 2018

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