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Anaesthesia

A horse may require a general anaesthetic for a routine planned procedure such as castration or for an emergency such as urgent colic surgery. When you are told that your horse needs surgery, there is a tendency to focus on the operation itself and forget the anaesthetic. It is easy to take anaesthesia for granted as an integral part of the procedure. However, for every horse, it is essential to consider the anaesthetic risks, when making the decision whether or not to proceed with surgery. A recent detailed survey on equine anaesthesia reviewed more than forty thousand anaesthetics. This investigated the perioperative deaths, i.e. those during and within seven days of general anaesthesia. This study revealed an overall death rate of 1.6%. If the sick colic cases were excluded, the death rate was still approximately 1%. One death in a hundred horses is frightening in comparison with the perioperative mortality rate in man, which is one in ten thousand. In companion animals (cats and dogs) it is about one in seven hundred. This means that there is a significant risk of any horse dying under any anaesthetic.
Although 1% might almost be an acceptable anaesthetic death rate for horses that are ill, it is alarming when it applies to normal healthy horses undergoing surgery for routine procedures. For this reason it is important that any anaesthesia and surgery is only ever performed when it genuinely justifiable rather than removing blemishes for cosmetic reasons. If it is not going to truly benefit the horse, you need to ask whether it is worth doing. Your vet should be able to advise you.

There are several reasons why horses are difficult to anaesthetise. For a start to have a general anaesthetic, the horse has to be made to lie down. Unlike people, horses are not already lying down quietly counting to ten, when the anaesthetic drug is administered. Care has to be taken to ensure that they do not harm themselves as they fall or when they struggle to stand after surgery. Modern anaesthetic techniques help, as do the well padded “knock down and recovery” boxes available at many equine hospitals, which are designed to reduce injury. At Bell Equine we constantly monitor every horse throughout anaesthesia and during recovery, and assist them to their feet, when required.


Unfortunately the horse’s large size complicates anaesthesia. They are extremely heavy, especially when they are lying still under anaesthesia for any length of time. The muscles on their underside can be squashed and the blood supply reduced by their bodyweight. When horses are positioned on their backs during an operation the large stomach and hindgut squash the lungs and make it more difficult for the horse to inhale enough oxygen with each breath. Their large hearts beat slowly anyway, and under anaesthesia the heart can all too easily stop altogether. Cardiac arrest was the commonest cause of anaesthetic death in the survey of perioperative equine fatalities.

 

In an emergency there will be no opportunity to plan ahead, but for a routine operation you should:

• Notify your insurers first. Always discuss the details of any operation with your insurers since you need their agreement to proceed. Some companies request an extra premium to cover the extra risk. In an emergency, the insurers should be notified as soon as it is possible to do so.

• Arrange for the shoes to be removed before any surgery, so that the horse does not damage itself when lying down or standing after the anaesthetic.

• Check with Bell Equine about starving the horse before surgery. Many vets routinely starve a horse overnight and sometimes longer for certain operations.

• Arrange for the operation to be performed in the safest possible place. In order to carry out major surgery, the horse should be admitted to the hospital where full theatre facilities and vets with specialist skills in both anaesthesia and surgery are available. This reduces the risks involved with an anaesthetic, and if problems do occur more equipment and expertise is readily available. Some surgery is done in the field, particularly relatively minor procedures such as castration, in which case ensure there is a clean empty paddock available. Check with us in advance exactly what will be required. Nowadays there are such good equine hospital facilities available that it is foolhardy to carry out major surgery in the field.

1. The vital planning phase: may include giving painkillers and any other treatments in advance of surgery. A thorough check-up is carried out to detect any potential dangers. The anaesthetic regime can then be modified to suit that horse or, if necessary, the operation postponed until the horse is fit enough to be anaesthetised. In addition the horse’s temperament can be assessed. Frequently the neck is clipped and a catheter inserted into the jugular vein to provide a pain free route for giving the anaesthetic injections and for intravenous fluid therapy. The area for surgery may also be clipped to save time later.

2. Pre-anaesthetic: this often includes giving the horse a “pre-med” injection of a tranquilliser, which reduces anxiety and provides protection for the horse's heart once anaesthetised. The horse is then groomed, the feet washed and the tail bandaged.

3. Induction or start: Ideally the anaesthetic is induced or started in a padded box, so that the horse lies down onto a soft surface. Initially the horse is heavily sedated and then several minutes later an anaesthetic injection is given to render the horse unconscious.

4. Maintenance of anaesthesia: Once the horse is lying down an "endotracheal tube" - a long hollow tube - is passed through the mouth, via the larynx (i.e. throat) and down into the windpipe. A cuff is blown up around the tube so that the horse only breathes the gases supplied via the anaesthetic machine. This is a mixture of oxygen and the anaesthetic gas that keeps the horse "asleep". Sometimes cocktails of intravenous anaesthetic drips are used instead of breathing in anaesthetic gases.
An overhead hoist may be used to move the horse from the padded box into the theatre - some patients may weigh 1000kg or more. The horse is then carefully positioned on the operating table, either on its side or back depending on the surgery to be performed. Positioning is very important, especially for operations lasting several hours, particularly to avoid a complication known as post-anaesthetic myopathy, where horses muscles to become swollen and painful as a result of poor blood supply whilst being "squashed" by the horse’s weight during the operation. For some horses, this can be a serious and distressing (and even life-threatening) problem.

5. Recovery: Once the operation is finished, the horse is hoisted back into a padded recovery room. Usually the horses are left alone to slowly "come round" in a darkened, quiet environment whilst being constantly and carefully watched from a safe distance and via CCTV. Sometimes it is necessary to give further sedative drugs to make the recovery smoother. Excitable horses that try to stand up before they are co-coordinated enough to do so, may require this. The recovery phase of the anaesthetic is a risky time when complications can occur. Occasionally a horse may fracture a leg whilst trying to stand. As the horse comes round from the anaesthetic its movements are unpredictable and it is dangerous for people to be in with them. It is not until the horse is safely standing up that the operation is considered to have been successfully completed.
The ideal anaesthetic prevents pain and stress for the horse before, during and after surgery. Some procedures can be performed with the horses standing under heavy sedation and by using local anaesthesia, but in many cases a general anaesthetic is essential for successful surgery to be performed. Pain relief is crucial throughout any procedure, as an unstressed and pain free patient will heal faster.
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 horses 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 extra treatment is given. Low blood pressure during surgery can result in major complications.