Whilst many procedures can be performed in standing sedated horses with the help of local anaesthetic drugs, there are occasions where full general anaesthesia is required for procedures to be carried out safely for both horse and surgeon.

Some short anaesthetics can be performed in the field, these instances are more often the exception than the rule and full theatre facilities are required for most procedures.

Pre-anaesthetic prep'

Before surgery is performed, there are multiple tasks that must be carried out:

If the procedure is elective, the horse will be starved overnight to ensure the stomach is free from food contents, in an emergency situation this is not vital and the priority must be given to treating the emergency over starving the horse.

Where possible, the horse will be weighed to ensure accurate drug calculations and therefore doses can be acquired. A thorough clinical examination is performed, with particular attention to the heart and lungs, ensuring that the horse is well enough to undergo surgery.

The shoes will be removed to prevent them slipping during recovery, pulling a shoe and damaging themselves, or damaging the padded recovery box floor.

An intravenous catheter is placed in the jugular vein to allow venous access throughout the procedure. This is usually placed on the left hand side of the neck, however if the procedure involves the right leg, it will be placed in the right vein, this allows access to the catheter when the horse is lying with its affected leg uppermost.

The catheter site is clipped and prepared in a sterile fashion. Other areas may be clipped to allow application of ECG pads and other pieces of monitoring equipment.

Pain relief and antibiotics, if appropriate will be administered pre-operatively.

Induction and maintenance of anaesthesia

Anaesthesia is induced via the intravenous catheter, the horse is first sedated and then a combination of two drugs is the usual regime.

Once the horse is asleep, an endotracheal tube is placed either from the mouth, or from the nasal passageways into the trachea (windpipe). This depends on the intended surgical procedure, for example tooth extractions will require the tube to come from the nose, in order to give the surgeon greatest access to the mouth.

Anaesthetic gas and oxygen are administered through this tube to the lungs to keep the horse asleep. The anaesthetist will alter the amount of anaesthetic gas, depending on how responsive the horse is.

The horse is then moved from the induction box to the theatre suit via a winch suspended from the ceiling. The operating table is well padded to prevent muscle damage.

Recovery

The horse is placed back into the recovery box (the padded room) and sedated to allow the horse to come round slowly from the anaesthetic drugs, to prevent the horse panicking and trying to stand too early.

A rope recovery system is commonly used to help stabilise the horse once it is back on its feet. In young horses, or un-handled horses, often the rope system won't be used as it can make them panic if they are not used to being restrained.

It takes approximately one hour for the horse to get back onto its feet, however this varies depending on the length of the surgery and therefore the amount of anaesthetic agents used. The horse will remain quite subdued/sleepy for several hours after returning to its stable.

Anaesthetic risk

General anaesthesia carries a risk of death or serious injury in any species, however the risk of mortality/morbidity in the horse is greater.

The problems most frequently encountered include injury during induction, or recovery from anaesthesia, unexpected drug reactions, heart or breathing difficulties and muscle or nerve damage.

But, the majority of horses undergoing anaesthesia do so safely and we use up to date monitoring equipment to ensure the horses cardiovascular system is stable. However, risk can never be completely eliminated.

As a general rule, 1% of healthy horses have complications during or following anaesthesia. This is even higher for compromised patients, such as very sick colics.

If the horse is insured, the insurance company should be informed of the intention to perform a general anaesthesia and many companies will not cover costs of complications if they did not have prior notice. In an emergency situation, where it is not possible to contact the insurers, exceptions can be made.

Foals

In many ways, the foal is very similar to the adult horse in response to anaesthetic agents.

Some of the risks that adults can encounter are not as much of a concern in foals, for example they are not as heavy and so they don’t encounter the same muscle damage as a large horse lying on its muscles and they are easier held manually to help them to their feet.

Newborn foals do bring their own complications – they need intravenous infusions of glucose throughout the procedure as they don’t have fat stores to rely upon for energy, they are susceptible to becoming hypothermic (too cold) again due to their lack of body fat.

The mare should be kept with the foal for as long as possible, even allowing the mare to see induction of the foal. The mare usually requires to be sedated for the duration of the surgery, to keep her calm.

In summary:

Anaesthesia in the horse is a complex procedure, as such it is more often than not carried out by a veterinary surgeon.

In order to reduce the risks associated with anaesthesia, advanced monitoring equipment is required to ensure the horse has as few complications as possible.

It can be a daunting and worrying time when your horse has to have an operation, having a good line of communication between the surgeon and anaesthetist before the procedure and raising any concerns you have should help to ease some of these concerns.