Being told that your horse needs an operation under general anaesthesia can be extremely stressful for horse owners. Horses are known to be ‘bad anaesthetic patients’ and have a higher danger of complications than with other animals. Richard Stephenson MRCVS explains how vets try to minimise the risks involved in anaesthetising a horse.
It is well known that the horse is an ‘at risk’ animal when it comes to general anaesthesia. The chances of complications occurring are far higher with horses than with any other type of commonly encountered animal. Partly this is due to their shear size and weight which makes handling difficult and also during prolonged procedures the blood flow to the muscles on the lower side of the horse can become restricted leading to muscle damage. But its not just weight that’s the problem – other large animals such as elephants manage to get through anaesthetics with far less problems. It is also the horse’s temperament. Horses are ‘flight’ animals – their natural instinct is to run away from danger or unfamiliar experiences. This means that they are prone to panic when confined or recovering from anaesthesia. Vets have developed systems to minimise the risks involved in anaesthetising these large, heavy, panic prone patients.
Prior to anaesthesia your horse will be given a clinical examination. Your vet will check for any heart or lung problems. It is usual to remove the shoes and clean any dirt from the feet. If the operation requires an area of hair to be clipped then this will usually be done prior to ‘induction’ (the process of sending the horse to ‘sleep’). An intravenous catheter will be inserted into one of the jugular veins – this is used to administer any necessary injections and also gives instant access to the blood system if an emergency occurs.
Most veterinary practices regularly anaesthetising horses will have a special ‘induction’ box. This will be heavily padded to prevent injury to the horse.
Prior to entering the induction area the horse will be heavily sedated. Every effort is made to keep the horse calm and quiet. Generally the feet are covered with tape both for cleanliness and to prevent them damaging the floor or causing self injury. Once the horse is drowsy it is manoeuvred behind a ‘crush door’. The door is heavily padded and used to support the horse against the wall so that it slides to the ground gently with minimal risk of injury to itself or the handlers.
At this stage anaesthesia is induced using injectable agents. This is a critical period and all the veterinary staff will be concentrating on getting the horse safely to the ground.
Once the horse is anaesthetised a rubber tube called an ‘endotracheal (ET) tube’ is inserted into the windpipe via the mouth so that the horse can be connected to an anaesthetic gas machine.
The horse must then be moved into the operating theatre (or preparation room). Although there are many different ways of doing this most clinics use an overhead railway line lifting the horse by hobbles on its legs. This is a very successful and safe system. Where the horse is being treated for a fracture it can be lifted with just three limbs.
Finally it will be placed on an operating table. The design of equine operating tables is also very important. Most are either ‘airbeds’ or have extensive padding to reduce the damage caused to the horses muscles by lying on them for prolonged periods.
During the operation the horse is monitored by a vet. An ECG (heart monitor) is connected, blood pressure is taken and the level of oxygen in the blood stream measured.
At the end of the procedure the horse is returned to a ‘recovery box’ a small padded area where it can recover from the anaesthetic slowly and calmly.
Is my horse too old to have a general anaesthetic?
Surprisingly older horses have a lower risk of serious complication during anaesthesia than young fit horses. If a horse is otherwise healthy then it is never too old for an anaesthetic.
What is the mortality rate for horses that have a general anaesthetic?
There have been a number of large studies into mortality under general anaesthesia. Generally a rate of 0.5% (i.e. 1 in 200) is about average. This is far higher than in any other species that vets deal with. Lengthy procedures such as fracture fixation or colic are associated with higher mortality rates, short procedures only requiring intravenous drugs are safer.
My horse has a heart murmur – will having an anaesthetic be too much for it?
A large proportion of normal healthy horses have audible heart murmurs. These are often called ‘physiological’ murmurs. Your vet will check your horse’s heart before anaesthesia and in all probability will fine that the murmur is of no consequence. On rare occasions a severe murmur is detected and then a careful risk assessment needs to be made balancing the need for surgery against the increased chances of complications occurring.
Should a horse be starved overnight before an anaesthetic?
Horses cannot vomit; therefore the danger of them regurgitating food into the windpipe during ‘induction’ is minimal. Therefore it is not necessary to starve horses for a prolonged period prior to anaesthesia – indeed as horses are very prone to gastric ulceration it is probably undesirable to with hold food. Most vets will stop feed for a couple of hours prior to an operation but this is not essential.
Why do you have to remove the shoes?
Anaesthetising a large animal like a horse can be dangerous for the handlers – especially if the horse has heavy metal shoes on. However in most instances we can now perform operations under GA with the shoes left on.